Solar Energy Glossary

Alternating current (AC) is a type of electrical current that periodically reverses direction. It is the standard form of electricity used in American homes, businesses, and the power grid. In the United States, AC power operates at 60 cycles per second (60 Hz).

Solar panels produce direct current (DC), not AC. For the electricity generated by your panels to power your appliances, lights, and HVAC system, it must first be converted from DC to AC by an inverter. This DC-to-AC conversion is a fundamental step in every solar energy system. AC electricity is preferred for power grids because it can be easily transformed to higher or lower voltages for efficient long-distance transmission. When your solar system is grid-tied, the AC electricity it produces either powers your home directly or flows back to the utility grid through net metering.

AC coupling is a system configuration where solar panels and battery storage are connected through the AC (alternating current) side of the electrical system. In an AC-coupled setup, the solar array has its own inverter (converting DC to AC), and the battery has a separate inverter/charger that converts AC back to DC for storage and then back to AC when discharging.

AC coupling is the most common method for adding battery storage to an existing solar system. Since the solar panels already have their own inverter, a battery with a built-in inverter can simply be connected to the home’s AC electrical panel — no need to rewire the solar array. This makes AC coupling ideal for retrofits. Popular AC-coupled batteries include the Tesla Powerwall and Enphase IQ Battery. The slight downside is a small efficiency loss from the double conversion (DC → AC → DC → AC), typically around 5–10%. The alternative, DC coupling, connects the battery directly to the solar panels on the DC side, which is slightly more efficient but requires a compatible hybrid inverter and is more common in new installations designed from scratch.

Additionality is a concept used in renewable energy and climate policy to determine whether a specific action (like purchasing RECs or investing in a solar project) leads to new renewable energy generation that would not have happened otherwise. A project has “additionality” if it would not have been built without the specific financial incentive or investment.

This concept is important for understanding the real climate impact of renewable energy purchases. For example, buying RECs from a decades-old wind farm doesn’t create any new clean energy — that farm would have operated regardless. But investing in a new community solar project that wouldn’t exist without subscriber funding does have additionality. For residential solar homeowners, installing panels on your roof has clear additionality — you are directly adding new clean energy generation capacity that would not exist without your investment. The concept becomes more relevant when evaluating corporate “100% renewable” claims, community solar programs, or green energy tariffs from utilities. Programs with genuine additionality have a measurable impact on reducing fossil fuel use, while programs without additionality may just be relabeling existing clean energy.

An ampere-hour (Ah) is a unit of measurement for battery capacity, indicating how much electric charge a battery can store and deliver over time. One ampere-hour means a battery can supply one ampere of current for one hour.

In the solar industry, ampere-hours are commonly used to rate the capacity of deep-cycle batteries in off-grid and hybrid solar systems. For example, a 200 Ah battery at 12 volts stores 2,400 watt-hours (2.4 kWh) of energy. To determine the total energy capacity in kWh (which is more intuitive for homeowners), you multiply the Ah rating by the battery voltage and divide by 1,000. While residential lithium-ion batteries like the Tesla Powerwall are more commonly rated in kWh (13.5 kWh for the Powerwall), lead-acid and some off-grid batteries still use Ah ratings. Understanding ampere-hours helps homeowners compare battery options and calculate how much backup power they will have during an outage.

The Authority Having Jurisdiction (AHJ) is the local government entity — typically the city or county building department — that has the legal authority to approve, inspect, and enforce building codes and permits for solar installations in your area. The AHJ is a central player in the permitting and approval process for every residential solar project.

Before your solar system can be installed, your installer must submit permit applications to the AHJ, including structural and electrical plans, equipment specifications, and site details. The AHJ reviews these plans to ensure compliance with local building codes, the National Electrical Code (NEC), and fire codes. After installation, the AHJ sends an inspector to verify that the system was built according to the approved plans. Only after the AHJ inspector approves the installation can the utility proceed with interconnection and Permission to Operate. AHJ requirements vary significantly — some jurisdictions have streamlined solar permitting with fast online approvals, while others require lengthy plan reviews and multiple inspections. Solar-friendly AHJs that have adopted the SolarAPP+ automated permitting platform can issue permits in hours rather than weeks.

Azimuth is the compass direction that a solar array faces, measured in degrees from north. In the United States (Northern Hemisphere), a south-facing azimuth of 180° is considered optimal because it captures the most sunlight throughout the day.

True south (180°) maximizes total daily production because the array faces the sun’s path across the southern sky. However, east-facing (90°) and west-facing (270°) arrays still produce significant energy — typically 80–85% of a south-facing array. Southwest-facing arrays (200°–240°) can actually be preferable for homeowners on time-of-use (TOU) rate plans, as they produce more electricity during expensive late-afternoon peak hours. North-facing arrays (315°–45°) are generally not recommended as they produce 30–50% less than south-facing panels. Most residential roofs don’t face exactly south, and solar installers account for your roof’s actual azimuth when modeling system production. Modern design software calculates the combined effect of tilt angle and azimuth to accurately predict annual energy output for your specific roof orientation.

Backup power refers to the ability of a solar-plus-battery system to continue providing electricity to your home during a utility grid outage. Without battery storage, most grid-tied solar systems automatically shut down during an outage for safety reasons — backup power solves this limitation.

When the grid goes down, a battery system with backup capability detects the outage and automatically disconnects from the grid (a process called islanding). The battery then powers your home’s essential circuits, and if sunlight is available, the solar panels continue generating electricity to recharge the battery. Backup power systems come in two main configurations: critical loads backup (powering only selected essential circuits through a dedicated subpanel) and whole-home backup (powering the entire house, which requires a larger battery system and higher-output inverter). The amount of backup time depends on battery capacity, your energy usage, and solar production. A single 13.5 kWh battery might power essential loads for 12–24 hours, while whole-home backup during heavy usage could drain the same battery in just a few hours. Backup power has become one of the top reasons homeowners add batteries to their solar systems, especially in areas prone to outages.

A backup power system is any combination of equipment designed to provide electricity to your home during a utility grid outage. In the solar context, a backup power system typically consists of solar panels, a battery storage system, and an inverter capable of operating independently from the grid.

Solar-plus-battery backup systems offer a clean, quiet, and automatic alternative to traditional gas generators. When the grid goes down, the system detects the outage, disconnects from the grid (via a transfer switch), and begins powering your home from the battery and solar panels. There are two main approaches: critical loads backup (powering only selected essential circuits) and whole-home backup (powering the entire house). The right choice depends on your budget, battery capacity, and how much power you need during an outage. A single battery (10–15 kWh) is typically sufficient for critical loads backup, while whole-home backup may require 2–4 batteries or more. Unlike generators, solar backup systems produce no noise, emit no fumes, and don’t require fuel — they recharge automatically from sunlight each day. This makes them especially valuable during extended outages lasting multiple days.

Balance of System (BOS) refers to all components of a solar energy system except the solar panels themselves. This includes the inverter, racking and mounting hardware, wiring, conduit, combiner boxes, disconnect switches, monitoring equipment, and the electrical connections to your home’s breaker panel and utility meter.

BOS components typically account for 40–60% of the total installed cost of a residential solar system. While solar panels get most of the attention, the quality and proper installation of BOS components are equally critical to system performance, safety, and longevity. A well-designed BOS ensures efficient power conversion, code-compliant wiring, reliable weather protection, and safe operation for decades. When comparing solar quotes, it’s important to look beyond just the panel brand — the inverter type, racking quality, and wire gauge all affect long-term system performance and maintenance costs.

A ballasted mount system is a type of solar racking that uses heavy weights (ballast) — typically concrete blocks — to hold panels in place on a flat or low-slope roof without penetrating the roof membrane. This approach avoids drilling holes in the roof, preserving the roof warranty.

Ballasted systems are most commonly used on commercial flat roofs but are also suitable for residential flat roofs and some low-slope roofs. The panels sit in trays or frames angled at 5°–15° and are held down by the weight of the ballast blocks. A structural engineer must verify that the roof can support the additional weight of the panels, racking, and ballast (typically 3–6 pounds per square foot). The main advantage is zero roof penetrations, which eliminates leak risk and simplifies installation. The downsides are the added weight and the inability to tilt panels at steeper angles that might optimize production. Wind uplift calculations are critical — in high-wind areas, more ballast weight or supplemental mechanical attachments may be required. Ballasted systems are generally easier to remove and relocate than penetrating roof mounts.

Base load (also called essential loads or always-on loads) refers to the minimum amount of electricity your home continuously consumes, even when you’re not actively using appliances. These are the devices that run 24/7 — refrigerator, HVAC fan, Wi-Fi router, security system, standby power for electronics, and lighting.

Understanding your base load is important for both solar system sizing and battery backup planning. A typical US home has a base load of 0.5–1.5 kW, consuming 12–36 kWh per day from always-on loads alone. When designing a battery backup system, your base load determines the minimum battery capacity needed to keep your home functional during an outage. For example, a 1 kW base load over 12 hours of nighttime equals 12 kWh — nearly one full Tesla Powerwall just for essential background consumption. Reducing your base load through energy-efficient appliances, eliminating phantom loads (unplugging devices that draw power when “off”), and using smart power strips can improve both your solar offset and your backup duration. Your installer may analyze your utility usage data to identify your base load pattern and size the system accordingly.

A battery bank is a group of batteries wired together to store electricity generated by a solar system. Battery banks provide backup power during grid outages and can store excess solar energy for use at night or during peak-rate periods.

Batteries in a bank can be wired in series (to increase voltage), in parallel (to increase capacity), or both. The total capacity of a battery bank is measured in kilowatt-hours (kWh). A typical residential battery bank ranges from 10 to 30 kWh — enough to power essential circuits for 8–24 hours depending on usage. Off-grid systems often require much larger battery banks (40–100+ kWh) since they must provide all power when the sun isn’t shining. Modern residential battery banks use lithium-ion chemistry (LFP or NMC), which offers high energy density, long cycle life, and low maintenance compared to older lead-acid batteries. Popular pre-packaged battery systems like the Tesla Powerwall (13.5 kWh) and Enphase IQ Battery (5 kWh per unit, stackable) have largely replaced custom-built battery banks in grid-tied residential systems, though DIY and off-grid installations still commonly use individual batteries assembled into banks.

A battery inverter is a dedicated device that manages the charging and discharging of a battery storage system. It converts AC electricity to DC for battery charging and then converts the stored DC back to AC when the battery discharges to power your home.

Battery inverters are used in AC-coupled solar-plus-storage systems, where the solar array has its own separate solar inverter. This configuration is common when adding battery storage to an existing solar system — you keep your existing solar inverter and add a battery with its own inverter. The battery inverter handles the bidirectional power flow: charging the battery from excess solar production or cheap grid power, and discharging to the home during outages or expensive peak-rate periods. Some batteries, like the Tesla Powerwall and Enphase IQ Battery, have the inverter built in, making installation simpler. Stand-alone battery inverters are used with battery banks that don’t include integrated electronics. The key specification to compare is continuous power output (measured in kW), which determines how many appliances the battery can power simultaneously.

A bidirectional meter (also called a net meter or bi-directional meter) is an electric meter that can measure electricity flowing in both directions — power drawn from the grid into your home and excess solar power sent from your home back to the grid. It is required for net metering.

Standard electric meters only measure consumption (one-way flow). When you install a grid-tied solar system, your utility replaces your old meter with a bidirectional meter (or reprograms a compatible smart meter) that tracks both import and export. The meter records how much grid electricity you use and how much solar electricity you send back, allowing the utility to calculate your net usage for billing. Some modern smart meters already have bidirectional capability and only need a firmware update. The meter replacement is typically handled by the utility as part of the interconnection process, often at no charge to the homeowner. Your bidirectional meter may display two readings: one for energy consumed from the grid and one for energy exported. The difference between these readings determines your net metering credit or charge for each billing period.

A building permit is an official authorization from your local government (the Authority Having Jurisdiction) that grants approval to install a solar energy system on your property. Obtaining a permit is legally required before any solar installation work can begin.

The permit application typically includes detailed plans showing the system layout, structural attachments, electrical wiring diagrams, equipment specifications, and fire code compliance. Your solar installer prepares and submits these documents on your behalf. Permit fees vary by jurisdiction — typically $100–$500 for residential solar, though some cities have reduced or waived fees to encourage solar adoption. Processing times range from same-day (for jurisdictions using automated platforms like SolarAPP+) to 2–6 weeks for manual review. Some jurisdictions require separate building and electrical permits, while others combine them into a single solar permit. Working without a permit is illegal and can result in fines, required system removal, voided insurance coverage, and problems when selling your home. The permit also triggers the inspection process that ultimately leads to Permission to Operate approval.

Capacity factor is the ratio of a solar system’s actual energy output over a period to its maximum possible output if it ran at full nameplate capacity 24 hours a day, 365 days a year. It is expressed as a percentage and reflects how effectively a system utilizes its rated capacity.

For residential solar, capacity factors in the US typically range from 15% to 25%, depending on location, weather, and system design. A 10 kW system running at 100% capacity for a full year would produce 87,600 kWh (10 kW × 8,760 hours). If it actually produces 15,000 kWh, its capacity factor is about 17%. This may sound low, but it’s normal — solar panels only produce during daylight hours and output varies with clouds, seasons, and panel orientation. Southwest states like Arizona and Nevada achieve higher capacity factors (20–25%) due to more sunlight hours, while northern or cloudier states may see 12–18%. Capacity factor is more commonly used for utility-scale solar projects and comparisons between energy technologies than for individual residential systems, where annual kWh production is the more practical metric.

A carbon footprint is the total amount of greenhouse gases — primarily carbon dioxide (CO₂) — generated by a person, household, organization, or activity. It is typically measured in metric tons of CO₂ equivalent per year. Burning fossil fuels for electricity, transportation, and heating are the largest contributors to most Americans’ carbon footprints.

Installing solar panels is one of the most impactful ways a homeowner can reduce their carbon footprint. The average US household emits approximately 7.5 metric tons of CO₂ annually from electricity use alone. A typical residential solar system can eliminate 80–100% of those electricity-related emissions. Over a 25-year lifespan, a single home solar system can prevent 100–150 metric tons of CO₂ from entering the atmosphere — the equivalent of planting thousands of trees. While manufacturing solar panels does produce some emissions, the “energy payback period” (the time it takes for a panel to generate enough clean energy to offset its manufacturing emissions) is just 1–3 years. After that, every kilowatt-hour the panel produces is essentially carbon-free electricity.

A change order is a formal modification to an existing solar installation contract that adjusts the project scope, equipment, cost, or timeline after the original agreement has been signed. Change orders are common in construction projects and may arise during any phase of a solar installation.

Common reasons for change orders in residential solar include: roof condition issues discovered during installation (requiring repairs before mounting panels), electrical panel upgrades needed to accommodate the solar connection (the 120% rule), structural reinforcement requirements identified during engineering review, changes in panel or inverter models due to supply chain availability, adjustments to system size or layout based on updated site survey findings, or homeowner-requested modifications. A legitimate change order should be documented in writing, clearly describe the change, explain why it’s necessary, state the cost impact (increase or decrease), and be signed by both parties before the additional work begins. Be cautious of installers who issue frequent or large change orders after offering a low initial quote — this practice, called “low-balling,” can significantly inflate the final cost. A thorough site survey before contract signing minimizes the likelihood of unexpected change orders.

