Solar Panel Calculator

It depends on three things: how much power you use, how sunny your location is, and the wattage of the panels you install. A US average home using 880 kWh per month with 4.5 peak sun hours per day and 400 W panels needs about a 8 kW system, which is roughly 21 panels covering 440 sq ft of roof. In a sunnier state like Arizona at 6.5 sun hours, the same home only needs about 14 panels.

Peak sun hours are the number of hours per day when solar irradiance averages 1,000 W per square meter. They are not the same as daylight hours. A location with 12 hours of daylight might only have 4-6 peak sun hours. NREL averages range from about 3.5 in Washington state to 6.5 in Arizona. Higher peak sun hours mean each panel produces more energy, so you need fewer panels for the same offset.

The residential Section 25D credit is not available for expenditures made after December 31, 2025 under current federal law. For a new 2026 project, set the calculator's federal credit input to 0 unless a tax professional confirms that a transition rule applies. State, local, and utility incentives may still be available.

Real solar systems lose energy between the panels and your meter. Inverter conversion loses 2-4%, DC and AC wiring lose 1-3%, panel temperature derating in summer can lose 5-10%, soiling and shading add a few more percent. The calculator uses an 80% default system efficiency, which is conservative for most rooftop installations. If you have a very clean roof with high-quality microinverters you might hit 85%, but heading below 75% is more realistic if you have any shading.

Simple payback is net project cost divided by estimated annual savings. Use current quotes, utility rates, export compensation, maintenance, degradation, financing, and incentives. For a new 2026 residential project, do not include the expired federal Section 25D credit unless a tax professional confirms eligibility.