Sofia has an extensive district heating system based around four combined heat and power (CHP) plants and boiler stations. Virtually the entire city (900,000 households and 5,900 companies) is centrally heated, using residual heat from electricity generation (3,000 MW) and gas- and oil-fired heating furnaces; total heat capacity is 4,640 MW. The heat distribution piping network is  long and comprises 14,000 substations and 10,000 heated buildings.