A circuit breaker is a resettable safety switch in your electrical panel that automatically trips (turns off) when it detects excessive current, protecting wires and equipment from overheating and fire. Unlike fuses, which must be replaced after they blow, circuit breakers can simply be reset.

In a solar installation, a dedicated solar circuit breaker is installed in your main service panel (or subpanel) to connect the solar system to your home’s electrical system. This breaker is typically sized at 20–40 amps for residential systems, depending on the inverter’s output. The NEC’s “120% rule” limits the combined amperage of the main breaker and the solar breaker — for a 200-amp main panel, the solar breaker cannot exceed 40 amps. If your panel doesn’t have space for an additional breaker or can’t accommodate the solar breaker under the 120% rule, your installer may need to upgrade the panel or add a subpanel. Some solar components also use DC-rated breakers, which are specifically designed to safely interrupt direct current — standard AC breakers cannot be used for DC circuits because DC arcs behave differently.

A combiner box is an electrical enclosure that merges the output of multiple strings of solar panels into a single feed before it reaches the inverter. Each string of panels connects to the combiner box through its own circuit, protected by individual fuses or breakers.

In larger residential and commercial solar systems with a string inverter, panels are wired into multiple strings. The combiner box consolidates these strings, simplifying the wiring between the array and the inverter. It also provides overcurrent protection for each string, making it easier to isolate and troubleshoot individual strings if an issue arises. Combiner boxes are weatherproof enclosures typically mounted near the solar array on the roof or on a wall below. Smaller residential systems with only one or two strings may not require a separate combiner box, as the connections can be made directly at the inverter. Systems using microinverters generally do not use combiner boxes since each panel converts to AC independently.

Commissioning is the final step in a solar installation where the system undergoes safety testing and verification before being officially activated. This process is typically performed by your utility company and/or a certified inspector to ensure the system meets all electrical codes and is safe to operate.

During commissioning, inspectors verify that all wiring is correct, grounding is proper, the inverter is functioning, and the system meets local building codes and the National Electrical Code (NEC). For grid-tied systems, the utility also installs or reprograms your electric meter to accommodate two-way power flow (for net metering) and verifies that anti-islanding protection is working — meaning your system will automatically shut off if the grid goes down, protecting utility workers from unexpected electrical backfeed. Commissioning typically takes 1–4 weeks after the physical installation is complete, depending on your utility’s scheduling and inspection process. Once commissioning is finished and your Permission to Operate (PTO) is granted, your solar system is officially live and you can start generating electricity and earning credits.

Community solar is a shared solar energy program that allows multiple participants to benefit from a single, centrally located solar installation — without needing to install panels on their own roof. Participants subscribe to a portion of the solar farm and receive credits on their electricity bills for their share of the energy produced.

Community solar programs are designed for people who cannot install rooftop solar, including renters, condo owners, homeowners with shaded or unsuitable roofs, and those who cannot afford the upfront cost of a personal system. Subscribers typically pay a monthly fee or purchase a share of the solar farm, and in return receive bill credits valued at the electricity their share generates. Most community solar programs offer savings of 5–15% on electricity costs with no installation or maintenance responsibilities. As of 2026, over 40 states have at least one community solar project, with states like Minnesota, New York, Massachusetts, and Colorado leading in program availability. Community solar is one of the fastest-growing segments of the US solar market, making clean energy accessible to a broader population.

Critical loads only backup is a battery storage configuration that powers only selected essential circuits during a grid outage — such as the refrigerator, key lighting, internet, phone chargers, and medical equipment — rather than the entire home. It is the most common and cost-effective approach to solar battery backup.

With critical loads backup, a dedicated subpanel (the critical loads panel) is installed and wired to only the circuits you deem essential. During an outage, the battery powers only these circuits, conserving energy and extending backup duration significantly. A single 13.5 kWh battery powering 1–2 kW of critical loads can last 8–15 hours overnight, and if the sun shines the next day, the solar panels recharge the battery for continued operation — potentially providing indefinite backup during multi-day outages. The main limitation is that high-power loads like central air conditioning, electric water heaters, and electric dryers are typically excluded from the critical loads panel because they would drain the battery too quickly. This trade-off keeps the system affordable (one battery vs. three or four for whole-home) while providing meaningful backup for the loads that matter most during an emergency.

A critical loads panel (also called a backup panel or essential loads panel) is a dedicated electrical subpanel that contains only the circuits you want to keep powered during a grid outage when using battery backup. It is a key component of any solar-plus-storage system designed to provide backup power.

When the grid goes down, your battery can only supply a limited amount of power. Rather than attempting to back up your entire home (which could drain the battery in hours), a critical loads panel lets you prioritize the most important circuits. Typical critical loads include the refrigerator, a few lighting circuits, Wi-Fi router, garage door opener, phone charging outlets, and any medical equipment. During an outage, the battery and solar panels supply power exclusively to this panel while the rest of the home remains off. Your solar installer will help you identify which loads to include based on your battery capacity and priorities. Some newer battery systems (like the Tesla Powerwall 3) can provide whole-home backup without a separate critical loads panel, but this depends on the battery’s power output and your home’s peak demand.

Cycle life refers to the number of complete charge-and-discharge cycles a battery can perform before its capacity degrades to a specified level (usually 70–80% of its original capacity). It is one of the most important metrics for evaluating the longevity of a solar battery.

One cycle equals one full discharge and recharge — though this doesn’t have to happen all at once. Discharging 50% and recharging, then discharging 50% and recharging again, counts as one full cycle. Modern lithium-ion home batteries typically offer 4,000–10,000+ cycles depending on chemistry and depth of discharge. Lithium iron phosphate (LFP) batteries generally have longer cycle lives (6,000–10,000+ cycles) than lithium nickel manganese cobalt (NMC) batteries (4,000–6,000 cycles). If a battery cycles once per day, 5,000 cycles equals roughly 13–14 years of use. Most battery warranties are expressed in both years and cycles — for example, “10 years or 4,000 cycles, whichever comes first.” When comparing batteries, consider both the cycle life and the guaranteed capacity retention at end of warranty.

Direct current (DC) is electrical current that flows in one constant direction. It is the type of electricity produced by solar panels and stored in batteries.

When sunlight hits a photovoltaic cell, the energy knocks electrons loose, creating a flow of current in a single direction — that’s DC electricity. However, your home’s electrical system and the utility grid operate on alternating current (AC), so the DC power from your solar panels must be converted to AC by an inverter before it can be used. If your solar system includes battery storage, the batteries store energy as DC and then convert it back to AC when discharging to power your home. DC electricity is also used in many modern electronics — your phone charger, laptop, and LED lights all convert AC back to DC internally. Understanding the DC-to-AC relationship helps homeowners appreciate why the inverter is such a critical (and valuable) component of any solar installation.

Degradation rate is the annual percentage decline in a solar panel’s power output over time. All solar panels gradually lose a small amount of their electricity-generating capacity each year due to natural aging of the silicon cells and materials.

The industry-average degradation rate for modern solar panels is approximately 0.3–0.5% per year. This means a panel producing 400 watts in its first year will produce about 398 watts in its second year, and so on. After 25 years at 0.5% annual degradation, a panel retains roughly 88% of its original output — still generating substantial electricity. Higher-quality panels from premium manufacturers often degrade more slowly (0.25–0.3% per year). Panel warranties reflect expected degradation: a typical warranty guarantees at least 84–87% of original output after 25 years. First-year degradation (called Light-Induced Degradation or LID) is slightly higher (1–3%) as the cells stabilize. When modeling long-term solar production and financial returns, your installer should factor in the specific degradation rate of the panels being proposed.

A demand charge is a fee on your electricity bill based on the highest rate of power usage (measured in kW) during a billing period, rather than total energy consumed (measured in kWh). Demand charges are common for commercial customers and are increasingly appearing on some residential rate plans.

While standard energy charges bill you for the total kilowatt-hours you use, demand charges bill you for your peak power draw — the maximum amount of electricity you consumed at any single point in time. For example, if you run your air conditioner, electric oven, dryer, and EV charger simultaneously, your peak demand might spike to 15 kW, even if it only lasts 15 minutes. That peak sets your demand charge for the entire month. Solar panels alone may not reduce demand charges effectively because your highest usage often occurs in the evening when panels aren’t producing. Battery storage is the most effective way to reduce demand charges — the battery can discharge during peak demand moments, effectively “shaving” your peak draw from the grid. For commercial solar customers, demand charge management is often a major part of the financial case for adding batteries.

Depreciation incentives are tax benefits that allow commercial and business solar system owners to recover the cost of their investment by deducting a portion of the system’s value from their taxable income each year. The most significant depreciation benefit for solar is the Modified Accelerated Cost Recovery System (MACRS).

Under MACRS, commercial solar systems can be depreciated over five years, allowing businesses to write off the system’s value much faster than its actual useful life. Additionally, bonus depreciation (currently at 80% in 2026, stepping down from 100% in previous years) lets businesses deduct a large portion of the cost in the first year. The depreciable basis is the total system cost minus half of the ITC amount. For example, on a $100,000 commercial system: after the 30% ITC ($30,000), the depreciable basis is $85,000 ($100,000 minus $15,000). Depreciation incentives are primarily a commercial benefit — homeowners using solar for personal use cannot claim MACRS. However, these incentives are a major driver of commercial and utility-scale solar adoption. Business owners should work with a tax professional to maximize these benefits.

Depth of Discharge (DoD) is the percentage of a battery’s total capacity that has been used. For example, if a 10 kWh battery has discharged 8 kWh, its depth of discharge is 80% (and its state of charge is 20%).

DoD is a critical specification when comparing home batteries because it determines how much of the battery’s rated capacity you can actually use. Most lithium-ion batteries have a recommended maximum DoD of 80–100%. A battery rated at 13.5 kWh with a 100% DoD gives you the full 13.5 kWh of usable energy, while a battery with an 80% DoD only provides 10.8 kWh. Deeper discharges (higher DoD) put more stress on the battery and can reduce its total cycle life. Battery manufacturers balance usable capacity and longevity by setting a recommended DoD in their warranty terms. For instance, a warranty might guarantee 70% capacity retention after 10 years at a maximum 80% DoD. When comparing batteries, always look at usable capacity (which accounts for DoD limits) rather than total capacity.

A derate factor is a multiplier used to reduce a solar system’s nameplate capacity to a more realistic estimate of actual output. It accounts for real-world losses that reduce production below the ideal laboratory conditions used for panel ratings.

Common sources of loss included in the derate factor are: inverter conversion efficiency (96–99%), wiring and connection losses (1–3%), soiling from dirt and dust (2–5%), temperature effects (5–15% in hot climates), shading (varies), panel mismatch (1–2%), and system age/degradation. The overall derate factor for a typical residential system is usually 0.75–0.85, meaning the system produces 75–85% of its nameplate DC rating as usable AC electricity. For example, an 8 kW system with a derate factor of 0.80 effectively produces about 6.4 kW AC under real conditions. Tools like PVWatts (developed by NREL) use derate factors to estimate annual solar production for specific locations. Understanding the derate factor helps homeowners set realistic expectations and compare solar proposals that may use different assumptions.

A solar developer is a company or entity that plans, finances, and manages the development of solar energy projects — from initial concept through permitting, financing, construction, and often long-term operation. Developers are most prominent in the commercial, community solar, and utility-scale segments of the market.

In the residential market, the term “developer” is less commonly used, but large solar companies like Sunrun and Vivint effectively operate as developer-installers for their PPA and lease programs. In the community solar market, developers build shared solar farms and recruit subscribers who receive bill credits. Utility-scale developers plan and build large solar power plants (50–500+ MW) that sell electricity to utilities through long-term Power Purchase Agreements. The developer role involves significant upfront risk — securing land rights, navigating permitting, arranging financing, and managing construction — before any revenue is generated. Developers may hire separate EPC contractors for the actual construction or have in-house installation capabilities. For homeowners, understanding the developer’s role is relevant primarily when evaluating community solar subscriptions or third-party-owned (PPA/lease) residential solar programs.

A disconnect switch is a manual safety device that allows the solar system’s electrical circuits to be quickly and completely shut off. Solar installations typically require both an AC disconnect (between the inverter and the electrical panel) and a DC disconnect (between the panels and the inverter).

Disconnect switches are required by the National Electrical Code (NEC) and local building codes to ensure that utility workers, firefighters, and maintenance technicians can safely de-energize the system. The AC disconnect is usually mounted on the exterior wall near your utility meter, giving the utility company easy access to isolate your solar system from the grid. The DC disconnect is typically located near the inverter and allows the high-voltage DC current from the panels to be safely interrupted. Both switches are lockable in the “off” position for safety during maintenance or emergencies. While modern rapid shutdown systems provide additional emergency de-energization capabilities, traditional disconnect switches remain a mandatory and fundamental safety component of every solar installation.

An electrical permit is a specific permit required for the electrical work involved in a solar installation, including wiring from the panels to the inverter, the inverter connection to the electrical panel, grounding, and any panel upgrades. In some jurisdictions, the electrical permit is separate from the building/structural permit.

The electrical permit ensures that all wiring and electrical connections comply with the National Electrical Code (NEC) and local electrical codes. The permit application includes a single-line electrical diagram showing the complete electrical path from the solar array through the inverter to the main service panel and utility meter. It specifies wire types and sizes, conduit, overcurrent protection, disconnects, grounding, and rapid shutdown equipment. An electrical inspector reviews the completed work during the inspection, checking for proper connections, code-compliant materials, correct labeling, and safe installation practices. Some jurisdictions require the electrical work to be performed by or supervised by a licensed electrician (separate from the solar installer’s contractor license). Electrical permit fees are typically $50–$300. Your solar installer handles the electrical permit process as part of the overall project management.

Electrification is the process of replacing fossil fuel-powered systems with electric alternatives — such as switching from gas furnaces to electric heat pumps, from gasoline cars to electric vehicles (EVs), and from gas stoves to induction cooktops. Electrification is a key strategy for reducing greenhouse gas emissions, especially when the electricity comes from renewable sources like solar.

The connection between electrification and solar is powerful: as you electrify more of your home (heating, cooling, cooking, transportation), your electricity usage increases — but if that electricity comes from your own solar panels, you’re effectively running your home and car on free, clean energy from the sun. Many homeowners who install solar panels later add EV chargers, heat pumps, or heat pump water heaters, further reducing their fossil fuel consumption and maximizing the value of their solar investment. The Inflation Reduction Act includes incentives for both solar and electrification technologies, making it financially attractive to combine them. When sizing a solar system, it’s worth considering future electrification plans — adding a slightly larger system now can accommodate an EV charger or heat pump you plan to install later, saving money on a potential system expansion.

An Energy Management System (EMS) is a smart controller that optimizes the flow of electricity between your solar panels, battery storage, home appliances, and the grid. An EMS goes beyond basic monitoring — it actively manages when and how energy is produced, stored, consumed, and exported to maximize savings and efficiency.

