Solar Panel Size Calculator (from Your Bill)

Your electric bill is the honest starting point for sizing solar, because it already reflects everything you actually run. The calculator takes your average monthly use in kilowatt-hours, multiplies by twelve for a yearly total, and divides by 365 to get an average day. If you only know the dollar amount, enter the bill and the price you pay per kWh and it backs out the kilowatt-hours first.

From there it uses the standard sizing identity: system size in kW equals daily kWh divided by your local peak sun-hours times a derate factor. Peak sun-hours is the number of hours per day your location gets the equivalent of full 1 kW/m² sun — roughly `3.5` in the cloudy North to `6` in the desert Southwest, from NREL's solar resource data. The derate (default `0.8`) knocks off about 20% for inverter, wiring, temperature, and soiling losses, so the number reflects real output rather than lab nameplate.

The result is a planning figure, not a quote. Roof orientation, shading, panel efficiency, and how much of your usage you want to offset all move it. Treat it as the ballpark you carry into an installer conversation.