Is there any way to tell what the start watts I need to run my ac units?

Like several others I have calls in to companies about getting a whole home generator but I dont want to buy way more than I need based purely off what they want to sell me. I had Envriromax install 2 new Trane single stage systems in Dec after a lighting strike so they are very new.

One thing I noticed in the first round of initial estimates is the sales reps added the starting loads of my two a/c units together - basically assuming every high usage device in my house starts at the exact same time to try to justify why I should buy a more expensive liquid cooled generator.

The A/C units in particular should be on load management (load management of two A/C units looks like it's built into the standard generac transfer switch) so the max load from A/C would be one running and one starting.

The only other electric appliance we have is a double oven (along with one 5 and one 2 ton a/c unit). Gas stovetop, gas dryer, gas heat, gas tankless water heater. And one guy tried to convince me I need a 38kW liquid cooled generator (I'm guessing that is the cheapest they had left in stock).

Hopefully someone here has experience with whole home generators but, with the two A/C units (and maybe the double electric oven) on load management, I don't see how the 24kW air cooled wouldn't be sufficient? I don't think you could get away with starting 1 a/c while the other is running and having one of the ovens on at the same time (along with other basic items like TV, fridge, etc) with anything under the 18kW and the price difference between that and the 24kW Generac seems negligible...

howdydamnit04 said:

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Is there any way to tell what the start watts I need to run my ac units?

Like several others I have calls in to companies about getting a whole home generator but I dont want to buy way more than I need based purely off what they want to sell me. I had Envriromax install 2 new Trane single stage systems in Dec after a lighting strike so they are very new.

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I can't remember if it's on the nameplate or not. It would be listed as "LRA" or locked rotor amps.

The "watts" would remain the same. You are concern about the "vars" which is high when starting. This can be lowered by installing a capacitor. I think they call them "hard start kits" in the HVAC world.

So LRA X 220 to compare to the generator size?

LostInLA07 said:

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So LRA X 220 to compare to the generator size?

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That would give you the starting VA of the A/C. Whether or not the generator can supply it and for how long is another question. Would probably require some sort of engineering study (transient stability). Welcome to the problem ERCOT was facing last week!

I would think there are rules of thumbs out there.

I guess I'll just do what ERCOT and the generators did - try and see what happens because I don't want to pay for more than the 24kW air cooled

LostInLA07 said:

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I guess I'll just do what ERCOT and the generators did - try and see what happens because I don't want to pay for more than the 24kW air cooled

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24kW is 100 amps. I would think that's more than enough to run a 5 ton, 2 ton, and an oven. Maybe get a meter clamp and clamp your main incoming conductor. Turn on your A/Cs and oven and see how much it's drawing.

Better yet, don't they make usage monitors you can plug into your panel? I'm not sure if they can see real time current draw and/or their sample rate.

If you get the smart load center, a Generac can start one A/C and then the other after the first has started and leveled out. Not sure the size of your A/C units but that start-up spike can be almost 3x the running wattage. That's why you typically see a 50 or 60amp double for a 5 Ton A/C. Two 5 ton units turning on at the exact same time is going to get very close to your max wattage for the genset. Just make sure they don't turn on at the exact same time and you'll have more than enough juice with a 24kw generator.

Have Generac and Koehler both come out. Overselling is a risky proposition. Is losing a $12k sale worth milking another $2k?

You threw fridges in just the other section but pretty sure their starting watts are pretty high. Running watts are low, though.

My ac units are a 2 and a 3 ton of that helps with back of the napkin math.

howdydamnit04 said:

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My ac units are a 2 and a 3 ton of that helps with back of the napkin math.

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FWIW - talked to a company out of Sealy about our home (3,400' 4 ton and 1.5 ton ac), fridge, deep freeze, gas furnaces, gas tankless H2O heater, etc.

Company said a 22kw would probably be fine, but assured a 24kw would be able to handle the entire home.

Thanks y'all.

For a generator, you primarily look at 220v loads. These include air conditioner, heat, water heater, range, oven, dryer, water heater. Remember heat strips use a lot of power.

You also need to decide if you can handle reduced power or not? Do you need to run both heaters at the same time all the time? Can you run one then the other? Do you need the dryer while the heater is running?

With a 22kw/24kw, you basically have 100 amps at 220V. So you can look at the amperage of various items. You can also look at the breakers. All of your 110V loads such as a refrigerator matter, but they don't use much current. For instance, a refrigerator maybe 9 amps, but it is at 110V. This means it uses 4.5 amps of your available 100. Most also drop to about 5-6 amps once running.

Also, remember you are burning more propane/gas with a larger generator so you need to consider that.

Putting central AC on generator is exponentially more $$$ install. If you can live with no AC for a week or two, you can run the rest of the house on a much smaller footprint setup.

JSKolache said:

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Putting central AC on generator is exponentially more $$$ install. If you can live with no AC for a week or two, you can run the rest of the house on a much smaller footprint setup.

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Agreed. And you can purchase a couple portable ac units. You should also save a decent amount on fuel. However, a huge portion of whole home generators is install. So not sure it'd save that much if going that route. Granted, I haven't ever priced them myself. I just have a portable generator setup.