Solar System Size Calculator (from Annual kWh)

If you already know your yearly electricity use — the total kilowatt-hours across all twelve bills, often printed on your annual summary — you can size an array in one step. This tool skips the monthly-to-annual conversion and works straight from that number.

The math is the same sizing identity used everywhere in solar: divide annual kWh by the energy one installed kilowatt produces in a year. That per-kW figure is your peak sun-hours times 365 days times a derate factor. At `4.5` sun-hours and a `0.8` derate, one kW makes about `1,314 kWh` a year, so a home using `10,800 kWh` needs roughly `10,800 ÷ 1,314 ≈ 8.2 kW`.

Peak sun-hours come from NREL's solar resource maps and vary with where you live; the derate accounts for the inverter, wiring, heat, and dust losses that separate nameplate DC from delivered AC. Change either input and the size updates live. As with any rule-of-thumb, roof orientation and shading still need a site visit to confirm.