GlenCalc

Solar panel calculator

Find your average monthly kWh on your electric bill, and your area’s peak sun hours (about 4 to 6 in most of the US).

14% is the default in NREL’s PVWatts (wiring, inverter, soiling, shading).

Your answer

System size7.65 kW
400 W panels20
Yearly production10,800 kWh
Roof area (about 20 sq ft per panel)400 sq ft

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How we worked it out

Updated as you change the numbers.

  1. Daily kWh = monthly use × 12 ÷ 365 × share covered. kW = daily kWh ÷ (sun hours × (1 − losses)). Panels = kW × 1,000 ÷ panel watts.

Method and assumptions

A 1 kW array produces about (peak sun hours × 1 kWh) a day, less system losses. NREL’s PVWatts tool models your exact location, tilt and shading and is the next step for a real quote.

Questions

How many solar panels for 900 kWh a month?

About 7.7 kW at 4.5 sun hours: 20 panels of 400 W.

Sources

Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.

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