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 federal Residential Clean Energy Credit (also called the Investment Tax Credit or ITC) lets you deduct 30% of the cost of a solar system from your federal income taxes. It applies to systems installed through 2032, then steps down. It covers the panels, inverter, wiring, mounting hardware, and labor. The credit reduces your tax bill dollar-for-dollar, so a $20,000 system gives you a $6,000 credit if you have enough tax liability to claim it.

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 cost divided by annual savings. For the US average example, a $20,100 system minus a $6,030 federal credit is $14,070 net. At 10,560 kWh per year and a $0.16 per kWh rate, you save about $1,690 per year, giving roughly an 8.3 year payback. Real payback is usually faster because electricity rates rise 2-5% per year, and slower if you finance the system.