Wikipedia
HSPF (Heating Seasonal Performance Factor) is a term used in the heating and cooling industry. HSPF is specifically used to measure the efficiency of air source heat pumps.
The efficiency of air conditioners is often rated by the HSPF as defined by the Air Conditioning, Heating, and Refrigeration Institute in its standard 210/240 Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.
The higher the HSPF rating of a unit, the more energy efficient it is. HSPF is a ratio of BTU heat output over the heating season to watt-hours of electricity used. It has units of BTU/watt-hr.
Depending on the system, an HSPF ≥ 8 can be considered high efficiency and worthy of a US Energy Tax Credit.
The HSPF is related to the non-dimensional Coefficient of Performance for a heat pump, which measures the ratio of heat energy delivered to electrical energy supplied, independently of the units used to measure energy. The HSPF can be converted to a seasonally-averaged COP by converting both the BTU heat output and the electrical input to a common energy unit (e.g. joules). Since 1 BTU = 1055.056 J, and 1 watt-hour = 3600 J, the seasonally-averaged COP is given by:
Avg COP = Heat transferred / electrical energy supplied = (HSPF * 1055.056 J/BTU) / (3600 J/watt-hour) = 0.29307111 HSPF.
Thus, a system which delivers an HSPF of 7.7 will transfer 2.25 times as much heat as electricity consumed over a season. In Europe the term Seasonal Performance Factor ("SPF") is used to mean the same as the average COP over the heating season. Thus a system which transfers 2.25 times as much heat as the electricity consumed is said to have an SPF of 2.25. A well designed ground source heat pump installation should achieve an SPF of 3.5, or over 5 if linked to a solar-assisted thermal bank.
Example: For a heat pump delivering 120,000,000 BTU during the season, when consuming 15,000 kWh, the HSPF can be calculated as :
HSPF = 120000000 (BTU) / (1000) / 15000 (kWh)) = HSPF = 8