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Solar Panel Calculator
Estimate the solar system size (kW), number of panels and roof area needed to cover your electricity use, from your monthly kWh and local sun hours.
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How to use it
- Enter your average monthly kWh (from 12 months of bills, if possible).
- Peak sun hours: roughly 3-4 in cloudy northern regions, 4-5 in much of the US, 5.5-6.5 in the Southwest.
- Losses of 14-20% cover inverter, wiring, temperature and soiling.
How it works
System kW = (monthly kWh × share ÷ 30.4) ÷ (sun hours × (1 − losses)). Panels = system watts ÷ panel watts. A 400 W panel is about 21 ft² (≈1.9 m²). For a site-specific estimate, NREL's free PVWatts tool uses local weather data.
Note: the US federal 30% residential clean energy credit (Section 25D) does not apply to systems with expenditures after Dec 31, 2025. State and utility incentives may still exist.
FAQ
How many solar panels do I need for 1,000 kWh a month?
With 4.5 sun hours and 20% losses, about 9.1 kW - roughly 23 panels of 400 W.
How much roof space do solar panels need?
About 20-22 ft² per 400 W panel plus spacing and setbacks.
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