A window air conditioner is the simplest way to cool one room: no ductwork, no outdoor unit to mount, and the hot side of the machine sits outside. Getting the size right matters more than any feature, and ENERGY STAR publishes a chart for it.
Size a window unit with ENERGY STAR's chart: about 6,000 BTU per hour for a 150 to 250 sq ft room, 10,000 for 400 to 450 sq ft and 12,000 for 450 to 550 sq ft. Add 10% for a very sunny room, take 10% off for a heavily shaded one, add 600 BTU for each regular occupant beyond two and 4,000 for a kitchen.
BTU Calculator
Estimated cooling capacity for any room.
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Sizing chart
ENERGY STAR's room air conditioner sizing chart:
| Room area | Cooling capacity |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/hr |
| 150 to 250 sq ft | 6,000 BTU/hr |
| 250 to 300 sq ft | 7,000 BTU/hr |
| 300 to 350 sq ft | 8,000 BTU/hr |
| 350 to 400 sq ft | 9,000 BTU/hr |
| 400 to 450 sq ft | 10,000 BTU/hr |
| 450 to 550 sq ft | 12,000 BTU/hr |
| 550 to 700 sq ft | 14,000 BTU/hr |
| 700 to 1,000 sq ft | 18,000 BTU/hr |
A unit that's too big cools the room quickly but shuts off before removing much humidity, leaving it cold and clammy; one too small runs constantly. Our BTU calculator applies the adjustments for you.
Efficiency: CEER
Window units are rated by their combined energy efficiency ratio (CEER), from a federal test that counts the power used while running and in standby. A higher CEER means more cooling per watt.
CEER is the cooling output in BTU per hour divided by the total watts drawn, so you can rearrange it to find the draw: watts = BTU ÷ CEER. A 10,000 BTU unit rated at CEER 12 pulls 10,000 ÷ 12, about 833 watts.
Compare CEER only between units of the same BTU size, since a bigger unit draws more watts even at the same efficiency. ENERGY STAR certifies models that beat the federal minimum. Our EER and CEER chart shows how the ratio maps to watts across sizes.
Inverter window units run their compressor at variable speed instead of switching on and off, which is usually quieter and more efficient at part load.
What it costs to run
The running cost follows one formula: watts × hours ÷ 1,000 × your rate. A unit drawing 700 watts for 8 hours a day uses 700 × 8 × 30 ÷ 1,000 = 168 kWh a month, about $30 at 18 cents per kWh.
To see how size changes that, suppose a unit rated at CEER 11, running 8 hours a day for a 30-day month at 18 cents per kWh. Each unit's watts come from BTU ÷ 11, and a higher CEER lowers every figure below.
| Size | Watts (at CEER 11) | kWh/month | Monthly cost |
|---|---|---|---|
| 5,000 BTU | ~455 W | 109 | ~$20 |
| 8,000 BTU | ~727 W | 175 | ~$31 |
| 12,000 BTU | ~1,091 W | 262 | ~$47 |
| 18,000 BTU | ~1,636 W | 393 | ~$71 |
Read the watts off your own unit's label rather than the estimate above, and look up your state's rate in our kWh cost calculator. Running fewer hours or cycling off when the room is empty cuts the bill in direct proportion.
Installation and window fit
- Measure the opening: width between the side tracks and the height with the sash raised, and check the unit's minimum and maximum dimensions.
- Support and tilt: use the mounting bracket or support the unit as its instructions say, tilted slightly outward so condensate drains outside; see how to tilt a window AC.
- Seal the gaps around the unit with the side panels and foam strips.
- Electrical: most smaller units plug into a standard 115-volt outlet; larger ones may need a 230-volt outlet. Check the plug and the circuit before buying.
The window type decides which unit can mount at all, because a standard unit relies on a sash that closes down onto its top from above.
| Window type | How it opens | What fits |
|---|---|---|
| Double-hung | Sashes slide up and down | Standard window unit; the lower sash closes onto it |
| Single-hung | Bottom sash raises | Standard window unit, same as double-hung |
| Horizontal slider | Sashes slide side to side | Vertical or casement-style unit, or a portable with a kit |
| Casement or crank | Hinged panel swings outward | Portable with a window kit, or a specialty vertical unit |
| U-shaped or saddle | Mounts in a double- or single-hung opening | U-shaped unit; the sash closes around its waist |
A slider or crank window has no sash that drops onto the unit, so a standard box has nothing to brace it. See portable vs. window AC for the portable route.
Noise
The compressor sits outside the window, which makes window units quieter in the room than portable units. Units with a U-shaped body that straddles the sash, and inverter compressors, are quieter still.
Maintenance
Clean or replace the filter every few weeks in cooling season, keep the outdoor coil fins clear, and check that condensate drains outside. A clogged filter forces the unit to run longer for the same cooling, so this is the single easiest efficiency step.
Lifespan and winterizing
A room air conditioner typically lasts 10 to 15 years, according to the U.S. Department of Energy's Home Cooling guide. Central systems last longer, about 15 to 20 years, because they run at lower stress and are serviced more often.
For winter you have two choices. Remove the unit and store it indoors, which fully closes the window. Otherwise leave it in place, fit an insulated cover over the outdoor side, and seal the cabinet and the gaps around it, since an uncovered unit is a direct path for cold drafts.
Frequently asked questions
What size window air conditioner do I need?
Use ENERGY STAR's chart: about 6,000 BTU per hour for 150 to 250 sq ft, 8,000 for 300 to 350 and 12,000 for 450 to 550, adjusted for sun, occupants and kitchens.
How much does it cost to run a window air conditioner?
A 700-watt unit running 8 hours a day costs about $30 a month at 18 cents per kWh.
Can a window AC go in a casement window?
Usually not. Casement and many vertical sliding windows need a portable unit with a matching window kit.
Are window air conditioners efficient?
Yes, compared with portable units, because their hot side is outdoors. Compare CEER ratings, and look for ENERGY STAR certification.
Do window air conditioners use a lot of electricity?
Not for cooling one room: a unit draws from a few hundred watts up to just over a kilowatt depending on size. At 18 cents per kWh and 8 hours a day, a 5,000 BTU unit runs around $20 a month and an 18,000 BTU unit around $71. Larger sizes and longer runtimes raise that proportionally.
What is a good CEER rating?
CEER is cooling output per watt, including standby power, so a higher number is better. Compare it only between units of the same BTU size, since a larger unit draws more watts at any efficiency. Look for ENERGY STAR certification, which sits above the federal minimum.
Can I leave a window AC in all winter?
You can, but an uncovered unit is a direct path for cold air and drafts. Either remove it and store it indoors, or fit an insulated cover over the outdoor side and seal the gaps around the cabinet.
Do window ACs need to be tilted?
Most standard units sit tilted slightly toward the outside so condensate drains outdoors rather than pooling inside. Follow the unit's instructions, since some models are designed to sit level and manage condensate internally. See how to tilt a window AC.