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Running my A/C on a generator...

TollerToller Member Posts: 9
I am looking into a NG standby generator; and am seeing if I can run my A/C off it.  Not a big deal, but if it is simply done...  I would be willing turn everything off for a few hours to run the A/C by itself to cool the house off a bit; and then turn the A/C off and everything else back on. 

It is a Trane X14 2.5 ton.  The generator installer "thinks" it will work with a 10kw, but that is not very encouraging.

I emailed Trane.  They say I should talk to my local dealer or generator installer.

I emailed my dealer.  He has no idea.

I emailed Generac.  They say I have to rely on the generator installer.

An answer to a question on the Generac website says a 3 ton may run on a 8kw depending on what else is going on.  That is encouraging, but the fact it is buried in the Q&A suggests it may not be authoritative.

Any ideas?


  • Jamie HallJamie Hall Member Posts: 6,708
    Somewhere on the nameplate...

    or other data plate on the air conditioner it should say what the starting and running amps are. That is what you are looking for. Your generator has to be able to handle that much current. Most generators have circuit breakers, not slow blow fuses or motor starter type breakers, and so your breaker rating on the generator has to be able to take the starting current.

    And that rating, times the voltage (usually 240 for bigger air conditioners) will give you the minimum wattage rating you need.  And that's running, not surge, watts (generator sales people sometimes like to quote surge watts...).  So say the starting current is 30 amps.  30 amps by 240 volts is 7.2 KW.

    Then add in anything else which might be running at the same time -- lights, stoves, whatever -- and the total will be your generator size.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    I am looking at natural gas fueled standby generators too.

    I looked in Consumer Reports, and various places on the web. Also several local dealers. They all suggest Kohler generators instead of Generac. Kohler generators cost more, but from what I read, they seem to be better.

    I will probably be getting this one:

    Even if you decide to get something else, you may learn a lot that carries over from one manufacturer to another by browsing this web site.

    And my contractor has this to say on one of the pages of his web site (this is for a smaller size than the one I will be getting):

    8.5 & 12 RES Competitive Advantages: The only generator in its class powerful enough to start and run a typical 4 ton central air conditioner
  • SWEISWEI Member Posts: 7,356
    residential backup gensets

    My generator guy (who has been around forever and knows where all the bodies are buried) likes these
  • heatpro02920heatpro02920 Member Posts: 991
    Best advice I can give is

    Get a genny that can run your entire house, with out having to shut off anything... The cost from a 10K and a 20K is only around $1000, so just get one that will run the entire load and be done with it.. I have liquid cooled LP unit that I don't even have to think about, it kicks on and off and does its thing with no questions asked... and honestly the smaller non automatic unit was only about $3000 less {BUT mine is huge, runs 3 a/c units, deep freeze, commercial refrigerator, electric dryer, all the lights, ect} so with a 10K vs a 20K {which will run most houses with a couple small ac units} why not spend the extra money.... Plus I was told going oversized makes them last longer....
  • Jean-David BeyerJean-David Beyer Member Posts: 2,532
    Looks very good.

    I never heard of Gillette, even looking over the Internet. And Consumer Reports did not test them. From their specification sheet, their units seem a little better than the Kohler. My guess is that they cost somewhat more. Both the Gillette and the Kohler units are made in USA. The Generac units seem to be made in China. I am prejudiced against Chinese made stuff. In the old days, Japanese made stuff was junk, but starting in the 1960s, they had a big program to improve the reputation of their products in international markets, and they succeeded. Think Japanese cameras and, later, Japanese made cars. They did not do this by advertising and public relations, but by improvements in Quality Control. I imagine China will do this, too.But it took Japan well over 10 years to do it with cameras and even longer for cars.
  • VinnyVinny Member Posts: 46
    edited March 2013
    I have a 17Kw Generac Guardian installed on my home

    and it will handle my entire home's electrical needs, including the central a/c. The Nexus transfer switch has something called load sharing which lets you run multiple a/c's on the generator. The thing you do have to worry about when powering a central a/c unit is it's start- up amperage, which is sometimes 5 times the normal amount required to run it.  At minimum, I needed a 14Kw unit to safely handle my demand and 17Kw was an even better choice.  It's not much of a difference cost wise.  Also, the Generacs are made in Wisconsin if buying a U.S. made product is important to you.

    Adding a soft start kit to your a/c may help with reducing the required start up torque:

    Also, all the information you want to know about Generac generators can be found on this site and the guys there are incredibly helpful and friendly.  You can join and ask them for advice as there are many codes you or your installer must follow:       

    Also, in this video you can hear a central a/c unit struggle a bit on start up and that is with a 14Kw genset:

    Just something to think about in your decision making process.
  • drhvacdrhvac Member Posts: 186

    When sizing a generator, after determining all the general lighting and appliance loads and coming up with the proper KW generator to use, the final step is making sure that the generator you use can handle the starting amps or lock rotor amps of the largest motor load. 95% of the time its the AC. For example - If your calculations say you need a 12kw generator. You then check the LRA of the AC. If A 12kw can handle 63 LRA, and your AC has an LRA of 80, then you would have to go up to the next size generator that could handle 80 or above LRA.
  • jimjim Member Posts: 68
    kohler generator

    We have the Kohler whole house 14K res generator. It is 2 years old. It runs great. Switches over to standby power 10 seconds after the outage. It also runs approx. 10 min after power comes on before switching back so you have no surges or power hits. Ours runs on LP. It runs a self test every week for 15 min. Has built in battery charger and heater for the carb. Very important in cold climates. Keeps carb from icing up after running. We change the oil annually along with the filter. Kohler support is excellent. We did a lot of research and the Kohler name kept coming up as the better generator along with a 5 year warranty. I also like the plastic enclosure as you never have to worry about rust issues. Good luck in your search.
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