Heat pumps collect heat from the air, water, or ground outside your home and concentrate it for use inside. Heat pumps operate in reverse to cool your home by collecting the heat inside your house and effectively pumping it outside. Heat pumps have both heating and cooling ratings-both in terms of capacity and efficiency. The HSPF tells you the ratio of the seasonal heating output in Btu's divided by the seasonal power consumption in Watt-hours.
Heat pump efficiency varies with outdoor temperature.
The performance of an air source heat pump in heating mode decreases with the drop in outside air temperature. In the cooling mode, a heat pump operates exactly like a central air conditioner. Federal efficiency standards require that conventional heat pumps have an HSPF rating of at least 6.8 and a SEER rating of at least 10.0. The most efficient air source heat pumps have an HSPF rating between 9.0 and 10.0 and a SEER above 14 or so.