Solar Panel Calculator

It depends on your usage and your sun. A US home using 900 kWh a month with 4.5 peak sun hours needs about an 8.4 kW system — 21 standard 400W panels, covering about 378 sq ft of roof. Enter your own bill in the calculator above for your exact number.

Divide your daily usage by your peak sun hours and by a system derate of 0.8 (for real-world losses): system kW = kWh per day ÷ sun hours ÷ 0.8. Then panel count = system kW × 1000 ÷ panel wattage, rounded up. For example, 30 kWh per day ÷ 4.5 sun hours ÷ 0.8 = 8.33 kW, which is 21 panels at 400W.

At a typical installed cost of $2.90 per watt, an 8.4 kW system costs about $24,360 before any local incentive. Costs vary by state, roof type and installer — get three local quotes. Note: the US 30% federal residential solar credit ended on 31 December 2025, so these figures use post-credit maths.

For the 900 kWh a month example above, estimated savings are about $1,856 a year at $0.17 per kWh, giving a simple payback of about 13 years — shown as a range of roughly 11 to 16 years, because costs and electricity prices vary. Payback is shorter where electricity is expensive and sun is strong, and longer where power is cheap.

Allow about 18 sq ft per panel including spacing. A 21-panel system needs about 378 sq ft of usable, unshaded roof — roughly a 19 ft by 20 ft area. South-facing roof with no shade is best in the northern hemisphere.

No. The 30% federal residential solar credit ended on 31 December 2025. This calculator uses post-credit maths throughout. Some states and utilities still offer their own incentives — check current local rules before you buy.