These conditions correspond to a clear day with sunlight incident upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon. STC specifies a temperature of 25 ☌ and an irradiance (G) of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum. The energy conversion efficiency (η) of a solar cell is the percentage of the solar energy to which the cell is exposed that is converted into electrical energy.īy convention, solar cell efficiencies are measured under standard test conditions ( STC) unless stated otherwise. Solar cell efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight.