Consumption of heating energy in residential buildings comprises the consumption of fuels, electricity, district heating, and ambient energy. Heating energy comprises the energy of the main heating system and those of other forms of supplementary heating, inclusive of devices associated with heating of domestic water, saunas, ventilation and heat distribution. In these statistics, supplementary heating is referred to as additional heating. Some of the electricity used by housing companies in residential properties has also been allocated to heating and ventilation systems. Consumption includes energy sources bought or otherwise acquired for housing (e.g. wood fuel produced for own use) and losses from the use of fuels (e.g. boiler losses).