A ducted heat pump system would be installed along with an integrated backup heat source – either electric, oil, propane or natural gas. A ductless system could rely on any previously existing heat source as the backup.
The heat pumps are the primary heat sources, the backups would only supplement them in colder temperatures. When you choose an ENERGY STAR® rated heat pump, some ductless units can still produce heat when the outdoor temperature is as low as -27° C.
When the outside temperature dips below the balance point, then your other fuel system kicks in. The balance point is the temperature at which the amount of heating provided by the heat pump equals the amount of heat lost from your house. At this point, the heat pump capacity matches the full heating needs of your home. Below this temperature, your supplementary or backup heat would be required.