What size generator do I need for a window AC?

A window AC can make a hot outage bearable, especially for kids, older folks or pets. The size of the AC, in BTU, tells you most of what you need to know.

Window AC watts by BTU

A window AC's compressor is a motor, so it needs a burst of extra power to start. Generator makers' charts only list a couple of sizes, so some rows below are scaled from those. We've labeled which is which.

AC size	Running watts	Starting watts	Where it comes from
5,000 BTU	450–600	900–1,200	Champion chart
6,000 BTU	about 550–720	about 1,100–1,450	Scaled from Champion's rows
8,000 BTU	about 720–960	about 1,450–2,000	Scaled; our calculator uses 750 / 2,000
10,000 BTU	900–1,200	1,800–2,400	Champion chart
10,000 BTU (older unit)	1,500	2,200	Honda chart
12,000 BTU	about 1,100–1,450	about 2,150–2,900	Scaled from Champion's 10,000 BTU row

Newer "inverter" window ACs use less. In the ENERGY STAR data set, every certified room AC has a variable-speed compressor. Their listed yearly energy use assumes 750 hours of compressor running. Divide it out and the typical model averages about 350 W at 5,000 BTU, 530 W at 8,000 BTU and 800 W at 12,000 BTU. That's an average, not a peak, but it shows how much a newer unit can save.

A worked example: 8,000 BTU AC and a fridge

• Running: 750 (AC) + 400 (fridge) = 1,150 W
• Biggest surge: the AC, 2,000 − 750 = 1,250 W
• Starting: 1,150 + 1,250 = 2,400 W
• With 20% headroom: about 1,400 running and 2,900 starting watts

A generator of about 3,000 W handles this pair with room for lights and a phone charger. The calculator above is pre-filled with it.

Quick answers by generator size

Generator	What it can usually run
2,000 W	One 5,000–6,000 BTU AC. An 8,000 BTU unit is borderline, with nothing else starting at the same time
3,000–3,500 W	One 8,000–12,000 BTU AC plus a fridge and lights
5,000 W+	Two window ACs, or one AC plus the home essentials

For a longer list, see what a 2,000 watt generator can run and what a 3,500 watt generator can run.

Will a 2,500 or 3,000 watt generator run a window AC?

• 2,500 W: yes for one 5,000–8,000 BTU unit, with a fridge if you start the AC first. A 10,000 BTU unit fits on its own.
• 3,000 W: yes for one unit up to about 12,000 BTU, plus a fridge and lights. Our 8,000 BTU plus fridge example needs about 2,400 starting watts, so there's room.
• 3,500 W: one large window AC plus the basics, or two small units that don't start at the same moment.
• 5,000 W: two window ACs, or one AC with the home essentials.

If your generator runs on propane, check its propane rating. It's usually a little lower than on gasoline.

How to find your AC's real numbers

Your AC's label beats any chart. It's usually on the side of the unit, or behind the front grille.

1. Find the amps. The label lists "rated amps" or "cooling current." Multiply by 115 or 120 to get running watts. A unit rated 6.5 A uses about 750–780 W.
2. Or use the EER or CEER. It's on the yellow EnergyGuide label. Running watts ≈ BTU ÷ EER. An 8,000 BTU unit with an EER of 11 uses about 730 W.
3. For starting watts, check the manual for "LRA" (locked rotor amps). If it's not listed, plan on about 2–2.5 times the running watts.

Tips to make a smaller generator work

• Start the AC first, before the fridge and anything else. Then add the rest one at a time.
• Pre-cool the room while the power is still steady, if you have warning.
• Use the AC's "fan only" setting for a minute before you switch to cool. The compressor then starts on its own, not together with the fan.
• Close blinds and doors. A smaller room with less sun means the compressor cycles less.
• Don't run the AC and a microwave together on a small generator.

Window AC or a mini split?

A mini split is quieter and more efficient, but many run on 240 V, which changes what you need from a generator. See will a generator run a mini split?

Plugging it in

Most window ACs up to about 12,000 BTU use a normal 120 V plug. Plug the AC straight into the generator, or use a heavy-duty outdoor cord rated for its amps. Some big window units (often 18,000 BTU and up) use a 240 V plug. Those need a generator with a matching 240 V outlet. Check the plug before you buy.

Never plug the generator into a wall outlet to power the house. That's called backfeeding, and it can electrocute a utility worker.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Don't set the generator near the window your AC sits in. Read our carbon monoxide guide.