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Thread: generator and air conditioner

  1. #1

    Default generator and air conditioner

    As the latest hurricane nears the coast, I have a question about which window air conditioner to buy.

    On most days the central hvac keeps up with the demand, as we like to keep the thermostat at 78 But if we have company over or do a lot of cooking, the central air can't handle the load. So I am thinking of buying a window unit to suppliment the central HVAC in the summer, and when hurricanes strike and the generator is needed, I can have some level of comfort. The generator runs everything in the house with no problems with the exception of the 70 amp central air which of course we do not run on the generator.

    What would be the suggested BTU of a window unit for a generator that puts out 6250 continuous watts. Since we are 2 adults, we can easily get our generator usage down to the window unit + fridge + some lights + small TV when the AC is running.

  2. #2


    one of the first steps is to figure out what your current expected load is, a gen putting out 6250 watts is only 52 amps, a typical fridge pulls anywhere from 3-15 amps, a mid range window AC can pull anywhere from 6-15 amps during a compressor cycle, add in lights, a small tv and lights can range a lot. So your first step is to figure out what your current load is, then figure out your wiggle room for the window AC

  3. #3


    It seems I am looking between 2 units. Both are LG models.

    One is 115v , 15 amp 12,000 BTU
    The second is 230v, 15amp 18,000 BTU

    Which would work better on the generator or isn't there a significant difference due to the amps?

  4. #4
    Join Date
    Dec 2003
    Kent, WA


    I would take the 240V one. The generator won't need to produce as many amps to run the 240V one leaving more margin for your other stuff. Always try to run the big motors and compressors at 240V if possible. I'd pick the smallest 240V items I could find (e.g. a 10A 240V unit over a 15A 120V unit). If the power draw or cooling capacity was equal, the 240V unit would draw half the amps of the 120V one. In your examples, the 240V AC is larger, but not twice as large. So it should draw less amps than the 120V unit. I'd look at amp nameplates on what you buy to compare the load.

    Remember that your generator is really a 240V model and has two separate legs that can generate about 25 amps. If you load down one leg with a 15A air conditioner, you need to manually balance everything else and put most of it on the other leg. A 240V 10A generator will take 10A from both legs leaving 15A on each (and the startup surge will be shorter and smaller on the 240V unit). You should then try to spread the other loads across both generator legs.

    With any AC unit on a generator, you need to understand which plugs go to which leg, and having power or amp meters on each leg would be a real plus to keep it more evenly loaded. Also, look into the surge rating of your generator. Some may not have the ability to start an airconditioner because the locked rotor startup amps are huge (50 to 100 amps). If the generator can produce for the quick startup time, the AC unit may not start at all. Ideally, the AC unit would have a nameplate amp rating of no more than half the amp capability of the generator.
    Kent, WA

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