CHP is a process in which both space/water heating and electricity are produced at the same time. Traditional CHP units have been operating at an industrial / community scale since the 1970s. Now since 2000, with rising energy prices, micro CHP designed for individual buildings has become economically viable. Effectively the micro CHP unit replaces the conventional gas central heating boiler providing heat and hot water as usual, but additionally providing some of the building's electricity needs. The European Cogeneration Directive defines micro-CHP as all units with an electrical capacity of less than 50 kW.