The electronvolt or electron volt, symbol eV, is used to measure energy. It is defined as the amount of energy an electron gains after being accelerated by 1 volt of electricity. Joules are used often for energy measurement, but it is sometimes useful to use electronvolts for very small amounts of energy. To convert joules into electronvolts, divide joules by the charge of an electron, which is about 1.602×10−19
E.g. 5 J = 5/1.602×10−19
5 J = 3.125×1019 eV.