Calorific values or heating values of a fuel express the heat obtained from one unit of the fuel. The gross calorific value (GCV), or high heat value, measures the total (maximum) amount of heat that is produced by combustion. However, part of this heat will be locked up in the latent heat of evaporation of any water present in the fuel before combustion (moisture), or generated in the combustion process. The latter comes from the combination of hydrogen present in the fuel with the oxidant oxygen (O2) present in the air to form H2O. This combination itself releases heat but this heat is partly used in the evaporation of the generated water.