An EMS uses data about your solar production, home consumption, battery charge level, electricity rates (including time-of-use schedules), and weather forecasts to make intelligent decisions in real time. For example, it might charge the battery from solar during midday (when TOU rates are low), then discharge the battery to power the home during expensive evening peak hours, and only draw from the grid when absolutely necessary. Some EMS platforms can also control smart home devices — scheduling EV charging during off-peak hours, pre-cooling the home with AC while solar is abundant, or delaying the dishwasher until solar production peaks. Popular EMS-capable platforms include Tesla’s Powerwall/app ecosystem, Enphase IQ system controller, Span smart electrical panel, and various third-party home energy managers. As electricity rate structures become more complex (TOU rates, demand charges, net billing), an EMS becomes increasingly valuable for maximizing solar-plus-storage savings.

An Energy Storage System (ESS) is the complete battery setup that stores electricity for later use, including the battery cells, battery management system (BMS), inverter/charger, enclosure, and monitoring software. In residential solar, ESS is used interchangeably with terms like “home battery” or “solar battery system.”

An ESS paired with solar panels allows homeowners to store excess solar energy generated during the day and use it at night, during peak electricity rate periods, or during grid outages. Key specifications to compare when shopping for an ESS include: usable capacity (kWh), continuous power output (kW), peak/surge power (kW), round-trip efficiency (%), cycle life (number of charge/discharge cycles), and warranty duration. Popular residential ESS options in the US market include the Tesla Powerwall, Enphase IQ Battery, LG RESU, Franklin WH, and Generac PWRcell. As of 2026, home batteries qualify for the 30% federal tax credit when installed with or added to a solar system. The cost of residential ESS typically ranges from $10,000 to $20,000 per unit installed, and multiple units can be stacked for greater capacity.

An EPC contractor handles all three phases of a solar project: Engineering (system design and permitting), Procurement (sourcing panels, inverters, and other equipment), and Construction (physical installation and commissioning). EPC contractors provide a complete, end-to-end solar installation service.

In the residential solar market, most solar companies operate as EPC contractors — they design your system, purchase the equipment, install the panels, handle permitting and inspections, and coordinate the utility interconnection. The EPC model gives homeowners a single point of contact and accountability for the entire project. In the commercial and utility-scale solar market, the EPC contractor role is more formally defined and often involves separate contracts for each phase. When evaluating solar companies, choosing one that handles all EPC functions in-house (rather than subcontracting) generally results in better quality control, faster timelines, and clearer warranty coverage. Key questions to ask an EPC: Do you use in-house installation crews or subcontractors? Who handles the permitting and utility paperwork? What does your workmanship warranty cover?

An EPC contract is a formal agreement between a solar customer and an EPC (Engineering, Procurement, and Construction) contractor that defines the scope of work, timeline, cost, equipment specifications, warranties, and responsibilities for a complete solar installation project.

In the residential market, the EPC contract is typically called a “solar installation agreement” or “solar purchase contract.” It should clearly specify: the system size (kW), exact panel and inverter models and quantities, mounting type, estimated annual production, total price and payment schedule, financing terms (if applicable), warranty coverage (product, performance, and workmanship), project timeline with key milestones, the installer’s obligations for permitting and interconnection, and conditions for changes or cancellations. A well-written contract protects both parties and sets clear expectations. Key clauses to review include: what happens if the project is delayed, how change orders are handled, what triggers a warranty claim, and whether production estimates are guaranteed. In the commercial market, EPC contracts are more detailed and may include liquidated damages for delays, performance testing protocols, and detailed commissioning procedures.

The Federal Investment Tax Credit (ITC) is the most significant financial incentive for residential solar in the United States. It allows homeowners to deduct 30% of the total cost of a solar energy system from their federal income taxes.

The ITC applies to the full installed cost of the system, including panels, inverters, racking, batteries (if installed at the same time), wiring, and installation labor. For example, if your solar system costs $25,000, the ITC provides a $7,500 tax credit — reducing the amount of federal income tax you owe dollar-for-dollar. The 30% rate is locked in through 2032 under the Inflation Reduction Act, after which it steps down to 26% in 2033 and 22% in 2034. The ITC is a tax credit, not a deduction, meaning it directly reduces your tax bill rather than just lowering your taxable income. If your tax liability is less than the credit amount, the remaining credit can be rolled forward to the following tax year. There is no cap on the credit amount for residential systems. Homeowners should consult a tax professional to ensure they qualify and can fully utilize the credit.

A feed-in tariff (FIT) is a policy mechanism that pays solar system owners a set rate for the electricity they feed back into the utility grid. Unlike net metering (which provides credits against your electricity consumption), a feed-in tariff pays you directly for every kilowatt-hour your system exports, often at a premium rate designed to encourage renewable energy adoption.

Feed-in tariffs were instrumental in driving early solar adoption in countries like Germany and Japan. In the United States, FIT programs are less common than net metering but do exist in some states and municipalities. The payment rate is typically fixed by contract for a set period (10–25 years), giving solar owners predictable income. Some FIT programs purchase all electricity the system generates, while others only purchase excess power not consumed on-site. The rates vary widely — from a few cents per kWh to rates that exceed retail electricity prices. As solar costs have dropped and net metering has become widespread, many US FIT programs have been phased out or reduced. However, feed-in tariffs remain an important policy tool in some markets, particularly for community solar and small commercial installations.

Flashing is a weatherproof metal or rubber component installed at every point where solar racking hardware penetrates the roof surface. Its purpose is to create a watertight seal that prevents leaks around the bolt holes used to secure the mounting system to the roof structure.

Proper flashing is one of the most critical aspects of a quality solar installation. Each roof attachment point involves drilling a lag bolt through the roofing material into the rafter below, and the flashing slides under the shingles (or sits on top of tile or metal roofing) to redirect water away from the penetration. Common flashing types include L-foot flashing for asphalt shingle roofs, tile hooks for tile roofs, and standing seam clamps for metal roofs (which don’t actually penetrate the roof). High-quality flashings are typically made of aluminum or stainless steel with an integrated rubber or EPDM gasket for a waterproof seal. A properly installed flashing should maintain a watertight seal for 25+ years — matching the lifespan of the solar system. Leaks from solar installations are rare when qualified installers use proper flashing techniques, but they can occur with poor workmanship, which is why choosing a reputable installer with strong workmanship warranties is important.

Greenhouse gas (GHG) emissions are gases released into the atmosphere that trap heat and contribute to global warming and climate change. The primary greenhouse gases include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and fluorinated gases. Burning fossil fuels for electricity generation is one of the largest sources of GHG emissions in the United States.

The US electric power sector is responsible for approximately 25% of total national GHG emissions. By generating electricity from solar panels instead of fossil fuel power plants, homeowners directly reduce GHG emissions. A typical residential solar system offsets 3–8 metric tons of CO₂ per year, depending on the local electricity grid mix and system size. Over a 25-year lifespan, that adds up to 75–200 metric tons of avoided emissions per household. While manufacturing solar panels does produce some GHGs, the emissions are recovered within the first 1–3 years of clean electricity generation. After that, every kilowatt-hour produced is essentially emissions-free. Solar energy’s role in reducing GHG emissions is one of the key environmental motivations for both individual homeowners and national energy policy.

Grid parity is the point at which the cost of generating electricity from solar panels equals or is less than the cost of purchasing electricity from the traditional power grid. When solar reaches grid parity in a given market, it becomes economically competitive with fossil-fuel-generated power without the need for subsidies.

Solar energy has already reached grid parity in most of the United States. The levelized cost of solar electricity (LCOE) — which factors in installation costs, maintenance, panel degradation, and financing over the system’s lifetime — has dropped below $0.05 per kWh in many regions. Compare that to the average residential electricity rate of around $0.16/kWh, and solar is significantly cheaper than grid power in most markets. This means that for the majority of American homeowners, solar panels will save money over their lifetime even without incentives like the federal tax credit. Grid parity is a milestone that has been a major driver of solar adoption worldwide. As panel prices continue to fall and electricity rates continue to rise, the economic advantage of solar only grows stronger.

Grid resilience refers to the ability of the electric power grid to withstand, adapt to, and quickly recover from disruptions — including severe weather events, equipment failures, cyberattacks, and demand spikes. Distributed solar energy and battery storage are increasingly recognized as tools for improving grid resilience at both the local and system-wide level.

When thousands of homes have their own solar panels and batteries, the overall grid becomes more resilient because it is less dependent on a small number of large, centralized power plants and long-distance transmission lines. During a grid outage, homes with solar-plus-storage can operate independently (in island mode), reducing strain on utility emergency resources. At the grid level, distributed solar reduces peak demand on transmission infrastructure and provides locally generated power close to where it is consumed. Utilities and regulators are increasingly designing programs that leverage residential batteries for grid support — such as virtual power plant (VPP) programs where homeowners allow their batteries to be discharged during grid emergencies in exchange for compensation. For homeowners, the personal resilience benefit is straightforward: solar-plus-storage keeps your lights on when the grid goes down.

A grid-tied solar system is the most common type of residential solar installation in the United States. It connects directly to your local utility grid, allowing you to draw power from the grid when your panels aren’t producing enough and send excess electricity back to the grid when they produce more than you need.

The primary advantage of a grid-tied system is net metering: surplus solar energy flows to the grid and earns you credits on your electric bill. This means you don’t need expensive battery storage to benefit from solar — the grid effectively acts as your backup. Grid-tied systems are simpler, less expensive, and easier to install than off-grid or hybrid systems. However, most grid-tied systems cannot power your home during a utility outage — when the grid goes down, your solar system automatically shuts off for safety reasons (to prevent backfeeding electricity to utility workers). If you want backup power during outages, you can add a battery storage system to create a grid-tied hybrid system. Over 95% of residential solar installations in the US are grid-tied.

A ground-mount solar system consists of solar panels installed on a racking structure with foundations in the ground, rather than on a rooftop. Ground-mounted arrays are an excellent option for homeowners with large properties but unsuitable roofs.

Ground-mount systems offer several advantages over rooftop installations. They can be oriented and tilted at the optimal angle for your latitude, maximizing energy production regardless of your roof’s direction or pitch. They are easier to access for maintenance, cleaning, and repairs. And they can be scaled to any size without being limited by roof space. There are two main types: fixed ground mounts (stationary frames set at a permanent angle) and tracking systems (motorized mounts that follow the sun throughout the day, boosting production by 15–25%). The drawbacks include higher installation costs due to foundation work and trenching for electrical conduit, the need for adequate yard space, and potential permitting requirements. Ground-mount systems typically cost $0.10–$0.30 more per watt than rooftop systems, but the increased energy production can offset the extra cost over time.

Grounding and bonding are essential electrical safety practices required in every solar installation. Grounding provides a safe path for fault current to flow to the earth, while bonding connects all metallic components (panels, racking, conduit, enclosures) to ensure they are at the same electrical potential.

Proper grounding protects people and equipment from electrical shock and fire in the event of a fault — such as a damaged wire touching the metal racking. An equipment grounding conductor (EGC) connects all exposed metal parts of the solar system to the home’s grounding electrode system (typically a ground rod driven into the earth). Bonding ensures that if any metal component accidentally becomes energized, the current flows safely through the grounding system rather than through a person who touches it. The NEC requires that all solar racking, panel frames, inverter enclosures, and junction boxes be properly bonded and grounded. Special grounding clips, lugs, and bonding jumpers rated for outdoor use are used throughout the array. Improper grounding is one of the most common code violations found during solar inspections and can create serious safety hazards.

A hybrid inverter (also called a multi-mode inverter or battery-ready inverter) is a single device that manages both solar panel electricity and battery storage. It handles DC-to-AC conversion from the solar panels, charges and discharges the battery, and manages the flow of power between the solar array, batteries, home, and grid.

Hybrid inverters simplify system design by combining the functions of a solar inverter and a battery inverter into one unit. This is more efficient than an AC-coupled setup because it avoids the double conversion losses (DC → AC → DC → AC). It is also more compact and typically less expensive than installing separate inverters. Popular hybrid inverters include models from SolarEdge, Enphase, Sol-Ark, and Tesla. If you plan to add batteries now or in the future, choosing a hybrid inverter upfront can save money and complexity. However, hybrid inverters are slightly more expensive than standard solar-only inverters, so they may not be cost-effective if you have no plans for battery storage. System sizes typically range from 5 kW to 12 kW for residential applications, matching the output of the solar array.

Insolation (incoming solar radiation) is the total amount of solar energy received on a given surface area over a specific period of time. It is typically measured in kilowatt-hours per square meter per day (kWh/m²/day) and is a key factor in estimating how much electricity a solar system will produce at a given location.

Insolation values vary dramatically across the United States. The Southwest (Arizona, Nevada, New Mexico) receives the highest insolation at 6–7+ kWh/m²/day, while the Pacific Northwest and upper Northeast receive 3–4 kWh/m²/day. These values are often expressed as “peak sun hours” — the equivalent number of hours per day at full 1,000 W/m² sunlight. A location with 5.0 kWh/m²/day of insolation gets 5 peak sun hours. Solar installers use insolation data from sources like NREL’s NSRDB (National Solar Radiation Database) and tools like PVWatts to estimate annual production for a specific site. Higher insolation means more solar production, but even areas with moderate insolation can generate excellent returns on solar investment due to higher electricity prices in those regions.

An inspection is the official on-site review of a completed solar installation by a local building or electrical inspector to verify that the system was installed according to the approved permit plans and meets all applicable codes and safety standards.

Most residential solar installations require at least one inspection, though some jurisdictions require two (a rough/structural inspection during installation and a final inspection after completion). The inspector checks a comprehensive list of items including: correct wire sizes and types, proper grounding and bonding, secure racking attachment, appropriate disconnect switches, accurate labeling, fire code setback compliance, rapid shutdown capability, and match to the approved electrical diagrams. If the inspector finds violations, you’ll receive a correction notice listing the issues that must be fixed before the installation can pass. Your solar installer is responsible for scheduling the inspection and correcting any deficiencies. A passed inspection is required before the utility will grant Permission to Operate (PTO). Inspection wait times vary from same-week to several weeks depending on the jurisdiction. Some areas have adopted virtual or photo-based inspections to speed the process.

An interconnection agreement is a formal contract between a solar system owner and their electric utility that establishes the terms and conditions for connecting a solar energy system to the public power grid. It is a required step for all grid-tied solar installations.

The interconnection agreement covers technical requirements (system size limits, safety equipment, insurance), operational rules (net metering terms, maintenance responsibilities), and liability provisions. Your solar installer typically handles the interconnection application on your behalf as part of the installation process. The application includes system specifications, single-line electrical diagrams, and equipment certifications. Processing times vary widely by utility — from a few days to several months. Some utilities charge interconnection application fees ($50–$500), while others process them for free. The agreement must be approved before your system can be legally connected to the grid and begin operating. In some cases, utilities may require system upgrades (like a new transformer) if the local grid cannot support additional solar capacity, which can delay the process. The interconnection agreement is separate from, but related to, the Permission to Operate (PTO) that formally activates your system.

