Quick answer: A typical 8,000 BTU window air conditioner draws about 730 watts while its compressor runs, which costs about $0.13 an hour at the US average rate of 18.2¢ per kWh, or about $31.89 a month at 8 hours a day.
Window units are the cheapest way to cool a single room. What you pay depends on the unit’s size, its efficiency and how many hours the compressor actually runs.
Window AC running costs by size
| Unit size | Power | Hours a day | US per hour | US per day | US per month |
|---|---|---|---|---|---|
| 5,000 BTU | 450 W | 8 | $0.08 | $0.66 | $19.66 |
| 8,000 BTU | 730 W | 8 | $0.13 | $1.06 | $31.89 |
| 10,000 BTU | 910 W | 8 | $0.17 | $1.32 | $39.75 |
| 12,000 BTU | 1,090 W | 8 | $0.20 | $1.59 | $47.61 |
Window AC cost calculator
Use your unit’s watts from the label, and the hours the compressor runs rather than the hours it is switched on.
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How to find your unit’s real wattage
- Check the label on the side or in the manual. It lists watts, or amps: multiply amps by 115 volts to get watts.
- Use the efficiency rating. CEER (or the older EER) is BTU per watt. Divide the BTU rating by the CEER: 8,000 ÷ 11 ≈ 730 W. A higher CEER means a cheaper unit to run.
- Measure it with a plug-in energy meter over a full day to capture how often the compressor cycles.
How to keep the cost down
- Pick the right size. ENERGY STAR sizing works out at roughly 20 BTU per square foot: about 8,000 BTU for 300 to 350 sq ft. An oversized unit cycles on and off and removes less humidity.
- Set it to 78°F when you are home, the setting the US Department of Energy suggests for balancing comfort and cost, and higher when you are out.
- Seal the gaps around the unit with the side panels and foam strips.
- Close blinds on sunny windows and use a ceiling fan so you feel cooler at a higher setting.
- Clean the filter every few weeks during the season.
Frequently asked questions
Is a window AC cheaper than central air?
To cool one or two rooms, yes. A 3-ton central system draws around 3,300 W, several times a window unit. See what central AC costs to run.
How many amps does a window AC use?
An 8,000 BTU unit at about 730 W draws around 6.3 amps at 115 volts. Large 12,000 BTU-plus units may need a dedicated circuit.
Does fan-only mode cost much?
No. The fan alone typically draws 50 to 100 W.
A real example: cooling a bedroom at night
Running an 8,000 BTU window unit in a bedroom for 8 hours a night in a hot month, with the compressor actually running about 60% of the time, uses about 105 kWh: roughly $19.13 a month. Set to a slightly warmer temperature, say 78°F instead of 72°F, it would run less often and could cost noticeably less.
Cooling one bedroom at night with a window unit, while letting the rest of the house warm up, can cost far less than running central air for the whole house.
Sizing your window AC
- Up to about 250 sq ft: 6,000 BTU
- 250 to 350 sq ft: 8,000 BTU
- 350 to 450 sq ft: 10,000 BTU
- 450 to 550 sq ft: 12,000 BTU
An oversized unit cools the room quickly but switches on and off often and removes less humidity, so the room can feel clammy. An undersized unit runs constantly.
More questions readers ask
Should I turn off the window AC when I leave?
Yes, for a few hours or more. A small room cools down quickly when you return, so leaving it running in an empty room wastes energy.
Are inverter window units worth it?
Inverter models vary their speed instead of switching fully on and off, which is usually quieter and more efficient, especially for long run times.
Sources
- ENERGY STAR room air conditioner sizing guidance.
- U.S. Department of Energy, Energy Saver guidance on thermostat settings.
- U.S. Energy Information Administration, Short-Term Energy Outlook.
- Our methodology.