Solar Panel Size Calculator

Estimate the solar system size and number of panels to cover some or all of your electricity use.

About 3.5 in the cloudy north to 6+ in the desert Southwest.
Wiring, inverter, heat, dust and shading. 14% is the PVWatts default.
Find monthly kWh on your electric bill.
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Panels needed
System size
Panels
Estimated production
Roof area (approx.)
Work

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How to Size a Solar System

System kW = daily kWh ÷ peak sun hours ÷ (1 − losses)   Panels = system W ÷ panel W

Peak sun hours are the equivalent hours of full 1,000 W/m² sunlight per day, averaged over the year. NREL’s free PVWatts tool gives the figure for your address, roof tilt and direction.

Worked Example

Tips

This is a rough estimate. Real output depends on roof angle, orientation, shading and climate. Utility rules (net metering) and local permits affect how big a system makes sense.