An inverter is an essential component of every solar energy system. It converts the direct current (DC) electricity produced by solar panels into alternating current (AC) electricity, which is the standard form of power used in American homes and the electrical grid.

There are three main types of solar inverters. String inverters (also called central inverters) connect multiple panels in a series and convert their combined DC output at one central location — they are the most affordable option. Microinverters are small units installed on each individual panel, allowing panel-level optimization and monitoring, which is especially useful for roofs with partial shading. Power optimizers pair with a string inverter but add panel-level DC optimization, offering a middle ground between cost and performance. Most residential inverters carry warranties of 12–25 years and are sized to match the total wattage of the solar array.

Inverter monitoring refers to the built-in data collection and communication capability of modern solar inverters that tracks system performance and reports it to a cloud-based monitoring platform. Since all solar electricity passes through the inverter, it is the natural point for measuring and monitoring system output.

All major inverter brands (Enphase, SolarEdge, SMA, Fronius, Tesla, Sol-Ark) include monitoring as a core feature. The inverter records data including: AC power output, DC input from the panels, energy production totals, grid voltage and frequency, operating temperatures, error codes, and (with microinverters or optimizers) individual panel performance. This data is transmitted via Wi-Fi, Ethernet, or cellular modem to the manufacturer’s cloud platform, where it is processed and made available through a web portal and mobile app. Inverter monitoring enables both the homeowner and the installer to track performance remotely. If the inverter detects a fault — such as a grid anomaly, over-temperature condition, or ground fault — it logs an error code and can send an automatic alert. Maintaining a stable internet connection to your inverter is important for continuous monitoring and timely fault notification.

Irradiance is the instantaneous power of solar radiation hitting a surface, measured in watts per square meter (W/m²). While insolation measures total solar energy over time, irradiance measures the intensity of sunlight at any given moment.

Solar panels are rated under Standard Test Conditions (STC) at an irradiance of 1,000 W/m², which represents bright, direct sunlight at noon on a clear day. In practice, irradiance changes constantly throughout the day — it’s zero at night, increases through the morning, peaks around solar noon, and decreases through the afternoon. Cloud cover, haze, and atmospheric conditions also affect irradiance moment by moment. A panel rated at 400W will produce 400 watts only when irradiance is exactly 1,000 W/m² and the cell temperature is 25°C. At 500 W/m² (overcast sky), the same panel produces roughly 200W. Understanding irradiance helps explain why solar production varies throughout the day and why your system’s real-time output shown on the monitoring app fluctuates. Solar design software uses historical irradiance data to model expected production for your specific location.

Island mode (also called islanding) is when a solar-plus-battery system disconnects from the utility grid and operates independently as a self-contained power source. The home becomes an “island” — generating, storing, and consuming its own electricity without any connection to the grid.

Islanding is what happens automatically when a grid outage is detected in a system with battery backup. The transfer switch disconnects the home from the grid (to prevent backfeeding electricity to utility workers), and the battery and solar panels take over powering the home. During island mode, the inverter must carefully manage power flow — balancing solar production, battery charge/discharge, and household demand in real time. The system continues operating in island mode until grid power is restored, at which point it automatically reconnects. Intentional islanding is the desired behavior during outages. However, unintentional islanding (where a solar system continues feeding the grid during an outage) is a safety hazard — which is why anti-islanding protection is required by code in all grid-tied inverters. Modern hybrid inverters and battery systems handle the transition to island mode seamlessly, often in under a second.

A kilowatt (kW) equals 1,000 watts and is the standard unit for measuring the size and power output of residential solar systems.

When solar installers describe a system as “8 kW,” they mean it can produce 8,000 watts of electricity under peak conditions. Typical residential solar systems in the United States range from 5 kW to 12 kW, depending on the household’s electricity usage, available roof space, and budget. A 5 kW system might suit a small home with modest energy needs, while a larger 10–12 kW system can power a home with high electricity consumption or an electric vehicle. Solar system costs are commonly quoted in dollars per kilowatt — for example, $2,770/kW is equivalent to $2.77 per watt. The system size you need depends on your annual kilowatt-hour (kWh) consumption, your roof’s sun exposure, and your local electricity rates.

A kilowatt-hour (kWh) is a unit of energy equal to using 1,000 watts of power for one hour. It is the standard measurement for both solar energy production and home electricity consumption — and it is what your utility company uses to calculate your electric bill.

The average American household consumes approximately 10,500 kWh per year, or about 875 kWh per month. When sizing a solar system, installers calculate how many kWh your panels need to produce annually to offset your electricity usage. A 8 kW solar system in a sunny location might produce 10,000–12,000 kWh per year. Your utility charges you per kWh consumed — the national average is around $0.16/kWh, though rates vary widely by state ($0.10–$0.35/kWh). Understanding your kWh usage is the first step in determining the right solar system size for your home. You can find your monthly kWh consumption on your electricity bill.

Levelized Cost of Energy (LCOE) is the average cost of generating one kilowatt-hour of electricity over the entire lifetime of a solar system, accounting for all costs including installation, financing, maintenance, and panel degradation. It is the most widely used metric for comparing the cost of different energy sources.

LCOE is calculated by dividing the total lifetime cost of the system by the total lifetime energy production. For residential solar in the US, LCOE typically ranges from $0.04 to $0.08 per kWh (after the federal tax credit), which is significantly lower than the average retail electricity rate of $0.16/kWh. This means solar electricity is 50–75% cheaper than grid electricity in most markets — the fundamental economic driver behind solar adoption. LCOE allows an apples-to-apples comparison between solar, wind, natural gas, coal, and other energy sources. As of 2026, utility-scale solar has the lowest LCOE of any new electricity generation source in the US. For homeowners, your personal LCOE depends on your system cost, financing terms, local sun hours, and system lifespan. A lower LCOE means a better long-term investment.

Lithium Iron Phosphate (LFP or LiFePO₄) is a type of lithium-ion battery chemistry that uses iron phosphate as the cathode material. LFP batteries are increasingly popular for residential solar storage due to their excellent safety, long cycle life, and thermal stability.

LFP batteries offer several advantages over other lithium-ion chemistries. They are inherently more stable and resistant to thermal runaway (overheating and fire), making them one of the safest battery options for home installation. They also have an exceptionally long cycle life — typically 5,000–10,000+ cycles — which translates to 15–25 years of daily use. The trade-off is a lower energy density compared to NMC batteries, meaning LFP batteries are physically larger and heavier for the same capacity. However, for stationary home storage where weight and size are less critical than safety and longevity, LFP is an excellent choice. The Tesla Powerwall 3, Enphase IQ Battery 5P, and many Sol-Ark compatible batteries use LFP chemistry. LFP batteries also use abundant, non-toxic materials (iron and phosphate) instead of cobalt and nickel, making them more environmentally friendly and less subject to supply chain concerns.

Lithium Nickel Manganese Cobalt (NMC) is a lithium-ion battery chemistry that uses a combination of nickel, manganese, and cobalt in the cathode. NMC batteries are widely used in electric vehicles and have been a popular choice for home solar storage systems.

The key advantage of NMC batteries is their high energy density — they can store more energy in a smaller, lighter package than LFP batteries. This makes them well-suited for applications where space is limited. NMC batteries also perform well at high charge and discharge rates. However, NMC batteries have a shorter cycle life than LFP (typically 4,000–6,000 cycles vs. 5,000–10,000+ for LFP) and are somewhat more susceptible to thermal instability, requiring robust battery management systems. The LG RESU and earlier Tesla Powerwall models used NMC chemistry. As the solar storage market has matured, many manufacturers have shifted toward LFP chemistry for residential applications due to its superior safety and longevity, though NMC remains common in products that prioritize compact size. The materials in NMC batteries (particularly cobalt) are more expensive and have greater environmental and ethical sourcing concerns than LFP materials.

A load center, commonly called a subpanel, is a secondary electrical distribution panel that receives power from the main service panel and distributes it to a specific group of circuits. In solar installations, subpanels are used when the main panel lacks space for a solar breaker or when specific circuits need to be isolated.

In battery-backed solar systems, a subpanel (often called a critical loads panel or backup panel) is used to separate the circuits you want powered during a grid outage from the rest of your home. Only the circuits wired to this subpanel will receive backup power from the battery. This approach is more practical and cost-effective than trying to back up the entire home, which would require a much larger (and more expensive) battery system. Common circuits moved to the critical loads subpanel include the refrigerator, select lights, internet router, phone chargers, and medical equipment. Your solar installer will work with you to decide which circuits are most important to keep running during an outage.

The Modified Accelerated Cost Recovery System (MACRS) is a federal tax depreciation schedule that allows commercial and business solar system owners to recover the cost of their investment through annual tax deductions over a 5-year period, even though the system lasts 25+ years.

MACRS is one of the most valuable incentives for commercial solar. Under the 5-year MACRS schedule, businesses can deduct the depreciable basis of their solar system (total cost minus half the ITC value) from taxable income using an accelerated schedule: approximately 20% in year 1, 32% in year 2, 19% in year 3, 12% in years 4 and 5, and 6% in year 6. Combined with bonus depreciation (80% first-year deduction in 2026, stepping down in subsequent years), a business can deduct a significant portion of the system cost in the first year alone. For a $200,000 commercial system with the 30% ITC ($60,000) and MACRS deductions, the combined tax benefits can cover 50–60% of the original cost within the first few years. MACRS does not apply to residential (personal-use) solar systems — only to systems used for business or commercial purposes.

The main service panel, also known as the breaker panel or electrical panel, is the central distribution hub for your home’s electrical system. It receives electricity from the utility (or your solar system) and distributes it through individual circuit breakers to the various circuits powering your lights, outlets, and appliances.

When a solar system is installed, it connects to the main service panel through a dedicated solar breaker. The panel’s total amperage rating (typically 100, 150, or 200 amps for residential homes) determines how much power can flow through it, and this is a critical factor in solar system design. The NEC’s “120% rule” limits the total amperage feeding into a panel — if your main breaker is 200 amps, the solar breaker cannot exceed 40 amps (200 × 120% = 240 amps total). If your existing panel is too small or too full to accommodate a solar connection, your installer may need to upgrade the panel or add a subpanel, which can add $1,000–$3,000 to the installation cost. A panel upgrade is one of the most common unexpected costs in solar projects.

A microinverter is a small inverter installed on the underside of each individual solar panel. Unlike a string inverter that handles the output of multiple panels at once, a microinverter converts DC to AC right at each panel, allowing every panel to operate independently.

The key advantage of microinverters is panel-level optimization. If one panel is shaded by a tree or chimney, only that panel’s output drops — the rest of the array continues producing at full capacity. Microinverters also enable panel-level monitoring, so homeowners can track the performance of each individual panel through an app or web portal. They are especially well-suited for complex roofs with multiple orientations, dormers, or partial shading. The trade-off is higher upfront cost compared to string inverters — typically $1,000–$2,000 more for an average residential system. However, many homeowners find the improved energy harvest and monitoring capabilities worth the investment. Enphase is the leading microinverter manufacturer in the US market.

Module-Level Power Electronics (MLPE) is a category of equipment installed at or near each individual solar panel to improve monitoring, safety, and energy production. The two most common types of MLPE are microinverters and DC power optimizers.

MLPE devices allow each panel in an array to perform independently. This is a significant advantage over traditional string inverter setups, where one underperforming panel can drag down the output of the entire string. With MLPE, shaded or dirty panels only affect their own output. MLPE also provides panel-level performance data, making it easier to identify issues and maintain the system. In addition, MLPE devices are required by the National Electrical Code (NEC) rapid shutdown requirements, which mandate that rooftop solar systems can quickly de-energize in an emergency for firefighter safety. While MLPE adds to the upfront system cost, the improved energy harvest and safety features make it the standard for most modern residential solar installations.

A monitoring portal (or monitoring app) is the web-based dashboard or smartphone application that displays your solar system’s performance data in an easy-to-understand visual format. It is your window into how your solar system is performing at any given moment.

Popular monitoring platforms include Enphase Enlighten (for microinverter systems), SolarEdge Monitoring Portal (for SolarEdge systems), Tesla app (for Tesla/Powerwall systems), and various third-party platforms. The portal typically shows: real-time energy production with live power output, daily/weekly/monthly/annual production charts, panel-level performance (if using MLPE), energy consumption and grid interaction, battery state of charge (if applicable), system alerts and error notifications, and environmental impact metrics (CO₂ offset, equivalent trees planted). Many portals allow you to set up email or push notification alerts for production anomalies, inverter errors, or when your system reaches production milestones. Access is usually free and included with the system for the life of the product. Checking your monitoring portal regularly (even just weekly) helps you stay informed about your system’s health and catch any performance issues early.

Monocrystalline solar panels are made from single-crystal silicon wafers, giving them a uniform dark appearance and the highest efficiency ratings among standard residential panel types. They are the most popular choice for home solar installations in the United States.

Each cell in a monocrystalline panel is cut from a single, continuous crystal of silicon, which allows electrons to flow more freely and produces higher efficiency — typically 20–23%, with premium models reaching 24% or more. This higher efficiency means monocrystalline panels generate more electricity per square foot, making them ideal for roofs with limited space. They also tend to perform better in high temperatures and low-light conditions compared to polycrystalline panels. The trade-off is a higher price per panel, though the gap has narrowed significantly in recent years. Monocrystalline panels typically carry 25–30 year performance warranties and can last well beyond that. Leading manufacturers include SunPower, LG, Panasonic, and REC.

Nameplate capacity is the maximum rated power output of a solar system or individual panel under ideal Standard Test Conditions (STC). It is the “official” size of your system, expressed in kilowatts (kW) or watts (W), and is the number used in system specifications, contracts, and incentive applications.

For example, a system with 20 panels rated at 400W each has a nameplate capacity of 8 kW (8,000 watts). However, real-world production is almost always lower than nameplate capacity because actual conditions rarely match the ideal laboratory settings used for STC ratings (25°C cell temperature, 1,000 W/m² irradiance). Factors like high temperatures, shading, soiling, wiring losses, and inverter clipping all reduce actual output below the nameplate rating. A system’s real-world output is typically 75–85% of its nameplate capacity. This difference is accounted for by the derate factor. When reviewing solar proposals, understand that the nameplate capacity represents the best-case peak output, not the constant output you should expect.

The National Electrical Code (NEC), also known as NFPA 70, is the comprehensive set of electrical safety standards adopted throughout the United States. It governs the safe installation and maintenance of all electrical systems, including solar photovoltaic systems.

The NEC is published by the National Fire Protection Association (NFPA) and updated every three years — the current edition is NEC 2023, though many jurisdictions still enforce the 2020 or 2017 edition. For solar installations, the NEC covers everything from wire sizing and overcurrent protection to grounding, disconnects, labeling, and rapid shutdown requirements. Article 690 specifically addresses solar PV systems, while Article 705 covers interconnected power sources (connecting solar to the grid). Compliance with the NEC is mandatory for all permitted solar installations and is verified during the building inspection. Your solar installer designs the system to meet the NEC edition adopted by your local AHJ. Key NEC requirements that affect residential solar include rapid shutdown (690.12), the 120% bus bar rule for panel connections (705.12), and equipment grounding (690.43).

