You can sketch annual heating energy from heating degree days only if you already have a design heat-loss number, or are willing to back one out from a bill. The usual degree-day relationship is that annual conductive and infiltration heat loss, in Btu, is approximately the design heat loss (Btu/h) times 24 hours times the heating degree days, divided by the temperature difference used to compute that design loss (indoor design temperature minus outdoor design temperature). If a house has a 40,000 Btu/h design loss at a 70°F indoor and 0°F outdoor design (a 70°F difference) and the climate has 6,500 heating degree days (base 65°F, a common public reporting base), the sketch is 40,000 × 24 × 6,500 ÷ 70 ≈ 89 million Btu of heat loss over the season. That is a labeled example, not your house, and mixing a base-65 HDD series with a 70°F indoor design already introduces a known approximation.
Limitations you should not paper over:
- HDD series (from NOAA or a utility) have a stated base temperature; the formula is only as good as the match between that base and how the house is actually operated.
- Internal gains, solar gains, and setbacks make real fuel use lower than a raw heat-loss integral.
- The formula does not size equipment. Manual J (or an equivalent) is still the design-load method.
- Without a design heat loss, people sometimes reverse the formula from last year’s heating therms. That is the same idea as this site’s “compare using my gas bill” page, and it still requires you to separate space heat from water heat and to use the furnace AFUE.
Once you have a useful-heat estimate in MMBtu, the calculator can turn it into a heat-pump kWh sketch at a stated HSPF2. Do not treat the degree-day sketch as a utility-grade forecast.
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.