Water heaters and boilers are residential and commercial appliances that provide a continuous supply of hot water throughout a building for conditioning or washing. The variation in names is attributed to their capacity in heating drinking water, use in the domestic or industrial sphere, and their respective energy sources. Fossil fuels are commonly used for heating water, but alternative energy, backed with fossil fuels or electricity, can also be implemented instead. Electric heat pump hot water heaters are another more efficient, but more expensive, option. Hot water for the purposes of space heating is heated in a boiler.