Solar Panel Calculator

You tell it how much electricity you use, either in kilowatt-hours or as a monthly bill, and it sizes a system to cover it. It turns your use into a daily figure, divides by the peak sun hours your roof gets and a real-world derate factor, and lands on a system size in kilowatts. From there it works out the panel count, a rough install cost after the federal credit, and a payback estimate. Everything runs in your browser, so nothing you enter leaves your device.

Take your daily kilowatt-hours, divide by your peak sun hours and about 0.8 for losses, and you've got the system size in kilowatts. Divide that by the panel wattage to get the count. A home using 900 kWh a month, with 4.5 sun hours and 400 watt panels, needs roughly an 8.3 kilowatt system, which is about 21 panels. Your roof space, shading, and goals shift that, so treat the number here as a solid starting point rather than a final design.

Peak sun hours are the number of hours a day your location gets full-strength sunlight, not just daylight. Sunny southwestern states see around 5.5 to 6, much of the country sits near 4 to 4.5, and cloudier northern areas run lower. It's the single biggest factor in system size, since fewer sun hours means more panels for the same output. If you don't know yours, 4.5 is a fair national starting point, and your installer can pin down the local figure.

Installed solar runs around $3 a watt before incentives as a rough US average, though it varies by state, installer, and equipment. So an 8 kilowatt system is roughly $24,000 gross, and the 30 percent federal tax credit knocks that down to about $16,800. This tool applies that credit and a default $3 a watt, but get real quotes, because prices and the exact incentive in your area can move the number a fair bit either way.

Payback is the net system cost divided by what you save on power each year. The calculator assumes the system offsets your usage, so your yearly saving is roughly your yearly bill at the rate you entered. A $16,800 system that saves about $1,800 a year pays back in around nine years. Rising power prices shorten that, while net-metering rules and maintenance can shift it, so read the payback as a ballpark, not a promise.

Yes, it's completely free, with no sign-up, and it runs right in your browser, so nothing you enter leaves your device. Type your power use, set your sun hours and panel size, and read the system size, panel count, cost, and payback in a tap. Bookmark it for comparing quotes or planning a roof, and treat the result as an estimate, since a real site visit accounts for things a calculator can't see.