Window and central AC wattage, start-up surge, and realistic cooling options when a heat wave and a blackout hit at the same time.
- Use the label watts and check the surge before you plug in.
- A central system needs a properly sized generator, not a portable battery.
- Fans and a cool room use far less power than air conditioning.
When the power fails during a heat wave, cooling is the thing everyone wants back. Air conditioners are also some of the hardest appliances to back up. This guide explains how much power they use, how window units differ from central systems, and what is realistic in a blackout.
Window units
A small window air conditioner draws far less than a large one, and size is usually measured in BTU per hour on the box. The running watts are printed on the label, along with the amps. Smaller units can run on a modest power source, while the largest window units draw as much as a big kitchen appliance. Always use the label rather than a rule of thumb.
Many window units list a cooling capacity and a separate electrical rating. Use the electrical rating, in watts or in amps and volts, for sizing your backup.
Central air
Central air conditioning is a much bigger load. The outdoor compressor and fan, plus the indoor blower, together draw far more than a window unit, and most systems run on 240 volts. A portable power station cannot run a central system. A properly sized generator connected through a transfer switch can, but the generator must be big enough for the start-up surge. See what size generator do I need and transfer switch vs extension cords.
Why surge matters so much
An air conditioner has a compressor, and a compressor needs a short burst of extra power to start. That burst can be two or three times the running watts. A power station can handle a window unit only if its surge rating covers that moment. Learn more in running watts vs starting watts.
Some newer units use inverter technology that starts more softly. The label or manual should say.
Runtime with a battery
Cooling a room uses a lot of energy over time. Take the running watts, multiply by hours, and compare with usable battery capacity as shown in how to calculate watt hours. Even a small window unit can empty a mid-size battery in a few hours of steady running. The unit cycles on and off with the thermostat, so real use is less than the full rating, but the answer is still usually hours and not days.
- Cool one small, well shaded room only.
- Close the door, curtains and blinds before turning it on.
- Set the temperature a little warmer than usual so it cycles off.
- Run it for a few hours at the hottest time of day, then rest.
Cooling that costs far less power
A fan uses a tiny fraction of the electricity an air conditioner does. Pair it with the habits in how to stay cool without air conditioning. Use the coolest room, usually on the lowest floor and on the shaded side of the house. Drink water regularly, take cool showers or use damp cloths, and wear light clothing. Our heat wave power outage guide covers the full plan.
What is realistic
- For a short outage, a fan and shade usually do the job.
- For a longer outage and a small room, a small window unit on a large enough battery or generator for a few hours at a time may be possible.
- For whole-home cooling, a generator with a transfer switch is the usual route, but fuel supply and noise should be planned ahead.
- If you cannot keep a safe temperature at home, go to a cooling centre, a library, a mall or a friend's place with power.
Generators must be used outdoors only, away from windows and vents. Use the outage planner to see how cooling loads compare with your other needs, and browse the power station hub for sizing ideas.
Quick answers
Can a power station run a window air conditioner?
A small unit sometimes can for a few hours if the surge and continuous ratings cover the label figures. Larger units usually cannot.
Can I run central air on a portable power station?
No. Central systems are 240 volt loads far above what portable stations supply.
What is the cheapest way to cool a room in an outage?
A battery fan, closed curtains, the coolest room in the house and plenty of water cost very little power.