Co-generation or CHP is the simultaneous generation of both electricity and heat from the same fuel, for useful purposes. The fuel varies greatly and can include coal, biomass, natural gas, nuclear material, the sun or the heat stored in the earth. In the South African context, cogeneration also refers to the production of electricity and useful heat from a fuel/energy source which is a co-product, by-product, waste product or residual product of some underlying industrial process. It differs from conventional generation in that it is coupled to an industrial process of the host plant.