How it works
An air conditioner's cooling capacity is rated in BTU per hour, and its efficiency rating (EER, CEER or SEER2) tells you how many BTU of cooling it delivers for each watt of electricity. Dividing one by the other gives the power it draws while the compressor runs.
On a hot afternoon the compressor may run almost non-stop. On a milder day the thermostat switches it off once the room is cool, so it might run half to three-quarters of the time. The calculator multiplies by that share, then by your hours, days and price per kWh.
Use EER or CEER for window and portable units and SEER2 for central systems and mini-splits. SEER2 is a seasonal average, so it gives a fair monthly estimate rather than the draw on the hottest hour.
The formula
watts = BTU per hour ÷ efficiency rating kWh per hour = watts ÷ 1,000 × share of time the compressor runs cost per hour = kWh per hour × price per kWh cost per month = cost per hour × hours a day × days a month
Worked example
A 10,000 BTU window unit with an EER of 12, running flat out for 8 hours a day at the US average of 18.2¢ per kWh.
| Step | Working | Result |
|---|---|---|
| Power draw | 10,000 ÷ 12 | 833 W |
| Per hour | 0.833 kWh × $0.182 | $0.15 |
| Per day | $0.1517 × 8 h | $1.21 |
| Per month | × 30 days | $36.40 |
| Four-month summer | × 4 | $145.60 |
A 3-ton (36,000 BTU) central system rated SEER2 14.3 draws about 2,517 W. Running three-quarters of the time for 8 hours a day, it costs about $82 a month at the same price.
Frequently asked questions
How much does it cost to run an AC for an hour?
A 10,000 BTU window unit draws about 833 W, which costs about 15¢ an hour at 18.2¢ per kWh. A 3-ton central system costs roughly 46¢ for every hour its compressor runs.
How much does it cost to run AC all day?
A window unit running non-stop for 24 hours costs about $3.64 at the US average price. In practice the thermostat cycles it off, so the real figure is usually lower.
Are portable air conditioners expensive to run?
They cost more than a window unit for the same cooling, because their efficiency rating is lower (often around 9 rather than 12) and the exhaust hose leaks heat back into the room.
Is it cheaper to leave the AC on all day or turn it off?
For most homes it is cheaper to raise the thermostat or switch off while you are out. A smart thermostat that starts cooling shortly before you get home gives you the saving without coming back to a hot house.
What do EER, CEER and SEER2 mean?
All three are BTU of cooling per watt-hour of electricity. EER is measured at one hot condition, CEER adds standby power for room units, and SEER2 averages a whole cooling season for central and mini-split systems. Higher is cheaper to run.
Sources
- Default prices: US average residential price, EIA 2026 forecast (checked Oct 2026); Ofgem price cap unit rate, 1 Oct – 31 Dec 2026, Direct Debit (VAT on electricity is 0% until 31 Mar 2027).
- ENERGY STAR, room air conditioners (CEER ratings).
- US Department of Energy, central air conditioning (SEER2 minimums).
Estimates are for planning only and are not financial advice. Get written quotes before you buy.