Understanding your result
The headline is the cost for the period, with the cost per hour of running and per day in the sub-line. The stats give the cost per hour, per day and per million BTU delivered. The table shows the efficiency used, the electricity per hour of running and, for a furnace, the fuel burned per hour, then the totals for the period: fuel, electricity, the heat or cooling delivered in million BTU, and the cost per million BTU.
The cost per million BTU is the number that compares systems. It does not depend on the house or the weather, only on the efficiency and the prices, so a furnace at $16.91 and a heat pump at $20.00 can be compared directly; multiply either by the season’s heat to get the season’s bill. The fuel cost per million BTU calculator ranks every fuel at once, and the heating cost by fuel calculator multiplies by a house’s season.
For air conditioners specifically, the AC running cost calculator adds the duty cycle and the peak-day draw at EER2; for the decision between a heat pump and a furnace, the heat pump versus furnace calculator adds backup heat and the break-even price.
How we calculate this
Each rating is defined as output over input, so dividing the output by the rating gives the input. SEER2 and HSPF2 are in BTU per watt-hour, so dividing by 1,000 more gives kilowatts; HSPF2 divided by 3.412 is the equivalent seasonal coefficient of performance. Fuel energy contents are the EIA figures on the fuel energy content and cost page (100,000 BTU per therm, 91,452 per gallon of propane, 138,500 per gallon of heating oil), and the ratings, minimums and conversions are on the efficiency ratings table.
In metric mode the output is shown in kilowatts and prices are not converted; the currency symbol is dropped because a visitor outside the US enters their own tariff.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Electricity price | $0.17 per kWh | US residential average, EIA Electric Power Monthly, 2025 |
| Natural gas price | $1.50 per therm | EIA residential natural gas price, 2025 |
| Propane, heating oil | $2.70, $3.70 per gallon | EIA residential prices, 2024–25 heating season |
| SEER2 / AFUE / HSPF2 | 15 / 95% / 8.5 | Common mid-tier ratings for new equipment |
| HSPF2 to COP | ÷ 3.412 | Unit conversion (BTU per Wh to W per W) |
| Furnace blower | 500 W | Typical PSC blower; variable-speed ECM blowers 100 to 300 W |
| Run time | 8 hours a day | Mid-season figure; replace with your thermostat’s history |
Assumptions last reviewed October 8, 2026.
The calculator uses one efficiency for the whole period. Real equipment is less efficient on the hottest and coldest days and more efficient in mild weather, a heat pump’s backup strips are not counted here, and an air conditioner’s indoor fan is included in its SEER2 while a furnace’s blower is not in its AFUE. The guide to SEER2, HSPF2, AFUE and COP explains what each rating covers, and the guide to comparing fuels per million BTU puts the results side by side.
Two worked examples
A gas furnace in January
76,000 BTU/hr output, 95% AFUE, natural gas at $1.50 per therm, 500 W blower, 10 hours a day for 31 days, electricity at $0.17.
- Gas: 76,000 ÷ 0.95 = 80,000 BTU/hr = 0.8 therms per hour × $1.50 = $1.20
- Blower: 0.5 kW × $0.17 = $0.085
- Per hour of running: $1.285; per day: $12.85
- For 31 days: $398.35 (248 therms, 155 kWh)
- Cost per million BTU: $16.91
The blower adds 7% to the gas cost, which a variable-speed blower at 150 W would cut to 2%. At 248 therms the month is a typical cold-climate January for a 2,000 sq ft house.
A heat pump in the same month
30,000 BTU/hr output, HSPF2 8.5, 12 hours a day for 31 days, electricity at $0.17.
- Seasonal COP: 8.5 ÷ 3.412 = 2.49
- Power: 30,000 ÷ (2.49 × 3,412) = 3.53 kW
- Per hour: $0.600; per day: $7.20
- For 31 days: $223.19 (1,313 kWh)
- Cost per million BTU: $20.00
Per million BTU the heat pump costs 18% more than the gas furnace above at these prices, and the note warns that January’s real COP is lower than the seasonal figure. With electricity at 14 cents instead of 17, it would cost $16.47 per million BTU and match gas.
Where to find your inputs
Output. For an air conditioner or heat pump, the capacity from the model number (36 = 36,000 BTU/hr). For a furnace, the output on the nameplate inside the cabinet, not the input.
Rating. SEER2 and HSPF2 on the yellow EnergyGuide label or the AHRI certificate; AFUE on the label or in the model’s specifications. Pre-2023 labels show SEER and HSPF: multiply SEER by 0.95 and HSPF by 0.85.
Run time. From a smart thermostat’s daily history, or estimate from the weather.
Prices. The all-in price per kWh and per therm from your bills, total divided by units used.
Common mistakes
- Using furnace input instead of output. The nameplate’s big number is input; the house receives the output.
- Ignoring the blower. A standard blower adds 5 to 10% to a gas furnace’s cost.
- Using the energy-only rate. Delivery and taxes are part of the price.
- Thermostat hours as run hours. Equipment cycles; use run time.
- Comparing cost per day across systems. Compare cost per million BTU.
- Trusting HSPF2 in a cold snap. Below about 20 °F a heat pump’s COP falls and strips may run.
Questions people ask
- How much does it cost to run a furnace per hour?
- Divide the output by the AFUE to get the gas burned, multiply by the price, and add the blower. A furnace delivering 76,000 BTU per hour at 95% AFUE burns 80,000 BTU, which is 0.8 therms, so $1.20 an hour at $1.50 per therm, plus about 8.5 cents for a 500 W blower at 17 cents per kWh, for $1.29 an hour of firing. Ten hours a day for a 31-day January is about $398.
- How much does a heat pump cost to run per hour?
- Output divided by the seasonal COP and by 3,412 gives the kilowatts, multiplied by the price. A heat pump delivering 30,000 BTU per hour at HSPF2 8.5 (a seasonal COP of 2.49) draws 3.53 kW, which costs 60 cents an hour at 17 cents per kWh. That is $20 per million BTU of heat; the same heat from electric baseboard costs about $50, and from a 95% gas furnace about $17 including the blower.
- How much does central air cost to run per hour?
- A 3-ton air conditioner at SEER2 15 draws about 2.4 kW while running, which is about 41 cents an hour at the US average of 17 cents per kWh. A 2-ton unit at SEER2 10 draws the same 2.4 kW for less cooling. On the hottest afternoons, when the unit runs at its EER2, the draw is about 12% higher than the seasonal figure.
- What is a good cost per million BTU?
- It is the price of a unit of delivered heat or cooling, so lower is better and it can be compared across systems. At 2025 US average prices, cooling with a SEER2 15 air conditioner costs about $11 per million BTU; heating with a 95% gas furnace about $16 to $17 with the blower; with a heat pump at HSPF2 8.5 about $20; with propane or oil about $31; with electric resistance about $50.
- Why is the run time so important?
- Because the cost scales directly with it, and it varies more than anything else. A furnace may fire 3 hours on a mild November day and 16 on a January cold snap; an air conditioner 4 hours in June and 12 in a heatwave. Smart thermostats report daily run time, which turns this calculator from an estimate into an accurate bill check. Hours set to "heat" or "cool" are not run hours.
- Does a bigger system cost more to run?
- Per hour, yes, but it runs fewer hours for the same weather, so for a correctly sized system the season costs about the same. An oversized single-stage system can cost a little more because it cycles more and spends more time in the inefficient start of each cycle; a variable-speed system costs less than either because it runs long at low power. The cost per million BTU is the fair comparison.