Calculate Solar Panel Efficiency with Simplest Method To calculate the solar panel efficiency, you need to divide the Wattage per unit area (Pmax/total area in sq.m.) of the solar module by the STC Irradiance value (1000W/sq.m.). This is the simplest way to get any solar PV module’s efficiency. Solar Panel Efficiency Formula: Efficiency (%) = (Pmax ÷ Area) ÷ (1000) x 100% where, Pmax = solar panel peak power (in Watts) Area = length x width of the solar panel (in sq.m.) 1000 = Standard Test Cond