They measure different things in different units. AFUE (annual fuel utilization efficiency) is the share of the fuel’s energy that becomes useful heat over a heating season — 95% AFUE means 95% of the gas energy, 5% out the flue or as other losses. HSPF2 (heating seasonal performance factor 2) is, in ENERGY STAR’s definition, the total space heating required in Region IV during the heating season, in Btu, divided by the electrical energy the heat pump uses, in watt-hours. You cannot say “8.1 is better than 95%” without prices and a conversion.
The conversions this site uses are:
furnace $ / MMBtu = (gas $ per therm × 10) / AFUE heat-pump $ / MMBtu = (1,000 / HSPF2) × ($ per kWh) implied seasonal COP ≈ HSPF2 / 3.412
An 8.1 HSPF2 is an implied seasonal COP of about 2.37. That is a Region IV test number from 10 CFR 430 Appendix M1, not a Duluth or Buffalo rating. Older “HSPF” (without the 2) used a different test and should not be mixed with HSPF2. Put both ratings into the calculator with real prices.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.