The electron-volt 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 electron-volts for very small amounts of energy, such as that carried by a single subatomic particle. To convert joules into electron-volts, divide joules by the charge of an electron, which is about 1.602×10−19 coulomb.
E.g. 5 J = 5/1.602×10−19
5 J = 3.125×10−19 eV.