The two different cost questions
"What does my air conditioner cost to run?" hides two questions that need different numbers.
Cost per hour while it runs is an instantaneous question, and it needs a full-load efficiency figure. EER is exactly that: BTU of cooling delivered per watt-hour of electricity, measured at one operating point — 95 °F outdoors, 80 °F dry bulb and 67 °F wet bulb indoors. Divide capacity by EER and you have watts. A 3-ton system at EER 12 draws 36,000 ÷ 12 = 3,000 W, so it costs three kilowatt-hours of electricity for every hour of running.
Cost over a season is a different question, because equipment does not run at full load most of the time. It cycles, it runs at part load, and modern variable-capacity equipment spends most of the summer well below its rated output — which is usually more efficient, not less. SEER is a weighted average over a standard season that captures all of that. Divide seasonal BTU by SEER and you get seasonal watt-hours.
Using the wrong one is the most common mistake in cooling cost estimates. Multiplying the hourly EER-based cost by a whole season's clock hours overstates the bill badly, because it assumes the compressor never modulates and never cycles off.
Why capacity divided by the rating gives watts
Both SEER and EER are ratios with the same units: BTU of cooling out per watt-hour of electricity in. That makes the arithmetic almost trivial.
Watts = BTU/h ÷ EER and Watt-hours = BTU ÷ SEER
The 1,000 in the formulas above just converts watts to kilowatts. Nothing else is going on.
The four rating scales. In 2023 the US Department of Energy moved to a new test procedure — Appendix M1 — which raises the external static pressure the equipment is tested against, so it now has to work against a duct system more like a real one. Ratings on the new procedure carry a "2". AHRI publishes approximate conversions for split systems: SEER2 ≈ 0.95 × SEER, EER2 ≈ 0.95 × EER, and HSPF2 ≈ 0.85 × HSPF. Those are approximations for comparing across the change; only a real M1 test result is authoritative for a specific model. The SEER to EER calculator handles all four conversions on their own.
Getting EER from SEER. When you have a SEER but no EER, this page estimates one using an empirical relationship widely used in residential energy modelling: EER = −0.02 × SEER² + 1.12 × SEER. It gives 11.2 at SEER 13 and 12.8 at SEER 16, which matches the shape of real equipment data: full-load efficiency improves more slowly than seasonal efficiency, because much of a high SEER rating comes from part-load performance the EER test never sees. If your AHRI certificate lists an EER2, enter that instead — a measured number beats a correlation.
Equivalent full-load hours is the input people get wrong. It is not how many hours the thermostat calls for cooling; it is how many hours of full-capacity running would deliver the same total cooling. A system cycling for 14 hours a day at 55% duty is about 7.7 equivalent full-load hours. Using clock hours instead roughly doubles the answer.
Worked example: a 3-ton system at 15 SEER
A 3-ton (36,000 BTU/h) split system rated 15 SEER, running 8 equivalent full-load hours a day for a 120-day season, at $0.15 per kilowatt-hour all in.
- Estimate EER from SEER. −0.02 × 15² + 1.12 × 15 = −4.50 + 16.80 = 12.30.
- Power draw. 36,000 ÷ (12.30 × 1,000) = 2.927 kW.
- Cost per running hour. 2.927 × $0.15 = $0.439.
- Equivalent full-load hours. 8 × 120 = 960 h.
- Season electricity. 36,000 × 960 ÷ (15 × 1,000) = 34,560,000 ÷ 15,000 = 2,304 kWh.
- Season cost. 2,304 × $0.15 = $345.60.
- Average per day in season. $345.60 ÷ 120 = $2.88.
Notice what happens if you mistakenly multiply the hourly figure by the season hours: $0.439 × 960 = $421, which is 22% higher. The gap is the difference between EER and SEER — the part-load credit that SEER contains and EER does not.
Now upgrade the same system to 20 SEER. Season electricity becomes 34,560,000 ÷ 20,000 = 1,728 kWh and the season cost falls to $259.20 — a saving of $86.40 a year at this rate and duty. Whether that pays for the equipment is what the SEER upgrade savings calculator is for.
How much of this to believe
The arithmetic is exact. The inputs are estimates, and two of them dominate the answer.
Run hours are the biggest source of error, and they are the one input nobody actually measures. If you want a real number rather than a guess, work backwards: take your summer electricity bill, subtract a comparable shoulder-season month to remove the baseline household load, and divide the difference in kWh by the seasonal kWh this calculator predicts. Adjusting the run hours until the two agree calibrates the model to your house.
