The unit of measurement for energy is the Joule, symbol: J. The former unit kilocalorie (kcal) was replaced by Joule on adoption of the international system of units. In nuclear physics the energy values are mainly indicated in electron volt (eV). 1 eV = 1.602ยท10-19 J. The measurement of energy values in kilowatt hours (kWh) is widely used: 1 kWh = 3.6ยท106 J. Related to the energy content of coal also the coal equivalent is usual in energy supply: 1 tonne coal equivalent is equal to 1 tonne coal with a calorific value of 29.3 billion joule = 7 million kcal.