Temperature affects the efficiency of solar cells in real-world conditions. The ideal temperature for panels is 25ºC (77ºF), which is the one used for STC. For crystalline solar panels, the temperature varies the efficiency of PV modules by -0.35 – 0.55%/ºC differing from the 25ºC STC. This means that solar panels at an external temperature of 35ºC (95ºF), would have an efficiency reduced by 3.5 – 5.5%.