Based on relativity theory, it is concluded that an equivalence relation exists between mass and energy. The energy is equal to the product of mass and the square of light velocity: E = mc2. The rest energy E0 is also the energy equivalent of a resting, i.e. immobile particle. Therefore, the rest energy of a proton for example is 938.257 MeV. The rest energy of 1 g mass is about 2.5·107 kWh.