A Nationally Recognized Testing Laboratory (NRTL) is an independent organization accredited by the Occupational Safety and Health Administration (OSHA) to test and certify that electrical products meet established safety standards. UL (Underwriters Laboratories) is the most well-known NRTL, but there are others.

The NEC requires that all electrical equipment used in solar installations — including panels, inverters, racking, combiner boxes, and disconnects — be listed by an NRTL. This ensures that every component has been independently tested for electrical safety, fire resistance, and durability. Besides UL, other NRTLs that certify solar equipment include: CSA Group (Canadian Standards Association), TÜV Rheinland, TÜV SUD, Intertek (ETL mark), and MET Laboratories. Products certified by any OSHA-recognized NRTL are equally acceptable under the NEC — a panel with a TÜV certification is just as valid as one with a UL mark. When reviewing solar equipment, look for an NRTL listing mark on the product label and verify the certification is current. Inspectors check for NRTL listings during the permitting and inspection process.

NEC Article 690 is the section of the National Electrical Code specifically dedicated to solar photovoltaic (PV) systems. It provides the detailed electrical safety requirements that every solar installation in the United States must follow.

Article 690 covers a wide range of technical requirements including: maximum system voltage calculations, conductor sizing and protection, disconnecting means (disconnect switches), grounding and bonding, overcurrent protection, labeling requirements, and the critical rapid shutdown provisions (Section 690.12). The rapid shutdown requirement — introduced in NEC 2017 and strengthened in NEC 2020 — mandates that all conductors within the array boundary must be reduced to 80 volts or less within 30 seconds of initiating shutdown. This requirement has effectively made module-level power electronics (microinverters or rapid shutdown devices) standard on all new installations. Article 690 is updated with each NEC edition to reflect advances in solar technology and safety practices. Solar installers and inspectors must be well-versed in Article 690, and electrical plans submitted for permits must demonstrate full compliance.

Net billing is a compensation structure for solar customers where excess electricity sent to the grid is credited at a rate lower than the retail electricity rate — as opposed to traditional net metering, which credits exports at the full retail rate. Net billing is becoming more common as states reform their solar compensation policies.

Under traditional net metering, a kWh exported to the grid is worth the same as a kWh consumed — full retail value. Under net billing, the export rate is typically set at the utility’s avoided cost, wholesale rate, or a reduced time-of-use value, which can be 40–75% less than the retail rate. California’s NEM 3.0 (implemented in 2023) is the most prominent example, reducing export credits by approximately 75% and shifting solar economics toward self-consumption and battery storage. Net billing makes batteries significantly more valuable because it’s financially better to store excess solar energy for personal use later than to export it at a low rate. As more states transition from net metering to net billing, the economic case for solar-plus-storage grows stronger, and system design increasingly focuses on maximizing self-consumption rather than grid exports.

Net metering is a billing arrangement between solar homeowners and their utility company. When your solar panels produce more electricity than your home consumes, the excess energy flows back into the public grid. Your utility meter effectively “runs backward,” and you receive a credit on your electricity bill for that surplus power.

Net metering is one of the most important financial benefits of going solar. During sunny daytime hours, your panels often generate more power than you need; those credits offset the electricity you draw from the grid at night or on cloudy days. Policies vary by state — some states offer full retail-rate credits, while others have moved to reduced rates or time-of-use structures. As of 2026, over 40 states have some form of net metering policy in place, though the specifics of compensation rates and program caps differ significantly. Homeowners should check their state and utility policies before installing a solar system.

Nominal Operating Cell Temperature (NOCT) is a more realistic set of testing conditions used to rate solar panel performance, closer to actual field conditions than Standard Test Conditions (STC). NOCT ratings help homeowners understand what their panels will produce in everyday use.

NOCT conditions are defined as: 800 W/m² irradiance (representing typical bright conditions rather than peak noon sun), 20°C ambient air temperature, and 1 m/s wind speed. Under these conditions, solar cells typically reach temperatures of 42–48°C, which is much more representative of real-world operation than STC’s idealized 25°C cell temperature. A panel rated at 400W STC might produce 295–310W under NOCT conditions — a 22–26% reduction that more accurately predicts typical hourly output. Checking a panel’s NOCT rating alongside its STC rating gives you a better understanding of its real-world performance. Panels with lower NOCT temperatures (closer to 42°C) perform better in the field because lower operating temperatures mean less power loss from heat. The NOCT value is listed on the panel’s datasheet alongside STC specifications.

Operations and Maintenance (O&M) encompasses all the ongoing activities required to keep a solar energy system running safely and at peak performance over its lifetime. For residential systems, O&M is relatively minimal compared to other energy technologies.

Residential solar O&M typically includes: monitoring system performance for anomalies (usually through an app or web portal), occasional panel cleaning (1–2 times per year in dusty areas), visual inspections for physical damage or loose connections, inverter maintenance or replacement (string inverters may need replacement at 12–15 years), vegetation management (trimming trees that create new shading), and verifying that production meets expectations. Most residential solar systems require very little active maintenance — there are no moving parts, and panels are designed to withstand decades of weather exposure. For systems under PPA or lease agreements, the solar company handles all O&M at no additional cost. For owned systems, many installers offer optional O&M service plans ($150–$500/year). In the commercial and utility-scale market, O&M is a more structured service with dedicated providers and detailed contracts covering response times, performance guarantees, and reporting.

An obstruction is any physical object that blocks sunlight from reaching solar panels or prevents panels from being installed in a particular location. Obstructions are identified during the site survey and directly impact system design, panel placement, and energy production estimates.

Roof obstructions include chimneys, plumbing vents, skylights, dormer windows, HVAC equipment, satellite dishes, and antenna masts. Ground-level obstructions include trees, neighboring buildings, fences, and utility poles. Fire code setbacks (required clear spaces around roof edges, ridges, and penetrations) also effectively act as obstructions by limiting where panels can be placed. Installers must design the array layout to avoid obstructions while maximizing the number of panels and minimizing shading impact. In some cases, obstructions can be relocated (satellite dishes) or removed (tree branches) to improve the installation area. 3D design software models the shadow patterns cast by obstructions throughout the year, ensuring the final layout delivers the production promised in the solar proposal.

An off-grid solar system operates independently from the utility grid, relying entirely on solar panels and battery storage to meet all of a household’s electricity needs. These systems are designed for energy self-sufficiency, with no connection to the power company.

Off-grid systems require careful design and significant battery capacity to ensure reliable power through nights, cloudy days, and seasonal variations. A typical off-grid setup includes a larger-than-average solar array, a battery bank (often 20–40 kWh or more), a charge controller to manage battery charging, and an inverter. Many off-grid homeowners also include a backup generator for extended periods of low sunlight. Off-grid systems are considerably more expensive than grid-tied systems — often 2–3 times the cost — due to the battery bank and additional components. They are most common in rural or remote locations where connecting to the utility grid is impractical or prohibitively expensive. While off-grid living offers complete energy independence, it also requires active energy management and system maintenance.

An Overcurrent Protection Device (OCPD) is any device designed to interrupt the flow of electrical current when it exceeds a safe level, protecting wiring, equipment, and people from damage or fire. Common OCPDs include circuit breakers and fuses.

In solar installations, overcurrent protection is required at multiple points in the system per the National Electrical Code (NEC). Fuses in the combiner box protect individual panel strings, a breaker at the inverter output protects the AC circuit, and a dedicated solar breaker in the main service panel protects the connection to your home’s electrical system. The NEC’s “120% rule” for bus bar ratings and proper OCPD sizing are critical design considerations that your solar installer must calculate correctly. Undersized OCPDs can trip nuisance faults and reduce system uptime, while oversized OCPDs fail to provide adequate protection. All OCPDs in a solar installation must be rated for the specific voltage and current type (AC or DC) they protect — DC-rated breakers and fuses are required on the panel side of the system since DC arcs are harder to extinguish than AC arcs.

Property Assessed Clean Energy (PACE) financing is a program that allows homeowners to fund solar installations (and other energy improvements) through a voluntary assessment on their property tax bill. The loan is repaid over 10–25 years as an additional line item on your annual property taxes.

PACE financing offers several unique advantages: there is typically no credit score requirement (approval is based on property equity and tax payment history), the loan stays with the property if you sell the home, and no down payment is required. However, PACE has significant drawbacks that homeowners should carefully consider. The interest rates are often higher than traditional solar loans (6–9%+), the PACE assessment creates a tax lien on your property that takes priority over your mortgage, and some mortgage lenders have restrictions on properties with PACE liens. Refinancing or selling a home with an active PACE assessment can be complicated. PACE programs are available in certain states and municipalities — California, Florida, and Missouri have the most active residential PACE programs. Before choosing PACE financing, compare the total cost with solar loans, which often have lower interest rates and don’t affect your property tax bill.

Payback period is the number of years it takes for the cumulative savings from your solar system to equal the total upfront cost of the system. After the payback period, every dollar saved on electricity is pure profit.

The average solar payback period in the United States is 6–10 years, depending on your location, electricity rates, system cost, available incentives, and financing method. In states with high electricity rates and strong incentives (like California, Massachusetts, New York, and New Jersey), payback periods can be as short as 4–6 years. In states with low electricity rates and fewer incentives, payback may take 10–12 years. The federal ITC (30% tax credit) significantly shortens the payback period by reducing the net system cost. For example, a $25,000 system with a $7,500 ITC credit has a net cost of $17,500. If you save $2,000 per year on electricity, the payback period is about 8.75 years. Since solar panels last 25–30+ years, the remaining 15–20+ years after payback represent pure savings. When evaluating solar proposals, ask for the projected payback period and the assumptions behind it — including electricity rate escalation, degradation rate, and incentive values.

A performance degradation warranty (also called a power output warranty or linear performance warranty) is a guarantee from the solar panel manufacturer that the panel will maintain a minimum percentage of its original rated output over a specified period — typically 25–30 years.

A standard performance warranty guarantees that a panel will produce at least 84–87% of its nameplate output after 25 years. Many premium manufacturers now offer 30-year warranties guaranteeing 88–92% of original output. “Linear” warranties guarantee a smooth, predictable decline each year (e.g., no more than 0.4% degradation per year), while older “stepped” warranties might allow up to 10% degradation in the first 10 years and 20% by year 25. If a panel’s tested output falls below the warranty threshold, the manufacturer must repair, replace, or compensate for the underperforming panel. However, filing a warranty claim can be complex — you need independent testing to prove the panel is underperforming, and the manufacturer may no longer be in business decades later. This is why choosing panels from established, financially stable manufacturers is important for long-term warranty security.

A performance monitoring system is the hardware and software that tracks your solar system’s energy production, consumption, and health in real time. Monitoring is an essential tool for ensuring your system is performing as expected and for quickly identifying any issues.

Most modern solar systems include monitoring as a standard feature, either through the inverter manufacturer’s platform (Enphase Enlighten, SolarEdge Monitoring, etc.) or a third-party solution. The system collects data from the inverter (and individual panels, if using microinverters or optimizers) and transmits it via Wi-Fi or cellular connection to a cloud-based platform. Homeowners can view production data, consumption patterns, and system status through a web portal or smartphone app. Key metrics displayed include: real-time power output (kW), daily/monthly/annual energy production (kWh), historical performance trends, and system alerts for inverter errors or production drops. Some advanced monitoring systems also track home energy consumption, grid imports/exports, and battery charge levels. Having monitoring enabled allows your installer to remotely diagnose issues and can speed up warranty claims if production falls below expected levels.

Performance Ratio (PR) is a measure of how efficiently a solar system converts available sunlight into electricity, accounting for all real-world losses. It compares actual energy output to the theoretical output based on the amount of solar radiation received and the system’s nameplate capacity.

A well-designed and maintained residential solar system typically achieves a performance ratio of 75–85%. A PR of 80% means the system is converting 80% of the available solar energy into usable electricity, with the other 20% lost to temperature effects, inverter conversion, wiring, shading, and other factors. PR is useful because it normalizes for weather and location — you can compare the performance of systems in different climates fairly. A system in cloudy Seattle with an 82% PR is performing better relative to its conditions than a system in sunny Phoenix with a 75% PR, even though the Phoenix system produces more total kWh. Monitoring your system’s PR over time helps identify degradation or maintenance issues. A sudden drop in PR could indicate shading from new tree growth, a malfunctioning inverter, or dirty panels that need cleaning.

A performance-based incentive (PBI) is a financial incentive that pays solar system owners based on the actual amount of electricity their system produces, rather than a flat upfront rebate. Payments are typically calculated per kilowatt-hour (kWh) generated over a set period of time.

PBI programs reward real energy production, which encourages homeowners to maintain their systems and ensure optimal performance. Payments are usually made monthly or quarterly and can last for 5–20 years depending on the program. The per-kWh rate varies by program and location — common rates range from $0.02 to $0.10 per kWh. Feed-in tariffs are a specific type of PBI where the utility purchases all or excess solar electricity at a predetermined rate. PBI programs are offered by some states, utilities, and municipalities. While not as widespread as net metering, PBIs can stack with other incentives like the federal ITC and SRECs, further improving the financial return of going solar. Check with your local utility and state energy office to see if PBI programs are available in your area.

Permission to Operate (PTO) is the official authorization from your electric utility that allows you to turn on your solar system and begin feeding electricity to the grid. PTO is granted after all inspections have passed, the interconnection agreement is finalized, and any required meter upgrades are complete.

PTO is the final milestone in the solar installation process and the moment when your system goes “live.” Until you receive PTO, you should not operate your solar system — running your system before PTO approval can violate your interconnection agreement and may result in penalties. The timeline from installation completion to PTO varies significantly: some utilities issue PTO within a few business days, while others take 4–8 weeks or longer. Common delays include utility inspection scheduling, meter replacement (installing a bidirectional net meter), and administrative processing. Your solar installer typically manages the PTO process and will notify you when approval is received. Once PTO is granted, your system begins generating electricity, your meter starts tracking two-way power flow, and you begin earning net metering credits on your utility bill.

Photovoltaic (PV) technology converts sunlight directly into electricity using semiconductor materials, most commonly silicon. The term comes from the Greek word “photo” (light) and “voltaic” (electricity, named after physicist Alessandro Volta). When photons from sunlight strike a PV cell, they knock electrons loose from atoms in the semiconductor, creating an electrical current.

PV cells are the fundamental building blocks of solar panels. A typical residential solar panel contains 60 to 72 individual PV cells wired together. Modern PV cells achieve efficiencies of 20–23% for standard panels, with premium models reaching up to 24%. The technology has been in use since the 1950s and continues to improve in both efficiency and affordability, making rooftop solar accessible to millions of American homeowners.

Polycrystalline solar panels are made from multiple silicon crystal fragments melted together, giving them a distinctive blue, speckled appearance. They offer a more affordable alternative to monocrystalline panels, though with slightly lower efficiency.

