HSPF is specifically used to measure the efficiency of air source heat pumps.
[3] Depending on the system, an HSPF ≥ 9 can be considered high efficiency and worthy of a US Energy Tax Credit.
[4] For instance, a system which delivers an HSPF of 7.7 will transfer 2.25 times as much heat as electricity consumed over a season.
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.
The HSPF can be converted to a seasonally-averaged COP assuming a lossless compressor and no heat loss by multiplying by the heat/energy equivalence factor .29307111 W·h per BTU.