The use of a single prime fuel source in a reciprocating engine or gas turbine to generate electrical and thermal energy in order to optimize the efficiency of the fuel used. The dominant demand for energy can be either electrical or thermal. Usually it is the latter with excess electrical energy, if any, being transmitted into the local power supply company's lines (with a reciprocal situation existing when electrical demands exceed the cogeneration plant's output). A parallel exists with total energy plants, which are typically designed for the electrical demands rather than thermal. Under the 1978 Public Utility Regulatory Policies Act (PURPA), regulated utilities are required to purchase electricity furnished by cogenerators and small power producers at rates set by regulatory bodies having jurisdiction over the utility receiving the electricity based on "full avoided cost."