What Size Generator To Run Window AC? (5000-12000 BTU)

A prolonged power outage or a short circuit could have devastating consequences for your AC on a hot summer’s day. The window AC is your saving grace, and if it goes out, you could be in for a long night. Follow along to learn what size generator you will need to run a window AC.

It is possible to run the window AC on a generator, but it all depends on the British thermal unit (BTU). The BTUs of window AC usually vary from 5000 to 12000. Once the BTU is established, you can select a matching wattage (W) generator.

The rule of thumb is 1W for every 10 BTU. Other factors include voltage and efficiency.

Generator Size for Window AC

5000 BTU Window AC

5000 BTU ACs are a popular choice for small homes or spaces. They are on the low end of cooling power, so the generator is also smaller.

A 5000 BTU window AC can get powered by a small, 500W generator. This assumes the AC efficiency units are standard (EER Rating of 10). If the efficiency rating is low (8), a 600W generator will be ok. If the efficiency rating is high (12.5), you only need a 400W generator.

The efficiency rating gets found on the side of the AC unit.

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6000 BTU Window AC

A 6000 BTU window AC needs at least a 600W generator. At standard efficiency, the 600W can provide adequate cooling to a small space.

Starting up a 6000 BTU window AC will require more wattage than 600. However, this is known as startup wattage. It only occurs for a few seconds. 600W generators can have up to 1200W peak output. A generator working at peak output will not be efficient. It gives the user a cushion for the startup watts.

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8000 BTU Window AC

An 8000 BTU provides enough power to cool a medium-sized space. A generator with 800W of energy will power an 8000 BTU window AC. A low-efficiency AC will need roughly 1200W of energy, and a high-efficiency AC can do 700W.

The 800W generator needs a maximum output of approximately 1000W minimum to account for the startup.

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10000 BTU Window AC

A 10000 BTU window AC is on the higher end of window ACs. At standard efficiency, a 1000W generator will sufficiently power a 10000 BTU window AC. The lowest efficiency 10000 BTU AC will need a 1400W generator, and the highest efficiency can operate with an 800W generator.

The generator needs to have a maximum output of 1200W minimum.

12000 BTU Window AC

A 12000 BTU window AC is about the most powerful window AC unit used. It can sufficiently cool down an entire single-bedroom apartment. A standard-efficiency 12000 BTU window AC needs a 1200W generator.

To account for startup wattage, the 1200W generator should have a minimum max output of 1500W.

The lowest efficiency of 12000 BTU window AC needs 1700W, and the highest efficiency only needs a 1000W generator.

Quick Comparison Of Generator Sizes For Window Air Conditioners

Window AC Size BTU Generator Size (Watts)
5000 500
6000 600
8000 800
10000 1000
12000 1200

How To Calculate The Size Of The Generator For My Window AC?

You can calculate the size of the generator that corresponds with your window AC yourself. The rule of thumb, 1W per 10 BTU, works well for standard efficiency.

You also have to account for the possibility the AC unit has low or high efficiency. The lowest efficiency rating is seven, and the highest is 12.5. You can calculate the wattage needed by dividing the BTU by the efficiency rating. So, a 5000 BTU AC with an efficiency rating of 7 is 5000/7= 714.2. The generator would need to be at least 750W before accounting for maximum output.

Finally, you need to confirm the maximum output of the generator can handle the compressor starting up. The max output gets accounted for on the generator. The maximum output is at least 20% of the wattage. For the AC unit that needs a 750W generator, there needs to be a 150W buffer for the startup. 20% of 750 is 150, so (750+150) the maximum output needs to be at least 900W.

If you already have a generator, you can calculate backward. For example, if you have a 3500W generator, you can estimate that it will power a 35000 BTU standard-efficiency air conditioner. A 2000W generator can power a 20000 BTU window AC.

Costs Associated With Using A Generator To Cool A Room

It is less expensive to power the window AC on the grid than a generator. A 5000 BTU window AC costs roughly $0.065 an hour to run. A 12000 BTU window AC costs roughly $0.143 an hour to run. Factors that change the cost are hours a day, efficiency, energy costs in your area, BTU, and model of AC.

The upfront cost of a generator varies between models, power, and fuel type. For example, a 500W battery-powered generator is at least $600. The gas-powered generator is significantly cheaper. They are often half the price. However, fuel costs eventually make them more expensive.

A 5000 BTU AC on the grid costs $0.50 a day to operate.

You can calculate how much gas your generator uses by filling it up fully, running it for an hour, and checking the tank. When you top it up, note how much gas is needed. That will be the amount it costs to run an hour.

Gas-powered generators will be the more costly and hour, but less cost upfront. Battery generators are expensive but cheaper in the long run.

Cost to Using a Generator for Window AC

People Also Ask (FAQ)

How long can a generator run a window AC?

The amount of time a generator can run a window AC depends on the AC and the generator. Small generators will only fuel an AC for five or six hours. Large gas generators can fuel a window AC for days.

Should you use a generator to power an AC year-round?

No. The cost of running the AC on the generator is too great. Hook up to the grid whenever available.

Where is the best place to put a generator?

Put your generator outside if the window is low. An RV or a main floor room can plug the AC into an outside generator. Never leave a gas generator inside.

How do you safely connect a generator to a window AC?

There are a few precautions to take when setting up the generator. Never leave it inside, never overload the generator, use extension cords, and store it in a safe place.


It is possible to power a window AC with a generator. As long as you know the BTU of your AC, you can match it to a generator. First, calculate the watts to determine what size generator to run a window AC.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.