Solar system size — methodology

What this calculator does

It converts a home's annual electricity use into a solar system: the size in kilowatts, the number of panels, the roof area required, the expected annual production and a rough installed-cost benchmark. You choose what share of your usage to cover; everything else follows from peak sun hours, a loss factor and your panel wattage.

The formulas

System size and panel count

target_kwh         = annual_consumption_kwh * (offset_pct / 100)
annual_kwh_per_kw  = peak_sun_hours_per_day * 365 * derate
required_kw        = target_kwh / annual_kwh_per_kw
panel_count        = ceil(required_kw * 1000 / panel_watts)
actual_kw          = panel_count * panel_watts / 1000

The panel count is rounded up to a whole panel, so the installed size can be a little larger than the ideal required_kw.

Roof area, production, cost and offset

roof_area_sqft         = panel_count * area_per_panel_sqft
annual_production_kwh   = actual_kw * annual_kwh_per_kw
est_gross_cost          = actual_kw * 1000 * cost_per_watt
offset_achieved         = annual_production_kwh / annual_consumption_kwh

We guard every division: if peak sun hours, the derate or the panel wattage is zero, the dependent results return “no result” rather than infinity.

Default values

Every default below is editable in the calculator and shown with its source in the calculator’s assumptions table. Peak sun hours is location-dependent and the cost is a broad benchmark — replace both with figures for your situation. Annual electricity use, panel wattage and the per-panel area are user inputs with sensible starting values (10,800 kWh, 400 W and 21 sq ft).

How we keep it honest

The calculation logic lives in a small, pure function that is unit-tested against normal, edge and invalid inputs (for example, zero peak sun hours returns “no result” rather than infinity). If you spot an error, tell us and we’ll fix it.