Understanding your result
The headline is the cost for the period you chose, with the kilowatt-hours and the daily cost in the sub-line. The stats give the power the unit draws while running, which is the figure to compare with a generator or a solar system, and the cost per hour and per day. The table adds the peak-day power at EER2 and the cost per ton-hour, which is the cleanest way to compare two systems of different sizes: a 2-ton system at SEER2 10 costs twice as much per ton-hour as a 3-ton system at SEER2 20.
The hours you enter are compressor run hours, and the duty-cycle field is there in case you only know how long the system is “on”. A system that is on for 12 hours but cycles 50% of the time runs 6 compressor hours; entering 12 hours and 50% duty gives the same answer as 6 hours and 100%. A smart thermostat’s run-time report gives the compressor hours directly.
If the result looks high, the SEER upgrade savings calculator shows what a more efficient unit would save and how long it takes to pay back, and the thermostat setback calculator shows what changing the setpoint saves without buying anything. For a heat pump or a furnace, the HVAC running cost calculator does the same arithmetic with heating ratings.
How we calculate this
SEER2 is defined as BTU of cooling delivered per watt-hour of electricity over the test season, so dividing capacity in BTU/hr by SEER2 gives watts, and dividing by 1,000 more gives kilowatts. The ratings and the conversion from the older SEER to SEER2 are set out on the efficiency ratings table; if your label shows SEER rather than SEER2, multiply by 0.95. The peak-day figure uses the typical ratio of EER2 to SEER2 of about 0.875.
Prices are entered per kilowatt-hour and are not converted in metric mode, since a visitor outside the US enters their own tariff; the currency symbol is simply dropped. The cooling capacity is shown in kilowatts as well as BTU/hr, using the factors on the unit conversions page.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Electricity price | $0.17 per kWh | US residential average, Energy Information Administration, Electric Power Monthly, 2025; replace with the all-in rate from your bill |
| SEER2 | 15 | A common mid-tier rating for new systems; the 2023 federal minimum is 13.4 (north) and 14.3 (south) |
| Compressor hours per day | 8 | Mid-range of a US cooling season; 4 to 16 depending on climate and house |
| EER2 ÷ SEER2 | 0.875 | Typical ratio for current residential equipment (AHRI directory listings) |
| SEER to SEER2 | × 0.95 | DOE 2023 test procedure change (M1), about 4.5 to 5% lower |
Assumptions last reviewed October 7, 2026.
The calculator does not model the fan’s electricity when it runs without the compressor, the degradation of efficiency from a dirty coil or low refrigerant, or time-of-use tariffs that charge more in the afternoon when the unit runs most. It also treats SEER2 as fixed, when a unit’s real seasonal efficiency depends on the climate it runs in; a system in Phoenix runs more of its hours at high outdoor temperatures and achieves less than its SEER2. The guide to SEER2, HSPF2, AFUE and COP explains the ratings, and the guide to degree days is the way to estimate run hours from your climate.
Two worked examples
A typical new system in a mixed climate
A 3-ton system at SEER2 15, 8 compressor hours a day for 30 days, electricity at $0.17 per kWh.
- Power: 36,000 ÷ 15,000 = 2.4 kW
- Energy per day: 2.4 × 8 = 19.2 kWh
- Cost per day: 19.2 × $0.17 = $3.26
- Cost for 30 days: $97.92 (576 kWh)
- Peak-day power: 36,000 ÷ (0.875 × 15,000) = 2.74 kW
About $100 a month is a typical July bill addition for a mid-sized house in St. Louis or Washington. On a 95 °F afternoon the unit draws 2.74 kW rather than 2.4, so a week of heat is about 14% more expensive per hour than the month’s average.
An old unit in a hot climate
A 2-ton system at SEER2 10 (a 20-year-old unit), 12 compressor hours a day for a 120-day season, electricity at $0.22 per kWh.
- Power: 24,000 ÷ 10,000 = 2.4 kW
- Energy per day: 2.4 × 12 = 28.8 kWh
- Cost per day: 28.8 × $0.22 = $6.34
- Cost for 120 days: $760.32 (3,456 kWh)
- Cost per ton-hour: $0.264, nearly twice the first example’s $0.136
The 2-ton unit draws as much power as the 3-ton unit above while delivering two-thirds of the cooling. The note points out that a 15.2 SEER2 replacement would use 34% less electricity, about $260 a season at these hours and prices; the SEER upgrade savings calculator shows the payback.
