metro atlanta primarily uses natural gas for central heating and water heaters, with the major exception of heat pumps in apartments built during and since the 1980s. this is because winters are mild, and large apartment buildings usually require little energy to heat. backup heat (also used during defrosting) is usually supplied by electric resistance heating, though some homes have hybrid heating units which use gas backup when it is cold. exurban homes may also use all-electric instead of gas, if gas mains have not been extended to an area.