☀️Solar Panel Calculator
From your monthly use (or bill), find the solar system size, panel count and roof space you'd need.
Your electricity use
You need about
8.3 kW system
21 panels · roughly 368 sq ft of roof
kWh / day
30.0
system kW
8.3
panels
21
roof sq ft
368
🛒 Shopping list
- Solar panels (400 W)21 panels
Peak sun hours range from about 3 in the cloudy Northwest to 6+ in the desert Southwest. Shade and roof direction matter a lot.
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How it works
- Daily use = monthly kWh ÷ 30 (or bill ÷ rate ÷ 30).
- System size (kW) = daily kWh ÷ peak sun hours ÷ 0.8 (the 0.8 covers real-world losses).
- Panels = system watts ÷ panel wattage, rounded up. Roof area ≈ panels × 17.5 sq ft.
Worked example
900 kWh a month, 4.5 sun hours, 400 W panels:
- 900 ÷ 30 = 30 kWh/day
- 30 ÷ 4.5 ÷ 0.8 = 8.3 kW
- 8,333 ÷ 400 = 21 panels, about 368 sq ft of roof
Questions people ask
The number of hours a day the sun delivers an average of 1,000 watts per square meter. Most of the US gets 3.5–6.
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Last updated October 8, 2026. These results are friendly estimates to help you plan — always double-check before buying or making big decisions.