The electricity rate must be the all-in rate. A bill with a $0.09 supply charge and a $0.07 delivery charge costs $0.16 per kilowatt-hour, not $0.09. Divide the total dollars on the bill by the total kilowatt-hours and use that. If you are on a time-of-use tariff, cooling runs disproportionately in the expensive afternoon window, so the flat average understates it.
The SEER rating assumes the equipment is installed correctly. A system with airflow well below design, a duct system leaking into an attic, or a refrigerant charge 15% off nameplate does not deliver its rated efficiency, and no amount of arithmetic recovers that. This calculator tells you what correctly installed equipment costs to run.
Heat pump heating figures cover the compressor only. HSPF is a seasonal average that already includes some backup-heat operation in the standard rating region, but it cannot know your building's balance point or how your controls stage the strip heat. In a cold climate, resistance backup below the balance point can easily exceed the compressor's own consumption — the balance point calculator shows where that transition happens for your building.
Cooling energy and cost per ton, per 1,000 full-load hours
| SEER | SEER2 | EER | kW per ton | kWh per ton per 1,000 h | Cost at $0.17/kWh |
|---|---|---|---|---|---|
| 13 | 12.35 | 11.18 | 1.073 | 923 | $156.92 |
| 14 | 13.30 | 11.76 | 1.020 | 857 | $145.71 |
| 15 | 14.25 | 12.30 | 0.976 | 800 | $136.00 |
| 16 | 15.20 | 12.80 | 0.938 | 750 | $127.50 |
| 18 | 17.10 | 13.68 | 0.877 | 667 | $113.33 |
| 20 | 19.00 | 14.40 | 0.833 | 600 | $102.00 |
| 22 | 20.90 | 14.96 | 0.802 | 545 | $92.73 |
Multiply the kWh column by your tonnage and by your full-load hours in thousands. The diminishing returns are visible in the last column: going from 13 to 16 SEER saves $29 per ton per 1,000 hours; going from 18 to 22 saves $21.
Mistakes that distort a cooling cost estimate
- Using clock hours instead of full-load-equivalent hours. A thermostat calling for cooling 14 hours a day does not mean 14 hours of full capacity. This single error commonly doubles the answer.
- Applying the hourly EER cost across a whole season. That deliberately discards the part-load credit that SEER exists to capture, and overstates seasonal cost by roughly 20% on typical equipment.
- Comparing a SEER2 rating against an old SEER number. A 14.3 SEER2 unit and a 15 SEER unit are approximately the same machine. Convert before comparing.
- Using the supply rate instead of the delivered rate. Delivery, distribution and fixed per-kWh riders often add half again to the energy charge.
- Forgetting the air handler in a heat pump comparison. Blower power is inside the SEER and HSPF ratings, but if you are comparing against a gas furnace, remember the furnace's blower runs on the same electricity while the gas is on top.
- Ignoring backup heat. On a heat pump in a cold climate, electric resistance backup below the balance point can dominate the winter bill and is not in the HSPF number in any way you can apply to your own building.
Where the ratings come from
SEER2, EER2 and HSPF2 are defined by the US Department of Energy test procedure at 10 CFR Part 430, Subpart B, Appendix M1, and certified performance is published by AHRI under AHRI Standard 210/240. Since 1 January 2023 all new residential central air conditioners and heat pumps sold in the United States are rated on the M1 procedure. Regional minimum efficiencies differ: split-system air conditioners must meet 13.4 SEER2 in the north and 14.3 SEER2 in the south and south-west, with additional EER2 minimums in the hot-dry region.
What to do with the number
Running cost is one input to three different decisions, and it carries different weight in each.
Deciding whether to replace working equipment. Compare the seasonal cost at your current rating against the same calculation at the rating you are considering. The difference is your annual saving; divide the installed price difference by it for a simple payback. On modest run hours and modest rates, the payback on a large efficiency jump often runs past the equipment's service life, which is a legitimate reason to buy the mid-range unit.
Sizing. Cost scales with capacity, which makes oversizing expensive twice over — a bigger unit costs more to buy and short-cycles, which costs efficiency and comfort. Size from a load calculation, not from square footage; the air conditioner BTU calculator is the starting point and a full Manual J is the right answer.
Operating decisions. Setback saves money roughly in proportion to the reduction in degree-hours the equipment has to overcome, which is why it saves more in a mild climate than in a brutal one — see the setback savings calculator. Cleaning the condenser coil, correcting airflow and fixing duct leaks all move the real efficiency toward the rated efficiency, and they are cheap.
If you want to work in COP rather than EER — common outside the United States and standard in engineering work — the EER to COP calculator converts between them: COP is simply EER divided by 3.412.