If you know how many therms went to space heating and the AFUE of the furnace that used them, useful heat in MMBtu is therms × 0.1 × AFUE. A heat pump delivering that same useful heat uses about (useful MMBtu × 1,000 / HSPF2) kWh. Multiply by your electricity price and compare with what you paid for those therms. Do not use the whole gas bill if it also includes water heating or a stove.
Example arithmetic with labeled round numbers, not a typical house: 800 therms of space heat through an 80% AFUE furnace is 800 × 0.1 × 0.80 = 64 MMBtu of useful heat. An 8.1 HSPF2 heat pump would use 64 × 1,000 / 8.1 ≈ 7,901 kWh. At the 2025 U.S. average 17.23¢/kWh that is about $1,361; the furnace therms at the U.S. implied 1.48/therm would have cost about $1,185. Your rates will change both sides. HSPF2 remains a Region IV seasonal rating.
The calculator has a “I have annual heating therms” mode that performs this scale-up after you enter AFUE, HSPF2, and prices. If you cannot separate water-heating therms, say so and treat the result as a rougher estimate.
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.