Efficiency in the application of electricity, by the end-user, continued use of nuclear power and support for renewable energy, are also important components in the Government's Strategy. The generation efficiency of a fossil-fired power station is normally quoted in terms of its thermal efficiency. This is the efficiency with which the heat energy available in the fuel consumed by the station is converted into electricity. The net calorific value is the total heat produced in burning the fuel less that needed to evaporate the water present in the fuel or produced during combustion. However, no machine is perfect.
All machines lose some energy through heat loss, friction and leakage.
It is the presence of such inefficiencies in even the best engineered systems that prevents us from building perpetual motion machines. They also reduce the attainable thermal efficiency of a power plant to a level somewhat below the theoretical maximum. Overall power station efficiency is further limited in practice by the energy requirements of equipment such as the coal mills, air heaters and cooling water pumps, which are considerable, and this electrical load reduces the amount of electricity available to be sold into the grid system. For example, a 4 x 500 MW coal-fired station requires over 65 MW of power to run its major equipment and lesser items such as lighting. North Tees was over twice as efficient as the nearby 1903 Carville station in County Durham, using under half the fuel for each unit of electricity generated.