The first thing to consider is the sort of fuel supply. The decision between natural gas and propane as a fuel source relies largely on the location of the property. Most city-connected households utilize natural gas to power space heating and other appliances, such as backup generators. Homes that are not connected to a gas supplu network utilize propane to power their standby generators. Propane is used because extending natural gas lines to existing residences is prohibitively expensive, and liquid propane may be kept on the sire in a tank.