Solar Array Sizing Calculator

About this calculator

Sizing a solar array starts from your target daily energy demand in kWh and works backward through two real-world loss factors: peak sun hours (the number of equivalent hours per day the sun delivers a full 1,000 W/m² of irradiance — a location-dependent value, typically 3–6 hours, that's usually much less than actual daylight hours) and a system derate factor (accounting for inverter losses, wiring resistance, panel soiling, temperature effects and mismatch, commonly around 0.75–0.80 per NREL's PVWatts methodology).

The calculator divides daily energy need by (panel wattage × peak sun hours × derate) to get the number of panels required, then multiplies panel count by wattage to report total array size in kW. This is the standard method solar installers and DIY off-grid planners use to size a system so it reliably meets — not just theoretically covers — actual daily energy demand under real irradiance and equipment losses.