BTU to watts: how many watts does an air conditioner use?

AC boxes say BTU. Generators say watts. The two look like they should convert with one simple number, and for heaters they do. For air conditioners, they don't, and that trips up a lot of people sizing a generator.

What is a BTU?

A BTU (British thermal unit) is an amount of heat. On an air conditioner, the rating is really BTU per hour: how much heat it can pull out of the room each hour. ENERGY STAR's sizing chart, for example, suggests about 8,000 BTU per hour for a 300 to 350 sq ft room.

The plain conversion: 1 W = 3.412 BTU/h

The U.S. Energy Information Administration lists 1 kilowatt-hour = 3,412 BTU. That works out to:

• 1 watt = 3.412 BTU per hour
• BTU/h ÷ 3.412 = watts

This is right for anything that turns electricity straight into heat. A 1,500 W space heater gives about 1,500 × 3.412 = 5,118 BTU/h.

Why an AC is different

An air conditioner doesn't make cold out of electricity. It uses a compressor to move heat from inside to outside. Moving heat takes far less power than making it. So a 12,000 BTU AC doesn't use 12,000 ÷ 3.412 = 3,517 W. It uses a fraction of that.

The number that links them is the AC's efficiency ratio. The Department of Energy's test method rates room ACs in BTU per watt-hour. You'll see it on labels as EER, CEER or, for mini splits and central AC, EER2.

Running watts ≈ BTU per hour ÷ EER

How many watts does a 12,000 BTU air conditioner use?

It depends on its EER. Here's the formula worked through, plus what ENERGY STAR's list shows:

AC size	At EER 10 (older)	At EER 12	ENERGY STAR room ACs (BTU ÷ CEER)
5,000 BTU	500 W	420 W	about 350–365 W
8,000 BTU	800 W	670 W	about 500–550 W
10,000 BTU	1,000 W	830 W	about 625–710 W
12,000 BTU	1,200 W	1,000 W	about 750–860 W

The last column is our math on ENERGY STAR's certified room AC list (October 2026): each model's capacity divided by its CEER, middle 80% of models. CEER is a combined, all-season figure. On a very hot day the AC can draw more than this, so plan on the higher columns for a generator.

Mini splits are often more efficient still. ENERGY STAR data puts a typical 12,000 BTU mini split at about 860 to 1,000 W at 95°F. See will a generator run a mini split.

Don't forget the start-up surge

A compressor is a motor, so it needs a burst of extra watts to start. Our calculator uses these figures:

• Window AC, 8,000 BTU: 750 running, 2,000 starting
• RV AC, 13,500 BTU: 1,500 running, 3,000 starting
• RV AC, 15,000 BTU: 2,000 running, 3,500 starting

RV rooftop units use more than a window AC of the same size. Champion's chart lists 1,500 to 2,000 running watts for a 13,500 BTU RV AC. Read why surges matter in running vs starting watts.

How to find your AC's real watts

1. Read the label on the side or back. Look for watts, or amps.
2. If it lists amps, multiply by the volts: 7 A × 115 V ≈ 800 W.
3. If it lists only BTU and EER, divide: BTU ÷ EER.
4. Then add the surge using the figures above, or a soft start if you have one.

Sizing a generator for AC

Use running watts for the AC plus everything else, then add the AC's surge if it's the biggest motor. The calculator above starts with an 8,000 BTU window AC, a fridge and lights. Our guides go deeper:

• Generator for a window AC
• Will a 2,000 watt generator run a 13,500 BTU RV AC?
• Will a generator run central air?

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.