Define net energy or useful energy with an example.

The difference between the output energy and the input energy is usually referred to as the net energy or useful energy. For example, when we use fuel, say coal or wood or kerosene, we have to provide some initial heat to ignite it, which in this case, is the input. The difference between the heat given off by the burning fuel i.e. the output and the input, will be the useful energy in this case.