HSPF (Heating Seasonal Performance Factor)
HSPF is an abbreviation for Heating Seasonal Performance Factor. It is the most commonly used measure of the heating efficiency of heat pumps. (The cooling efficiency of a heat pump is measured by its SEER.)
The important takeaway from this definition is that the HSPF is a seasonal measure (meaning that it takes into account the fact that the heat pumps rarely for as long as is optimal during Spring and Fall). A heat pump with a high HSPF is more efficient than a heat pump with a low HSPF.
New heat pumps manufactured after Jan 1, 2015 are required to have an HSPF of at least 8.2. The most efficient heat pumps have an HSPF of 13.