Where to find your inputs
System size. The model number on the outdoor unit encodes the capacity in thousands of BTU/hr: 24 is 2 tons, 36 is 3 tons, 48 is 4 tons.
SEER2. On the yellow EnergyGuide label or the AHRI certificate for the matched system. For a unit installed before 2023 the label shows SEER; multiply by 0.95. If you cannot find it, 10 is a fair guess for a unit from 2000 to 2010 and 13 for 2010 to 2022.
Run hours. A smart thermostat’s history shows “cooling run time” by day. Without one, a run-time hour meter wired to the condenser contactor costs a few dollars, or estimate from the hub FAQ: 4 to 6 hours in a mild climate, 8 to 10 in a mixed one, 12 to 16 in a hot one in July.
Electricity price. Total bill divided by kilowatt-hours used, so that delivery charges and taxes are included; the energy charge alone is often half the true rate.
Common mistakes
- Entering thermostat hours as run hours. The compressor runs 50 to 70% of the time the thermostat calls on a hot day and less on a mild one. Use the duty cycle, or read the run time from the thermostat.
- Using the energy charge without delivery charges. The all-in price on the bill is what you pay.
- Mixing SEER and SEER2. A SEER 16 unit is about SEER2 15.2; using 16 understates the cost by 5%.
- Assuming the same hours every month. Run time in June is often half of July’s. Run the calculator per month for a season estimate.
- Ignoring the peak-day draw. A generator or a solar battery must handle the EER2 figure plus the compressor’s starting surge, not the seasonal average.
- Comparing systems by cost per day. Compare per ton-hour; a bigger system costs more per day because it does more.
Questions people ask
- How much electricity does a 3-ton air conditioner use?
- Divide the capacity by the SEER2 rating. A 3-ton unit is 36,000 BTU per hour; at SEER2 15 it draws 36,000 ÷ 15,000 = 2.4 kW while running, and at SEER2 10 (a system from the early 2000s) it draws 3.6 kW. Over 8 hours of compressor time a day that is 19 and 29 kWh respectively, or about $3.26 and $4.90 a day at 17 cents per kWh.
- How much does it cost to run an air conditioner for a month?
- For a 3-ton system at SEER2 15 running 8 hours a day at the US average of 17 cents per kWh, about $98 for a 30-day month. A 2-ton unit at SEER2 10 running 12 hours a day in a hot climate at 22 cents costs about $190 a month. The spread is wide because run time varies from 4 hours a day in a mild June to 16 in a Phoenix July, which is why the calculator asks for it rather than assuming it.
- How many hours a day does an air conditioner run?
- A correctly sized system runs 50 to 70% of the time on a design day (the hottest typical afternoon) and much less on an average day; across a cooling season most US systems average 6 to 10 compressor hours a day, with the Gulf Coast and desert Southwest at 12 or more. A smart thermostat's history, or a run-time meter on the condenser, gives your own figure. The compressor's hours, not the thermostat's "cool" hours, are what cost money.
- What is the difference between SEER2 and EER2?
- SEER2 is the seasonal average efficiency over a range of outdoor temperatures weighted towards mild ones; EER2 is the efficiency at a single hot condition, 95 °F outdoors. EER2 is typically 85 to 90% of SEER2, so on the hottest afternoons a SEER2 15 system runs at about EER2 13 and draws about 12% more power than the seasonal figure suggests. The calculator shows both so a peak-day bill does not surprise you.
- Does a variable-speed air conditioner cost less to run?
- Yes, for two reasons. Its SEER2 is higher, often 18 to 22, so it uses less electricity for the same cooling, and it runs long and slow at part load, which keeps the house more evenly cool and removes more moisture. Enter its SEER2 and the longer run hours; the cost comes out lower even though the unit runs more of the day, because it draws a fraction of its full power most of the time.
- Can I lower the cost without replacing the system?
- The cheapest savings are in run time. A thermostat setback saves about 3% per degree Fahrenheit for an 8-hour setback according to the Department of Energy for cooling, shading west windows cuts the load, and a clean filter and coil keep the SEER2 you paid for. Sealing ducts that leak into the attic can cut run time by 20%. The thermostat setback and duct leakage calculators put numbers on each.