In the manufacturing process, molten silicon is poured into a mold and allowed to cool, forming multiple crystal structures within each cell. This simpler process reduces production costs but results in slightly lower efficiency — typically 15–18% — because the boundaries between crystals create resistance to electron flow. Polycrystalline panels require more roof space to produce the same amount of electricity as monocrystalline panels, but they still deliver solid performance and reliable energy production. They are a practical choice for homeowners with ample roof space who want to minimize upfront costs. Like monocrystalline panels, polycrystalline panels carry 25-year warranties and have a long operational lifespan. However, as the price difference between mono and poly panels has shrunk over the years, monocrystalline panels have become the dominant choice in the US residential market.

A power optimizer is a DC-to-DC converter installed on each individual solar panel that maximizes the power output of that panel before sending the electricity to a central string inverter. Optimizers are a type of module-level power electronics (MLPE) that offer a middle ground between basic string inverters and full microinverters.

Power optimizers work by performing Maximum Power Point Tracking (MPPT) at each panel, ensuring every panel operates at its optimal voltage and current regardless of shading, soiling, or manufacturing variations. Unlike microinverters, optimizers do not convert DC to AC — they condition the DC output and send it to a compatible string inverter for the DC-to-AC conversion. This two-stage approach is slightly more efficient than microinverters in ideal conditions while still providing panel-level optimization and monitoring. The leading manufacturer is SolarEdge, whose optimizers pair exclusively with SolarEdge inverters. Power optimizer systems also satisfy NEC rapid shutdown requirements. The typical cost premium over a basic string inverter is $500–$1,500 for a residential system, less than a full microinverter setup but with many of the same benefits.

A Power Purchase Agreement (PPA) is a financing arrangement where a third-party company installs, owns, and maintains a solar system on your roof, and you agree to purchase the electricity it produces at a predetermined rate — typically lower than your current utility rate.

With a PPA, you do not own the solar panels. The solar company retains ownership and receives the tax credits and incentives. In return, you pay for the electricity the panels generate, usually at a rate 10–30% below your utility rate, with a small annual escalator (1–3% per year). PPAs typically last 20–25 years. The advantage is zero upfront cost and immediate electricity savings with no maintenance responsibilities. The downsides include not receiving the federal ITC yourself, the annual rate escalator that may eventually approach or exceed utility rates, and potential complications if you sell your home (the PPA must be transferred to the new buyer or bought out). PPAs are best suited for homeowners who want solar savings without the upfront investment and don’t qualify for or want to use the tax credit themselves. They are available in about 25 states — some states prohibit third-party ownership of solar systems.

A product warranty (also called a materials or equipment warranty) covers defects in manufacturing, materials, and workmanship of individual solar components — panels, inverters, racking, and batteries. It is separate from the performance degradation warranty and the installer’s workmanship warranty.

For solar panels, product warranties typically last 12–25 years and cover issues like delamination, junction box failure, glass breakage (from manufacturing defects), cell cracking, connector failures, and frame corrosion. If a panel fails due to a manufacturing defect within the warranty period, the manufacturer will repair or replace it. For inverters, product warranties range from 12 to 25 years — Enphase microinverters offer 25-year warranties, while many string inverters come with 12-year warranties that can be extended to 20–25 years for an additional cost. Battery product warranties typically cover 10–15 years. When comparing solar proposals, pay attention to product warranty lengths for all major components and consider whether extended warranties are worth the cost. Also note whether the warranty covers only the replacement equipment or also includes the labor cost of removing and installing the replacement.

A production guarantee is a warranty offered by some solar installers that ensures your system will produce a minimum amount of electricity over a specified period. If the system underperforms the guaranteed level, the installer compensates you — typically through a cash payment, bill credit, or additional equipment.

Production guarantees give homeowners peace of mind that their solar investment will deliver the promised energy savings. A common guarantee might promise that your system will produce at least 90% of the projected annual kWh output over the first 10 years. This is different from a manufacturer’s panel warranty, which covers defects and degradation rates for individual panels. A production guarantee covers the entire system’s real-world performance, accounting for factors like weather, shading, and equipment efficiency. Not all installers offer production guarantees, so it’s worth asking when comparing solar quotes. Companies that do offer them often use monitoring systems to track production in real time and proactively address any issues that arise. Read the fine print carefully — some guarantees have exclusions for unusual weather events or changes to surrounding structures (like a neighbor building an addition that creates new shading).

A production meter is a dedicated electric meter that measures the total electricity generated by a solar system, independent of how much is consumed on-site or sent to the grid. It is separate from the utility’s bidirectional meter, which tracks net energy flow.

Production meters are required in some states and utility territories, particularly where solar incentives (like SRECs or performance-based incentives) are paid based on actual system production rather than net metering credits. The production meter is typically installed between the inverter and the main electrical panel and records every kilowatt-hour the system generates, regardless of whether that energy powers your home or flows to the grid. Some utilities or incentive programs provide a revenue-grade production meter at no cost, while others require the homeowner or installer to purchase one. In many modern systems, the inverter’s built-in monitoring serves as a de facto production meter — some incentive programs accept inverter data as proof of production in lieu of a separate meter. Check with your utility and incentive programs to determine if a dedicated production meter is required for your installation.

Racking refers to the mounting system that secures solar panels to rooftops, building surfaces, or the ground. It is a critical component of every solar installation, providing structural support while positioning panels at the optimal angle for sunlight exposure.

Roof-mounted racking systems attach to the roof structure using bolts and flashing to maintain a waterproof seal. They typically hold panels a few inches above the roof surface to allow airflow for cooling. Ground-mount racking uses posts driven into the ground or set in concrete footings, with adjustable frames that can be tilted to the ideal angle for your latitude. Some ground-mount systems include single-axis or dual-axis tracking motors that follow the sun throughout the day, increasing energy production by 15–25% compared to fixed mounts. Racking materials are usually anodized aluminum or galvanized steel, chosen for their durability and corrosion resistance. Racking costs typically account for 5–10% of the total solar system price.

Rail-based racking is the traditional and most common mounting system for residential rooftop solar. It uses long aluminum rails that run horizontally (or vertically) across the roof, attached at multiple points to the roof structure with flashed lag bolts. Solar panels are then clamped to these rails.

The rails distribute the weight of the panels evenly across multiple attachment points and provide a secure, adjustable framework for panel placement. Rail systems are versatile — they work on virtually any roof type and pitch, and can accommodate different panel sizes and orientations. The main components are the rails themselves, L-feet or standoff brackets that attach the rails to the roof, and mid-clamps and end-clamps that secure the panels to the rails. Rail-based systems are proven, well-understood by installers, and supported by extensive engineering data for wind and snow load calculations. The downside is slightly higher material cost and longer installation time compared to rail-less systems. Major rail-based racking manufacturers include IronRidge, Unirac, and SnapNrack.

Rail-less racking is a mounting system that attaches solar panels directly to the roof structure without using continuous horizontal rails. Instead, individual mounting brackets or standoffs connect directly from the roof to the panels, often clamping to the panel frames or using integrated mounting hardware.

Rail-less systems have gained popularity because they reduce material costs (no rails to purchase), lower shipping weight, and significantly speed up installation time — often cutting rooftop labor by 30–50%. With fewer components, the visual profile is cleaner and lower to the roof surface. However, rail-less systems require more precise roof attachment point placement (since there are no rails to provide adjustment flexibility), and they may have more limited engineering approvals for high wind or snow load zones. Rail-less racking works best on standard asphalt shingle roofs with consistent rafter spacing. Leading rail-less racking brands include Quick Mount PV’s Quick Rack, Pegasus, and various proprietary systems from large installers. The choice between rail-based and rail-less racking often comes down to installer preference, local code requirements, and roof conditions.

Rapid shutdown is a safety requirement mandated by the National Electrical Code (NEC) that ensures rooftop solar systems can quickly reduce voltage to safe levels in an emergency. This protects firefighters, utility workers, and first responders from the risk of electrical shock when accessing a roof with solar panels.

Since 2017, the NEC (Section 690.12) has required that conductors within the array boundary be reduced to 80 volts or less within 30 seconds of activating the rapid shutdown. This means each panel must have a module-level device — either a microinverter, a power optimizer with rapid shutdown capability, or a dedicated rapid shutdown device — that de-energizes the panel on command. The shutdown is typically initiated by turning off the inverter or activating a dedicated rapid shutdown switch. This US-specific requirement has made module-level power electronics (MLPE) effectively standard on most new residential installations. Older systems installed before the requirement may not have rapid shutdown capability. If you’re getting a new system installed, rapid shutdown compliance is handled automatically by your installer’s equipment choices.

Renewable energy is energy generated from natural sources that are constantly replenished and virtually inexhaustible on a human timescale. The main types of renewable energy include solar, wind, hydroelectric, geothermal, and biomass. Unlike fossil fuels (coal, oil, natural gas), renewable sources produce little to no greenhouse gas emissions during operation.

Solar energy is the fastest-growing form of renewable energy in the United States. In 2025, renewable sources accounted for over 20% of US electricity generation, with solar contributing a rapidly increasing share. The shift to renewable energy is driven by falling costs (solar electricity is now cheaper than fossil fuel power in most markets), federal and state incentives, climate goals, and consumer demand for cleaner energy. For homeowners, installing solar panels is the most accessible way to use renewable energy — it reduces reliance on fossil fuels, lowers electricity costs, and shrinks your household’s carbon footprint. Many states have Renewable Portfolio Standards (RPS) that require utilities to source a percentage of their electricity from renewables, further driving demand for solar and other clean energy sources.

A Renewable Energy Certificate (REC) is a market-based instrument that represents the environmental attributes of one megawatt-hour (MWh) of electricity generated from a renewable energy source. RECs can be bought and sold separately from the physical electricity, allowing organizations to claim they are using renewable energy even if their electricity comes from the grid.

RECs are the backbone of the US renewable energy tracking system. When your solar system generates 1 MWh (1,000 kWh) of electricity, it creates one REC. The REC can be “retired” (claimed by you or your utility for renewable energy compliance) or sold on the open market. Solar-specific RECs are called SRECs and are typically more valuable than general RECs because some state Renewable Portfolio Standards require a specific amount of solar energy. General REC prices are relatively low ($1–$5 per MWh) because supply is abundant. SRECs in states with solar carve-outs can be worth $10–$300+ per MWh. For most residential solar homeowners in net metering states, the RECs are automatically transferred to the utility as part of the net metering agreement. In SREC markets, homeowners retain and sell their SRECs for additional income.

A Renewable Portfolio Standard (RPS) is a state-level regulation that requires electric utilities to source a specified percentage of their electricity from renewable energy sources by a target date. RPS policies are one of the primary drivers of solar energy growth in the United States.

As of 2026, over 30 states plus Washington DC have enacted RPS policies, with targets ranging from 25% to 100% renewable energy by various deadlines. For example, California’s RPS requires 100% clean electricity by 2045, while New York targets 70% renewable by 2030. Some states include a specific “solar carve-out” within their RPS, requiring a portion of the renewable target to come specifically from solar energy — this creates dedicated demand for solar and drives SREC markets. To comply with RPS requirements, utilities either build or purchase renewable energy or buy Renewable Energy Certificates (RECs) from solar system owners. The existence of an RPS in your state generally supports higher SREC values, stronger net metering policies, and more utility-sponsored solar incentive programs, all of which benefit residential solar customers.

Return on Investment (ROI) for solar measures the total financial gain from your solar system compared to the initial cost, expressed as a percentage. A positive ROI means your solar system saves you more money than it costs over its lifetime.

Solar ROI is calculated by taking the total lifetime savings (electricity bill savings + incentives + increased home value) minus the total system cost (purchase price + maintenance - tax credits), divided by the net system cost. The average residential solar system in the US delivers a lifetime ROI of 100–300%, depending on location, electricity rates, and incentives. In high-rate states, ROI can exceed 300% — meaning the system saves three times more than it costs. Factors that improve ROI include: high local electricity rates, strong net metering policies, available state/local incentives, paying cash (no interest charges), and rising utility rates over time. Factors that reduce ROI include: low electricity rates, unfavorable net metering changes, high financing interest rates, and significant shading. Solar ROI typically outperforms many traditional investments when factored over the 25–30 year lifespan of the system, especially since the “returns” (electricity savings) are tax-free.

A roof mount system is the most common type of residential solar installation, with panels attached to the home’s roof using a racking system secured to the roof structure. Over 90% of residential solar installations in the US are roof-mounted.

Roof mount systems are popular because they use existing space (your roof) without requiring additional land, keep panels out of the way, and are generally less expensive to install than ground-mount alternatives. The racking is attached to the roof rafters or trusses using lag bolts and sealed with flashing to prevent leaks. Panels are positioned a few inches above the roof surface to allow airflow for cooling. The ideal roof for solar is south-facing with a pitch of 15°–40°, minimal shading, and made of composition shingle, standing seam metal, or concrete tile. Flat roofs use ballasted or tilted racking to achieve an optimal angle. Roof condition is important — if your roof needs replacement within the next 5–10 years, it’s best to re-roof before the solar installation to avoid the cost of removing and reinstalling panels later. Most roof mount installations can be completed in 1–2 days.

A roof penetration is any hole drilled through the roof surface to attach solar racking hardware to the underlying roof structure. Each penetration point is sealed with flashing to maintain the roof’s waterproof integrity.

A typical residential solar installation requires 30–60 roof penetrations, depending on the number of panels and the type of racking system used. Each penetration is made by drilling a lag bolt or structural screw through the roofing material (shingles, tile, etc.) into a rafter or truss. This is the standard method for securing solar panels and is proven to be safe and reliable when done correctly. Homeowners often worry about roof leaks from penetrations, but with proper flashing and sealant, leaks are extremely rare — reputable installers offer 10–25 year workmanship warranties covering any leak issues. Some mounting solutions minimize or eliminate penetrations entirely: standing seam metal roofs use non-penetrating clamps, flat roofs can use ballasted systems, and some newer racking designs reduce the number of attachment points needed. If minimizing roof penetrations is a priority, discuss options with your installer during the proposal stage.

Roof pitch (or roof slope) describes the steepness of a roof, typically expressed as a ratio of vertical rise to horizontal run — for example, a 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. This ratio directly determines the tilt angle of roof-mounted solar panels.

Common residential roof pitches range from 3:12 (low slope, about 14°) to 12:12 (steep, 45°). A 6:12 pitch (about 27°) is one of the most common and happens to be close to optimal for solar in many US locations. Roof pitch affects solar production, installation methods, and safety requirements. Low-pitch roofs (3:12 or less) may benefit from tilted racking to increase the panel angle, while very steep roofs (10:12+) can pose safety challenges during installation and may require specialized equipment. The optimal pitch for solar depends on your latitude — as a general rule, a pitch equal to your latitude angle maximizes annual production. However, variations of 10–15° from optimal typically reduce production by less than 5%, so most roof pitches work well for solar. Your installer will account for your specific roof pitch when calculating expected system production.

Round-trip efficiency is the percentage of energy that can be retrieved from a battery compared to the energy that was put in. It accounts for the energy losses that occur during the charging and discharging process.

