Quick answer: Window air conditioner watts are 397 at 5,000 BTU, rising to 933 watts at 15,000 BTU, while the compressor is running. But it does not run all the time: the certified annual figures imply about 750 hours of compressor a year for a mid-sized unit, and it is the hours, not the watts, that land on your bill.
Unlike most entries in our appliance wattage chart, this one has a published answer for every model on sale, though it is rarely printed as watts. It is derived from a number called CEER, and once you can read it the whole category becomes legible.
We pulled all 438 certified window units to write this. One finding surprised us enough that it changes the advice at the end.
Window air conditioner watts by size, from the certified data
CEER is the Combined Energy Efficiency Ratio: cooling capacity in BTU per hour divided by the electricity it takes to produce it, in watts. Turn that around and you get the number you want. A 10,000 BTU unit with a CEER of 15.0 draws 10,000 ÷ 15.0 = 667 W while cooling.
| Cooling capacity | Models | Typical watts | Median CEER | Per hour US | An 8-hour night |
|---|---|---|---|---|---|
| 5,000–6,000 BTU | 36 | 397 W | 13.7 | 7.3¢ | $0.58 |
| 8,000–9,000 BTU | 111 | 533 W | 15.0 | 9.8¢ | $0.78 |
| 10,000–12,000 BTU | 196 | 750 W | 15.0 | 13.8¢ | $1.10 |
| 14,000–15,000 BTU | 37 | 933 W | 15.0 | 17.1¢ | $1.37 |
The spread inside each size is worth more than the size itself. Among 8,000 to 9,000 BTU units, CEER runs from 13.2 to 17.6. Scaled to exactly 8,000 BTU that is 455 W for the best and 606 W for the worst: a third more electricity, for the same cold air, for the life of the machine.
Per hour is not what you pay
An air conditioner cycles. The figures above are watts while the compressor is turning, and no unit does that continuously except in a heatwave in an undersized room.
The certified database also publishes annual consumption, and comparing the two tells you the assumption behind it. A typical 10,000 to 12,000 BTU unit is certified at 562 kWh a year against 750 W running, which works out at 750 hours of compressor a year, or roughly two hours a day averaged across all twelve months.
That is a test assumption, not your summer. If you cool one room every night from June to September the real figure will be several times higher. But it does mean the honest annual number for that unit is about $103 in the US at current prices, not the $403 you would get by assuming 750 W around the clock through a four-month summer.
The finding that changes the advice
Every single one of the 438 certified window air conditioners has a variable-speed compressor. Not most. All of them, and all 47 certified through-the-wall units as well.
This matters far beyond efficiency. A fixed-speed compressor is thrown across the line at full voltage and pulls a large inrush current, which is the reason air conditioners have a reputation for defeating backup power. A variable-speed unit ramps up under electronic control instead, in the same way the ECM furnace blower does. The violent startup that surge ratings exist for is largely gone.
Two cautions before you rely on that. Certification is not the whole market: plenty of uncertified fixed-speed units are still sold, and the one already in your window is probably older than this change. And no manufacturer here publishes a locked-rotor figure either way, so this is an argument from motor type, not from a number on a sheet.
Running one in an outage
Summer outages and air conditioning arrive together, so the arithmetic matters. A 10,000 to 12,000 BTU unit at 750 W will run for about 2.7 hours of compressor time on a 2 kWh portable power station. At a realistic sixty percent duty cycle that is closer to four and a half hours on the clock.
Compared with the other loads on this site it sits in the middle: heavier than a chest freezer, lighter than a 1,500 W space heater. And unlike the heater, there is no 90 W alternative that does the same job: cooling has no equivalent of running a gas furnace blower. A smaller unit cooling one room is the sensible outage choice; a 15,000 BTU machine is not.
Check the voltage before you plan anything. Most certified window units are 115 V, but 65 of the 438 are 230 V, and a 230 V unit will not run from an ordinary portable power station at all.
Safety. A window air conditioner is heavy, often sits in an opening above ground level, and is a load large enough to matter. Fitting or removing one is a two-person job and the manufacturer’s bracket is not optional. Never run it through an extension lead or a domestic power strip; a 230 V unit needs its own dedicated circuit installed by a licensed electrician.
If you are cooling during an outage, a generator goes outside and well away from any opening — and note that a window air conditioner is an opening into the house. Carbon monoxide from a portable generator kills quickly and cannot be smelled. In heat emergencies, checking on elderly neighbours matters more than any of the arithmetic above.
Frequently asked questions
How many watts does a window air conditioner use per hour?
While the compressor runs, about 397 W at 5,000 BTU and 933 W at 15,000 BTU. Divide the cooling capacity by the unit’s CEER to get the figure for a specific model.
What is CEER and why does it matter?
Cooling capacity divided by electrical input, including standby. It is the one number that separates two units of the same size: at 8,000 BTU, CEER 17.6 means 455 W and CEER 13.2 means 606 W for identical cooling.
Will a portable power station run a window air conditioner?
A 115 V unit, yes, for a few hours: 2 kWh gives roughly 2.7 hours of compressor time on a mid-sized model. Every certified window unit now has a variable-speed compressor, so the old startup surge problem is much reduced. A 230 V unit will not run from one at all.
How much does a window AC cost to run per month?
It depends entirely on hours. A mid-sized unit costs about 13.8 cents for each hour the compressor runs. Cooling one bedroom for eight hours a night comes to roughly $33 a month at US prices, before the compressor cycling that reduces it.
Is a bigger unit more efficient?
Per BTU delivered, slightly: median CEER is 13.7 in the smallest band and 15.0 in the rest. But an oversized unit cools the room fast, switches off, and leaves it humid, so buying larger than the room needs is not a saving.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.
