Solar Panel Array Calculator

Sizing a solar array starts from how much daily energy you want to offset and works backward through system losses to a nameplate DC capacity. This calculator divides your Daily Energy Requirement by the site's Peak Sun Hours (the NREL PVWatts figure for equivalent full-sun hours per day, which varies from roughly 3 in the Pacific Northwest to 6+ in the desert Southwest) times a combined system efficiency factor — (1 − system losses%) × inverter efficiency% — to get the theoretical array size in kW. Because panels come in fixed wattage increments, the panel count is rounded up to the next whole panel, which means the Array Size, Daily Production, and Annual Production outputs are all recalculated from that rounded panel count rather than the theoretical ideal, so your actual system will typically slightly overshoot your target.

System Losses defaults to 14%, NREL PVWatts' standard breakdown of soiling, shading, wiring, mismatch, and light-induced degradation losses; Inverter Efficiency covers the DC-to-AC conversion stage separately. Annual Production is a simple 365-day multiplication of the daily figure and doesn't account for seasonal variation in sun hours — winter production will be meaningfully lower than summer in most climates even though the annual total should still be a reasonable estimate. Required Roof Area assumes about 18 square feet per panel (roughly a 1.7 m² panel footprint), a reasonable approximation for standard 60-cell residential modules but one that should be checked against your specific panel's actual dimensions before finalizing a layout.