Furnace Efficiency Calculator — AFUE to Annual Fuel Cost
Enter your furnace's AFUE rating, annual heat requirement, and local fuel price to estimate your yearly heating fuel cost.
Annual Fuel Utilization Efficiency — found on your furnace label or manual. Typical range: 56%–98.5%.
U.S. average natural gas: ~$1.10–$1.50/therm.
Typical U.S. home: 40–150 MMBtu/year depending on climate and size.
Formula
Annual Fuel Cost = (Annual Heat Load ÷ AFUE) ÷ BTU_per_unit × Fuel_Price
- Annual Heat Load — total useful heat energy your home needs each year (BTU/yr or MMBtu/yr).
- AFUE (Annual Fuel Utilization Efficiency) — fraction of fuel energy converted to useful heat (e.g., 0.80 for 80%).
- Annual Fuel Input = Annual Heat Load ÷ AFUE — accounts for combustion losses up the flue.
- Annual Fuel Units = Annual Fuel Input (BTU) ÷ BTU content per unit of fuel.
- Annual Cost = Annual Fuel Units × Price per unit.
Degree-Day Heat Load Method:
Annual Heat Load (BTU/yr) = UA × HDD × 24
- UA — overall building heat loss coefficient (BTU/hr·°F); sum of all envelope conductances.
- HDD — annual Heating Degree Days (base 65°F); measures heating season severity.
- 24 — converts degree-days (°F·day) to degree-hours (°F·hr) to match BTU/hr·°F units.
Fuel BTU Content (HHV): Natural Gas ≈ 100,000 BTU/therm | Propane ≈ 91,452 BTU/gallon | Heating Oil ≈ 138,500 BTU/gallon | Electricity ≈ 3,412 BTU/kWh.
Assumptions & References
- AFUE is defined by ANSI/ASHRAE Standard 103 and regulated by the U.S. Department of Energy (DOE). Minimum federal standard for gas furnaces is 80% AFUE (78% in some regions); high-efficiency units reach 95%–98.5%.
- Fuel BTU values are higher heating values (HHV) per EIA (U.S. Energy Information Administration).
- Heating Degree Day data sourced from NOAA Climate Data Online or DegreeDays.net (base 65°F standard).
- The UA × HDD × 24 method is a standard simplified steady-state heat loss model per ASHRAE Fundamentals Handbook.
- This calculator estimates fuel cost only; it does not account for distribution losses (duct leakage), thermostat setbacks, internal gains, or solar gains, which can reduce actual fuel use by 10%–30%.
- Electricity AFUE applies to electric resistance furnaces (AFUE ≈ 100%). Heat pumps use COP/HSPF ratings instead and are not modeled here.
- Fuel prices are national averages and vary significantly by region and season. Check current prices at EIA.gov.
- Results are estimates for budgeting and comparison purposes. Actual costs depend on thermostat settings, home air-sealing, occupant behavior, and utility rate structures.