Heating Season Performance Factor (HSPF)
The Heating Season Performance Factor, or HSPF, can be thought of as the "average COP" for an entire heating season. HSPF is calculated by taking the total annual heating requirements, including all energy inputs (defrost and back-up heating energy included) divided by the total electric power used.
The industry standard rating system compares BTUs of heat output to watts of electrical energy input. Since there are 3.4 BTUs per watt of electricity, HSPFs of 8 or 9 are considered good, and the higher the better the efficiency.