Annual Heating Energy and Fuel Cost from Degree-Days
The annual heating bill an upgrade is measured against.
Example
You enter
- Heat-loss coefficient UA (Btu/h-F) 500
- Heating degree-days (base 65 F) 5000
- System efficiency (AFUE or COP; blank = 1.00 electric, 0.80 fuel) 0.8
- Fuel gas
- Fuel unit price ($/unit, optional) 1.2
You get
- Delivered heating energy 60 MMBtu/yr
- Fuel use 750
- Annual cost 900
Checked against published worked examples from: University of Washington (ARCH 331/431).
Details, formula, and sources
ScopeQ = 24 x HDD x UA turns the building UA and the climate's heating degree-days into delivered energy, then fuel = Q/efficiency and cost = fuel x price. A UA 500 house in a 5,000-HDD climate uses 60 MMBtu; at 80% gas that is 750 therms (~$900/yr), or at COP 3.0 electric 5,862 kWh (~$879) - the comparison a fuel-switching decision turns on. Energy scales directly with UA, so a 20% envelope cut is a 20% lower bill. Base-65 steady-state, no gains. An estimate, not a calibrated model.
Formula or tableQ = 24 x HDD x UA; fuel = Q/eff; units = fuel / {therm 1e5, oil 138,500, kWh 3,412}; cost = units x price.
Source and editionThe degree-day annual-heating-energy method Q = 24 x HDD x UA (base-65 degF) with the fuel and cost conversions, as compiled in the ASHRAE degree-day / RESNET references, by name.
Free accessThe degree-day method and the fuel-energy conversions are public building-science results.
What governsEstimate. AHJ and licensed professional govern.
API field namesua_btuhf, hdd, eff, fuel, price, q_mmbtu, fuel_units, cost
- Degree-day energy Q = 24 x HDD x UA at base-65 degF heating degree-daysASHRAE degree-day method
- Fuel and cost fuel = Q/efficiency, then unit-converted and pricedenergy-audit practice
- Steady-state ignores internal and solar gains that lower the true balance pointdegree-day limitation