How Many Watts Does a Window Air Conditioner Use?
Updated July 13, 2026
Just want the number? Use the energy cost calculator.
A window air conditioner uses roughly 450 watts for a small 5,000 BTU unit and about 1,400 to 1,500 watts for a large 15,000 BTU unit. The exact wattage depends on two things: the BTU rating printed on the box and the unit’s energy efficiency ratio. Divide BTU by efficiency and you have watts, and from watts you can calculate exactly what the unit costs per hour to run.
The formula: BTU divided by EER
Window ACs are rated in BTU per hour (the cooling output) and EER, the energy efficiency ratio. EER tells you how many BTU of cooling you get for every watt of electricity consumed. The formula is:
watts = BTU rating / EER
A 10,000 BTU unit with an EER of 10 draws 1,000 watts. The same output at EER 12 drops to about 833 watts, about 17 percent less energy for the same cooling. Older or budget units tend to land around EER 8 to 10. Energy Star certified models typically reach EER 12 or higher. The yellow EnergyGuide label on the unit carries both the BTU rating and the estimated annual energy use. The U.S. Department of Energy publishes guidance on sizing and operating room air conditioners and explains how EER is measured.
Wattage by common window AC size
The table below uses EER 10 for a standard unit and EER 12 for an efficient one. Most window ACs sold today fall somewhere in that range.
| BTU rating | EER 10 (standard) | EER 12 (efficient) | Typical room size |
|---|---|---|---|
| 5,000 BTU | about 500 W | about 417 W | small bedroom (100 to 150 sq ft) |
| 8,000 BTU | about 800 W | about 667 W | medium bedroom (200 to 300 sq ft) |
| 10,000 BTU | about 1,000 W | about 833 W | large bedroom or small living room |
| 12,000 BTU | about 1,200 W | about 1,000 W | large living room or open plan |
| 15,000 BTU | about 1,500 W | about 1,250 W | very large room |
What it costs to run per hour and per month
Electricity is billed by the kilowatt-hour. Divide the wattage by 1,000 to get kilowatts, then multiply by your electricity rate and by hours of use.
A 10,000 BTU unit at 1,000 watts running at the U.S. average residential rate of about 17 cents per kWh costs about 17 cents per hour. Run it 8 hours a day and the monthly cost is around $41. The U.S. Energy Information Administration tracks residential electricity rates by state and publishes monthly updates; rates in many northeastern and western states run well above the national average, which pushes those numbers meaningfully higher.
Why the actual bill is lower than the nameplate suggests
Window air conditioners cycle on and off rather than running at full output the entire time the unit is switched on. Once a room reaches the setpoint temperature, the compressor shuts off and the unit draws only a few watts for the fan. On a mild day in a well-insulated room, the compressor might run less than half the time, which cuts the effective average wattage well below the rated draw.
The practical upshot: the table above shows the draw when the compressor is running. Your real monthly cost depends on how many total hours the compressor actually runs, not on how many hours the unit stays switched on. Keeping the thermostat set no colder than needed and closing windows and doors while the unit runs are the two easiest ways to reduce compressor runtime.
How this compares to central AC and mini-splits
A 3-ton central air conditioner draws about 3,000 to 3,500 watts for an entire house. A 12,000 BTU window unit draws about 1,000 to 1,200 watts for a single room. If you are cooling only one or two rooms, a window unit almost always costs less per hour than running the whole central system. The trade-off shifts when you need to cool most of the house, where the central system handles far more square footage for a similar total draw. That comparison is covered in the central air conditioning running cost guide.
Mini-splits offer a third option. They are typically more efficient than window units, they avoid blocking a window, and they dehumidify more effectively because their inverter compressors run in longer, lower-intensity cycles rather than hard on and off. What size mini-split do I need for a room covers the BTU sizing math if you are weighing the switch.
Find your exact number
Take the wattage from your unit’s nameplate or EnergyGuide label, enter it in the energy cost calculator along with your electricity rate from a recent bill and the hours per day you run the unit. The calculator returns cost per hour, per day, and per month with the math shown. Adjust the hours to match your real usage and you get a seasonal estimate rather than a worst-case figure.