thermal power plants (including those that use fissile elements or burn coal, petroleum, or natural gas), and heat engines in general, do not convert all of their available energy into electricity. in most heat engines, a bit more than half is wasted as excess heat (see: second law of thermodynamics). by capturing the excess heat, chp uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 70%, compared with at most 40% for the conventional plants. this means that less fuel needs to be consumed to produce the same amount of useful energy. also, less pollution is produced for a given economic benefit.