Combined heat and power (CHP) is the simultaneous generation of heat and power (usually electricity) in a single process. In a standard power plant a huge amount of energy is lost as waste heat. CHP plants harness this 'waste' heat for a useful purpose (heating a block of flats for example). As a result CHP plants can be up to 95% efficient, almost twice as efficient as gas or coal fired power stations. The CHP process is therefore both green and historically Climate Change Levy exempt.