For example, if you store 10 kWh of solar energy in a battery with 90% round-trip efficiency, you can retrieve 9 kWh — the other 1 kWh is lost as heat during the conversion process. Modern lithium-ion home batteries achieve round-trip efficiencies of 85–95%, meaning relatively little energy is wasted. Higher efficiency means more of your solar energy is usable and less money is lost to conversion losses. When comparing battery options, even a 5% difference in efficiency adds up over thousands of cycles and years of operation. AC-coupled battery systems typically have slightly lower round-trip efficiency than DC-coupled systems because AC coupling involves an additional conversion step (DC → AC → DC → AC). The Tesla Powerwall has a round-trip efficiency of approximately 90%, while Enphase IQ batteries achieve around 89%.

Section 25D of the Internal Revenue Code is the specific legal provision that authorizes the Residential Clean Energy Credit — the federal tax credit for homeowners who install solar panels and other qualifying clean energy systems on their primary or secondary residences.

Under Section 25D, homeowners can claim a tax credit of 30% of the total cost of a qualifying solar energy system installed between 2022 and 2032. The credit applies to solar electric (PV) systems, solar water heating, battery storage (3 kWh or larger), geothermal heat pumps, small wind turbines, and fuel cells. The 30% rate was extended and enhanced by the Inflation Reduction Act of 2022. After 2032, the credit steps down to 26% in 2033 and 22% in 2034, and is scheduled to expire for residential systems after 2034 (unless extended by Congress). There is no maximum dollar cap on the Section 25D credit for residential systems. The credit is non-refundable, meaning it can reduce your tax liability to zero but cannot generate a refund — however, unused credits can be carried forward to future tax years. This is the same credit commonly known as the “solar tax credit” or “ITC.”

Section 48 of the Internal Revenue Code provides the Investment Tax Credit (ITC) for commercial, industrial, and utility-scale solar energy systems. It is the commercial counterpart to the residential Section 25D credit and offers a base credit of 30% with potential bonus adders.

Under the Inflation Reduction Act, Section 48 provides a base credit of 6% for commercial solar projects, which increases to 30% if the project meets prevailing wage and apprenticeship requirements (which most commercial projects do). Additional bonus credits are available: +10% for domestic content (using US-manufactured components), +10% for projects in energy communities (areas affected by fossil fuel industry closures), and +10–20% for projects in low-income communities. This means qualifying commercial solar projects can receive total tax credits of 40–60% or more. Section 48 is available for businesses, nonprofits (via direct pay provisions in the IRA), and tax-exempt entities. Unlike the residential credit, commercial systems using Section 48 can also benefit from MACRS depreciation, further improving the financial return. The combined value of the Section 48 ITC and MACRS depreciation can offset 50–70% of the total system cost.

A setback is the required minimum distance between solar panels and the edges, ridges, valleys, or other features of a roof. Setback requirements are mandated by fire codes and local building codes to provide clear pathways for firefighter access and emergency ventilation.

In most US jurisdictions following the International Fire Code (IFC), residential solar installations must maintain setbacks of 18–36 inches from the roof ridge and 18 inches from the roof edges and hips. Some jurisdictions also require a 3-foot-wide clear pathway from the eave to the ridge on at least one roof slope for firefighter access. These setbacks reduce the usable roof area for panels, which is why a 1,500 square foot roof may only support panels on 60–70% of its surface. Setback requirements vary by jurisdiction — some cities have adopted more lenient rules, while others are stricter. Your solar installer designs the array layout to comply with local setback codes, which is one of the first things verified during the permitting and inspection process. Newer rooftop fire safety technologies and UL-listed racking systems may allow reduced setbacks in some jurisdictions.

Shading refers to any obstruction that blocks sunlight from reaching solar panels, reducing their energy output. Even partial shading on a small portion of a solar array can significantly impact the entire system’s performance, making shade analysis a critical part of solar design.

Common sources of shading include trees, chimneys, dormers, neighboring buildings, utility poles, vent pipes, and satellite dishes. In a string inverter system, shading on just one panel can reduce the output of the entire string because panels are wired in series. Microinverters and power optimizers mitigate this problem by allowing each panel to operate independently — only the shaded panel is affected. During the site assessment, your solar installer uses tools like the Solmetric SunEye, satellite imagery, or 3D modeling software (like Aurora Solar) to analyze shading patterns across all seasons and times of day. Even deciduous trees that lose their leaves in winter can reduce annual production by 10–25% if they shade the array during summer months. In some cases, trimming or removing trees may be recommended to optimize solar production and return on investment.

Single-phase power is the standard type of electrical service delivered to most residential homes in the United States. It provides 120/240 volts through two hot wires and a neutral wire, supplying power for lights, outlets, appliances, and HVAC systems.

The vast majority of residential solar installations in the US connect to single-phase electrical service. Solar inverters for residential use are designed to output single-phase AC power that synchronizes with your home’s electrical system and the utility grid. When sizing a solar system, the inverter must match the voltage and phase configuration of your home’s service. Single-phase service typically supports up to 200 amps, which is sufficient for most homes including those with solar and battery storage. Larger homes or those with particularly high electrical loads (such as multiple EV chargers, pools, and large HVAC systems) may have or need three-phase service. If you are unsure of your service type, check your main service panel or ask your utility — nearly all standard suburban and rural homes in America have single-phase power.

A site assessment is the initial evaluation of a property’s suitability for solar panels, often performed remotely using satellite imagery, 3D modeling software, and utility data before any physical visit. It is typically the first step in the solar sales process and forms the basis for the initial proposal.

Modern site assessments use high-resolution satellite and aerial imagery (from sources like Google Earth, Nearmap, or EagleView) combined with 3D modeling tools (like Aurora Solar or Helioscope) to create a detailed digital model of your roof. The assessment evaluates: roof size and available area, orientation and tilt, estimated shading from trees and structures, structural feasibility, and estimated solar energy production. Combined with your utility bill data (showing your electricity usage patterns and costs), the site assessment enables the installer to generate a preliminary proposal with system size, production estimates, and savings projections — all without setting foot on your property. While remote site assessments have become remarkably accurate, they can miss details like roof condition, hidden shading, or electrical panel limitations. This is why a follow-up physical site survey is conducted before finalizing the installation plan.

A site survey is an on-site evaluation of your property conducted by a solar installer (or technician) to gather the detailed information needed to finalize the system design. It is typically performed after you’ve received an initial proposal and before the final contract is signed.

During the site survey, the technician will: examine the roof condition, age, and material; measure roof dimensions, pitch, and orientation; identify obstructions (vents, chimneys, skylights, satellite dishes); perform a shade analysis using specialized tools; inspect the main electrical panel (brand, age, amperage, available breaker spaces); check the utility meter configuration; evaluate the path for conduit runs from the roof to the panel; assess attic access and rafter spacing for structural attachments; and take photos documenting all findings. The site survey may reveal conditions that weren’t visible from satellite imagery — such as roof damage that needs repair, an undersized electrical panel requiring an upgrade, or shading from nearby structures. These findings can lead to adjustments in the system design or additional costs. A thorough site survey is the best protection against unexpected change orders and installation delays.

Solar access refers to the amount of unobstructed sunlight available to a solar installation site, measured as a percentage of the total possible sunlight. A solar access rating of 90% means the site receives 90% of the sunlight it would get with zero obstructions.

Solar access is assessed during the site survey and is a key factor in determining whether a roof or ground location is suitable for solar panels. An ideal installation site has solar access of 80% or higher. Sites with lower solar access may still be viable but will produce less electricity, reducing the financial return. Solar access varies across different areas of the same roof — a south-facing section may have 95% access while a tree-shaded west section has only 60%. Installers measure solar access using shade analysis tools that account for obstructions at every angle and elevation, factoring in the sun’s path across all seasons. Some states and municipalities have solar access laws or solar easements that protect homeowners’ right to sunlight by preventing neighbors from planting trees or constructing buildings that would shade an existing solar installation.

A solar array is a collection of multiple solar panels wired together and mounted as a single electricity-producing system. While a single solar panel generates a limited amount of power, combining panels into an array allows homeowners to scale their system to match their energy needs.

A typical residential solar array in the United States consists of 15–25 panels, producing 6–10 kW of power. The panels in an array can be wired in series (to increase voltage), in parallel (to increase current), or in a combination of both. Arrays are mounted on rooftops using racking systems or installed on the ground using ground-mount frames. The size of your array depends on your electricity usage, available roof or ground space, local sun hours, and budget. A properly sized array can offset 80–100% of a household’s annual electricity consumption.

A solar cell is the basic building unit of a solar panel. Each cell generates an electrical charge when struck by sunlight through a process called the photovoltaic effect. When photons hit the cell’s semiconductor material (usually silicon), they dislodge electrons and create an electric current.

Individual solar cells produce only a small amount of voltage — typically around 0.5 volts each. To generate useful amounts of electricity, dozens of cells are wired together in series and parallel within a single solar panel. A standard residential panel contains 60 or 72 cells. Solar cells come in different types: monocrystalline cells (cut from a single silicon crystal, higher efficiency) and polycrystalline cells (made from multiple silicon fragments, lower cost). Newer technologies such as half-cut cells and shingled cells improve performance by reducing electrical resistance and minimizing the impact of shading.

A solar charge controller is a device that regulates the flow of electricity from solar panels to a battery bank, preventing overcharging and extending battery life. It is an essential component in off-grid and some hybrid solar systems where batteries are charged directly from the panels’ DC output.

There are two main types of charge controllers. PWM (Pulse Width Modulation) controllers are simpler and less expensive — they work best when the panel voltage closely matches the battery voltage. MPPT (Maximum Power Point Tracking) controllers are more advanced and efficient, capable of converting higher-voltage panel output to the optimal charging voltage for the battery. MPPT controllers can extract 15–30% more energy from the panels compared to PWM controllers, making them the preferred choice for most systems. Charge controllers are sized based on the array’s voltage and amperage. In modern grid-tied systems with AC-coupled batteries (like the Tesla Powerwall), a separate charge controller is not needed because the battery has its own built-in charge management. Charge controllers are most commonly found in off-grid cabins, RVs, boats, and standalone power systems.

Solar energy is the electromagnetic radiation emitted by the sun that reaches Earth. It is the most abundant energy resource on the planet — in just one hour, the sun delivers enough energy to Earth’s surface to power the entire world for a year. Solar energy can be harnessed for electricity (using photovoltaic panels) or for heating (using solar thermal systems).

In the context of residential solar, solar energy refers specifically to the electricity generated by rooftop or ground-mounted photovoltaic panels. The amount of solar energy available at a given location depends on geographic latitude, local climate, time of year, and time of day. Solar energy production is measured in kilowatt-hours (kWh) or megawatt-hours (MWh). The Southwestern United States receives the most solar radiation, but solar panels work effectively in every state — even cloudy regions like the Pacific Northwest produce significant solar energy. Solar energy is clean, produces zero emissions during generation, requires minimal maintenance, and provides predictable electricity costs for 25+ years. It is the foundation of the growing US transition toward renewable energy and energy independence.

A solar installer (or solar contractor) is a licensed company or professional that designs, installs, and maintains residential or commercial solar energy systems. Choosing the right installer is one of the most important decisions in going solar, as it affects system quality, pricing, warranty coverage, and long-term performance.

Solar installers range from large national companies (like SunPower, Sunrun, and Tesla) to regional and local contractors. National companies often offer standardized pricing and financing options, while local installers may provide more personalized service and competitive pricing. Key qualifications to look for include: a state contractor’s license (electrical or general, depending on state), NABCEP (North American Board of Certified Energy Practitioners) certification, positive reviews and references, several years of installation experience, clear warranty terms (both product and workmanship), and proper insurance (liability and workers’ compensation). It is recommended to obtain at least three quotes from different installers before making a decision. The installer should conduct a thorough site survey, provide a detailed proposal, handle all permitting, and offer transparent pricing that clearly itemizes equipment, labor, and other costs.

A solar lease is a financing arrangement where you pay a fixed monthly fee to use a solar system installed on your roof by a third-party company. Unlike a PPA (where you pay per kilowatt-hour generated), a lease payment is the same each month regardless of how much electricity the panels produce.

Solar leases typically require zero money down and last 20–25 years. The monthly lease payment is set to be lower than your expected electricity savings, so you save money from day one. Like PPAs, the solar company owns the system, handles all maintenance, and claims the tax credits. The predictable monthly payment makes budgeting simple, but you don’t benefit from the federal ITC or any increase in home value from the panels (since you don’t own them). At the end of the lease term, you typically have the option to renew, purchase the system at fair market value, or have it removed. If you sell your home before the lease ends, the new buyer must agree to assume the lease or you may need to buy out the remaining term. Solar leases have become less popular as solar loan financing has improved, allowing more homeowners to purchase systems with zero down payment while still claiming the tax credit.

Solar offset is the percentage of your total annual electricity consumption that is covered by your solar system’s production. A 100% solar offset means your panels produce as much electricity over the course of a year as your home consumes.

Solar offset is one of the most intuitive metrics for understanding the value of a solar installation. For example, if your home uses 12,000 kWh per year and your solar system produces 10,000 kWh, your solar offset is 83%. Most solar installers design systems to achieve an 80–100% offset, balancing maximum savings with practical constraints like available roof space and budget. A 100% offset doesn’t mean you never use grid electricity — you’ll still draw from the grid at night and during cloudy periods, but your annual production matches your annual consumption through net metering credits. In some cases, homeowners intentionally size systems for 100–110% offset to account for future electricity needs (like an EV charger) or to bank extra credits. Your installer should clearly state the projected solar offset percentage in your proposal, along with the assumptions used for the calculation.

A solar panel (also called a solar module) is a device made up of multiple photovoltaic cells that captures sunlight and converts it into electricity. The terms “solar panel” and “solar module” are interchangeable in residential solar contexts.

A standard residential solar panel contains 60 or 72 silicon-based PV cells laminated between protective glass and a backsheet, framed in aluminum for durability. Modern panels typically produce 350–450 watts each and measure roughly 65 × 39 inches. Most manufacturers guarantee at least 80–85% of original output after 25 years, though real-world degradation rates average just 0.3–0.5% per year. Panels are rated under Standard Test Conditions (STC) at 25°C and 1,000 W/m² of sunlight, so actual output varies with temperature, shading, and orientation.

A solar proposal (also called a solar quote) is a detailed document from a solar installer that outlines the recommended system design, expected performance, pricing, financing options, and projected savings for your specific home. It is the primary tool for evaluating and comparing solar offers.

A thorough solar proposal should include: system size (kW), number and model of panels, inverter type and model, mounting method, estimated annual energy production (kWh), projected solar offset percentage, total system cost (gross and net after incentives), itemized breakdown of costs (equipment, labor, permits, etc.), available financing options with monthly payment estimates, projected electricity savings over 25 years, estimated payback period and ROI, warranty coverage details, and a preliminary system layout showing panel placement on your roof. Quality proposals are based on actual site data (from a site survey or satellite imagery analysis) and use your real utility bills to project savings. When comparing proposals from different installers, ensure you’re comparing equivalent system sizes, similar equipment quality, and the same assumptions for electricity rate escalation and production estimates. Getting at least three proposals is recommended to ensure competitive pricing and find the best fit.

A Solar Renewable Energy Credit (SREC) is a tradable certificate that represents the environmental benefits of generating one megawatt-hour (1,000 kWh) of electricity from solar energy. SREC programs exist in several states, and they can provide significant additional income to solar homeowners.

Here’s how SRECs work: for every 1,000 kWh your solar system produces, you earn one SREC. You can then sell that SREC on the open market to utilities that need to meet state renewable energy mandates (called Renewable Portfolio Standards). The value of SRECs varies dramatically by state and market conditions — ranging from under $10 in some states to over $300 in states like New Jersey or Massachusetts where demand is high. States with active SREC markets include New Jersey, Massachusetts, Maryland, Pennsylvania, Ohio, Illinois, and Washington DC. Not all states have SREC programs, and the rules differ in each market. In states where SRECs are valuable, they can add thousands of dollars in income over the life of your solar system, significantly improving your return on investment.

A solar system (also called a solar PV system or solar energy system) is the complete set of hardware that generates electricity from sunlight for your home. It includes solar panels, an inverter, racking/mounting, wiring, a monitoring system, and the electrical connections to your home’s breaker panel and utility meter.

A residential solar system works as an integrated unit: the panels capture sunlight and convert it to DC electricity, the inverter converts DC to AC, and the wiring delivers that AC power to your home’s electrical system. If connected to the grid (as most systems are), excess electricity flows to the utility through a bi-directional meter. Optional components include battery storage for backup power, power optimizers for panel-level optimization, and a monitoring app or portal to track energy production. Residential solar systems in the US typically range from 5 kW to 12 kW in size, cost $15,000–$35,000 before incentives, and are designed to last 25–30 years or more. The entire system is covered by a combination of manufacturer warranties (for panels and inverters) and installer workmanship warranties (for labor and installation quality).

Standard Test Conditions (STC) are the laboratory conditions under which solar panels are tested and rated for their power output. STC provides a universal benchmark that allows fair comparison of panels from different manufacturers.

The three parameters that define STC are: cell temperature of 25°C (77°F), solar irradiance of 1,000 W/m² (equivalent to bright noon sunlight), and an air mass of 1.5 (representing the spectrum of sunlight passing through 1.5 atmospheres). When a panel is rated at 400W STC, it means it produces 400 watts under these exact conditions. However, real-world conditions rarely match STC — rooftop panels typically operate at 40–65°C (not 25°C), and irradiance fluctuates throughout the day. This is why actual production is lower than nameplate ratings. To provide a more realistic performance estimate, panels are also rated under Nominal Operating Cell Temperature (NOCT) conditions, which better represent typical field conditions. When comparing solar panels, STC ratings are useful for apples-to-apples comparisons, but NOCT ratings give a more realistic picture of expected daily output.

State of Charge (SoC) is a measurement that indicates how full a battery is at any given moment, expressed as a percentage from 0% (completely empty) to 100% (fully charged). It is the battery equivalent of a fuel gauge.

SoC is one of the most important metrics displayed on your solar battery’s monitoring app or portal. Knowing your battery’s SoC helps you understand how much backup energy is available and how your battery is cycling throughout the day. Most home battery systems are programmed to maintain a minimum SoC reserve (typically 10–20%) to protect the battery from deep discharge, which can reduce its lifespan. For example, if your battery has a 20% reserve, it will stop discharging at 20% SoC even though it technically has energy remaining. During normal operation, your battery might charge to 100% from solar during the day, discharge to 20–30% overnight, and recharge the next morning. Monitoring SoC patterns over time can help you determine if your battery is sized appropriately for your household’s needs.

A string inverter, also called a central inverter, converts DC electricity from multiple solar panels wired together in a “string” into AC electricity at one central location, usually mounted on a wall near your electrical panel.

String inverters are the most established and cost-effective inverter technology for residential solar. In a string configuration, panels are connected in series — like lights on a string — so the DC output of all panels flows to a single inverter. This design is simple and reliable, with fewer components than microinverter setups. However, string inverters have a drawback: if one panel in the string is shaded or underperforming, it can reduce the output of the entire string. For this reason, string inverters work best on roofs with consistent sun exposure and minimal shading. Many homeowners pair string inverters with power optimizers to get panel-level performance while keeping costs lower than a full microinverter system.

Temperature coefficient is a specification that indicates how much a solar panel’s power output decreases for each degree Celsius above 25°C (77°F). It is expressed as a negative percentage per degree, such as −0.35%/°C.

Solar panels are rated at a cell temperature of 25°C under Standard Test Conditions, but on a hot summer day, rooftop panel temperatures can reach 50–70°C or higher. Every degree above 25°C reduces output by the temperature coefficient percentage. For a panel with a −0.35%/°C coefficient operating at 55°C, the output drops by about 10.5% (30°C × 0.35%). This is why solar panels in hot climates like Arizona, while receiving more sunlight, may produce less per peak sun hour than expected. The temperature coefficient varies by panel type: monocrystalline panels typically range from −0.30% to −0.40%/°C, while some premium panels achieve −0.25%/°C or better. Lower (closer to zero) temperature coefficients are better, especially in hot climates. Adequate airflow between panels and the roof surface also helps reduce operating temperatures and mitigate temperature-related losses.

Three-phase power is an electrical service that uses three alternating currents offset by 120 degrees, delivering more consistent and higher-capacity power than single-phase. It is standard for commercial and industrial buildings but uncommon in residential homes in the United States.

Three-phase power is occasionally found in larger residences, farms, or homes with commercial-grade equipment (like large workshops or agricultural operations). Solar installations on three-phase properties require a three-phase inverter or multiple single-phase inverters balanced across all three phases. Three-phase inverters are more common in commercial solar and are typically available in larger sizes (10 kW and above). If your property has three-phase service and you’re installing residential solar, make sure your installer is aware — using a single-phase inverter on a three-phase service may require load balancing and could affect utility interconnection requirements. The cost difference is usually minimal, but the equipment selection and system design must account for the three-phase configuration to ensure code compliance and proper grid connection.

Tilt angle is the angle at which solar panels are oriented relative to the horizontal ground. The optimal tilt angle for a fixed solar array is generally close to the latitude of the installation location, maximizing the amount of direct sunlight captured throughout the year.

For most of the continental United States (latitudes 25°–48°), the ideal tilt angle is between 25° and 45°. A home in Houston, TX (latitude 30°) benefits from a tilt of about 30°, while a home in Minneapolis, MN (latitude 45°) performs best at around 40–45°. Roof-mounted panels are typically installed at whatever angle the roof provides (most residential roofs are pitched at 15°–40°), and modest deviations from the optimal angle reduce production by only 5–15%. Ground-mount systems offer the flexibility to set the ideal tilt angle precisely. Some advanced ground-mount systems use single-axis tracking to automatically adjust the tilt throughout the day or seasonally. When evaluating a solar proposal, your installer should model the expected production based on your actual roof tilt rather than just using ideal conditions.

Time-of-Use (TOU) rates are an electricity pricing structure where the cost per kilowatt-hour varies depending on the time of day. Electricity is more expensive during peak demand periods (typically late afternoon and evening) and cheaper during off-peak hours (overnight and early morning).

TOU rates significantly affect the economics of solar and battery storage. Solar panels produce the most electricity during midday hours, which under TOU pricing are often mid-peak or even off-peak — meaning the credits you earn for excess solar may be worth less than the electricity you buy during the expensive evening peak. This is where battery storage becomes especially valuable: you can store midday solar production and discharge the battery during peak evening hours when electricity costs 2–4 times more. In California, which has mandatory TOU rates for solar customers, the difference between off-peak ($0.15/kWh) and peak ($0.45/kWh) rates is substantial. TOU rate plans are becoming more common across the US as utilities modernize their pricing. Understanding your TOU schedule is essential for maximizing solar savings, properly sizing battery storage, and programming your system’s charge/discharge schedule.

A transfer switch is an electrical device that safely switches a home’s power source between the utility grid and a backup source — such as a solar battery system or generator. It prevents the two power sources from being connected simultaneously, which could cause dangerous backfeeding to the grid.

In solar-plus-storage installations, the transfer switch is a critical safety component. When the grid goes down, the transfer switch disconnects the home from the utility and connects it to the battery/solar system. When grid power is restored, it switches back. There are two types: manual transfer switches require someone to physically flip the switch, while automatic transfer switches (ATS) detect the outage and switch over within milliseconds, often before you even notice the power was out. Most modern home battery systems (Tesla Powerwall, Enphase, etc.) include an integrated automatic transfer switch or equivalent gateway device. For whole-home backup systems, the transfer switch is installed at the main service panel. For critical-loads-only backup, it is installed between the main panel and the critical loads subpanel. Transfer switches must be rated for the amperage they handle and installed by a licensed electrician to meet NEC requirements.

A turnkey solar system is a complete, ready-to-operate installation where one company handles every aspect of the project — from system design and equipment procurement to permitting, installation, inspection, and utility interconnection. The homeowner simply “turns the key” and the system is ready to generate electricity.

Most reputable residential solar installers offer turnkey solutions as their standard service. A true turnkey experience means the homeowner does not need to separately hire designers, electricians, or deal with permitting offices and utility paperwork. The installer manages the entire process, including: conducting the site survey, designing the system, pulling building and electrical permits, procuring all equipment, performing the physical installation, scheduling inspections, and coordinating the utility interconnection for Permission to Operate. Turnkey pricing is typically quoted as a single, all-inclusive dollar-per-watt figure that covers everything. When comparing solar proposals, confirm what is included — some “turnkey” quotes may exclude costs like electrical panel upgrades, tree removal, or roof repairs that could add unexpected expenses. A good turnkey installer provides clear communication throughout the project, keeping you informed of progress at each stage.

UL 1703 and UL 61730 are safety standards for photovoltaic (PV) solar panels published by Underwriters Laboratories (UL). These standards define the testing and certification requirements that solar panels must pass before they can be sold and installed in the United States.

UL 1703 was the original US safety standard for flat-plate PV modules, covering electrical safety, fire resistance, and mechanical stress testing. It has been largely replaced by UL 61730, which harmonizes with the international IEC 61730 standard while adding US-specific requirements. UL 61730 includes two parts: Part 1 covers construction requirements (materials, spacing, labeling) and Part 2 covers safety testing (dielectric withstand, wet leakage, temperature cycling, humidity freeze, mechanical load, fire resistance, and more). As of 2023, UL 61730 is the primary standard referenced in the NEC for PV module safety certification. Panels that were certified under UL 1703 remain valid, and many carry dual certification. For homeowners, the key takeaway is simple: always ensure the panels in your solar proposal carry UL 61730 or UL 1703 certification — this is non-negotiable for a code-compliant installation.

UL listing (or UL certification) means that a product has been tested and certified by Underwriters Laboratories (UL) — an independent safety science organization — to meet specific safety standards. For solar equipment, UL listing is required by the NEC and most building codes before products can be installed in the United States.

Solar panels must be UL listed to UL 61730 (or the older UL 1703 standard), which covers safety and performance requirements for PV modules. Inverters must meet UL 1741, which includes grid-interconnection safety features. Racking, combiner boxes, disconnects, and other components also have their respective UL standards. When a product bears the UL mark, it means samples have been tested for fire, electrical shock, and mechanical hazards, and the manufacturing facility undergoes periodic inspections. Using non-UL-listed equipment is a code violation that will fail inspection and may void your insurance coverage. When reviewing solar proposals, verify that all major components (panels, inverter, racking) are UL listed — reputable equipment from established manufacturers always carries UL certification.

Voltage drop is the reduction in electrical voltage that occurs as current flows through wires over a distance. In solar installations, excessive voltage drop means energy is wasted as heat in the wiring rather than being delivered to the inverter or electrical panel.

The NEC recommends keeping voltage drop below 3% for any individual circuit and below 5% for the total system (from panels to the point of connection). Factors that affect voltage drop include wire gauge (thickness), wire length, current flow, and temperature. Longer wire runs between the solar array and the inverter require thicker (lower gauge) wire to keep voltage drop within acceptable limits. This is especially important for ground-mount systems, where the array may be 50–200 feet from the house. Excessive voltage drop reduces system efficiency, wastes energy, and can cause inverters to underperform or shut down. Your solar installer calculates voltage drop during system design and selects appropriate wire sizes accordingly. Using properly sized conductors adds a small cost but ensures your system delivers the maximum energy from your panels.

A watt (W) is the standard unit of measurement for electrical power. It represents the rate at which energy is produced or consumed at any given moment.

In solar energy, watts are used to describe the power output of individual solar panels under standard test conditions. A typical residential solar panel produces 350–450 watts. When you see a panel rated at “400W,” it means that panel can generate 400 watts of power at peak sunlight. System sizes are usually expressed in kilowatts (kW), where 1 kW equals 1,000 watts. The cost of solar is often quoted in dollars per watt ($/W) — for example, $2.75/W means a 8,000-watt (8 kW) system would cost $22,000 before incentives. Understanding watts vs. watt-hours is important: watts measure instantaneous power, while watt-hours measure total energy produced or consumed over time.

Whole-home backup is a battery storage configuration that powers your entire house — every circuit and appliance — during a grid outage, rather than only selected essential loads. It provides the same experience as having uninterrupted grid power, but supplied by your solar and battery system.

Whole-home backup requires a larger battery system and a more powerful inverter than critical-loads-only backup. While a single 13.5 kWh battery might sustain essential loads (refrigerator, lights, Wi-Fi, outlets) for 12–24 hours, powering an entire home — including air conditioning, electric water heater, dryer, oven, and EV charger — can draw 5–10 kW or more continuously, potentially draining the same battery in just 1–3 hours. True whole-home backup typically requires 2–4 batteries (27–54+ kWh) and an inverter system rated for your home’s peak demand. Some newer battery systems, like the Tesla Powerwall 3 (with up to 11.5 kW continuous output), are designed specifically for whole-home backup without a critical loads subpanel. The cost of whole-home backup ($20,000–$50,000+) is significantly higher than critical-loads backup ($12,000–$20,000), so homeowners should carefully evaluate which loads truly need backup power.

A workmanship warranty (also called an installation warranty or labor warranty) is a guarantee from the solar installer covering the quality of the physical installation work — including roof attachment, wiring, conduit, grounding, and overall system assembly. It is separate from the manufacturer’s product and performance warranties.

Workmanship warranties typically range from 2 to 25 years, depending on the installer. Most reputable companies offer at least 10 years, with many top-tier installers providing 25 years to match the lifespan of the panels. This warranty covers issues that arise from improper installation, such as: roof leaks at attachment points, loose or corroded electrical connections, improperly secured racking, wiring that doesn’t meet code, and any installation-related defects. The workmanship warranty is arguably the most important warranty to evaluate when choosing an installer because manufacturer warranties cover equipment defects but not installation errors. A strong workmanship warranty signals the installer’s confidence in their work quality. Key questions to ask: How long is the workmanship warranty? Does it cover roof leak repairs? Is the warranty backed by insurance or a bond? What happens if the installer goes out of business?