Sizing a backup generator? Install costs?

[frule]

I would really reconsider getting the Rheem and look at a Generac or Kohler. Generac has an 11kW unit that is reasonable. I think I have read that some Generacs are available with transfer switches that can help "balance the load". Seems to me that a 10kW will start one AC unit (2-ton) but not 2 at  the same time. So the type of transfer switch may be the key.The running amps for 2 should be OK. My 8kW produces 33amps of 240W continuous power.

Like I said, see if you can find a HVAC installer that is a generator dealer as well. You'd get better advice on the AC units. Another consideration is that a 17-20kW unit will use twice the fuel (natural gas, I assume?). Typically these units run most of the time at way less than full load, thus less fuel use, but still way more fuel in larger units.

[Snipe Hunter]

I can't speak for any other brand than Generac (we have two) but in order to have any warranty coverage on them they must be installed and serviced by an approved Generac technician. They aren't hard to find. They are good units and very reliable but (read service reviews online) you won't get any factory support without following the letter of the contract. I'm a 99.9% do-it-yourselfer so I have personal struggles buying anything I can't fix when it breaks. Generac parts aren't cheap and they're proprietary, you can only buy them through Generac. Even their engines are designed and built by Generac. That isn't necessarily a bad thing, it's just reality. I've never needed to buy a part or repair one.

We bought the last one through Amazon. Shipping was free with Amazon Prime. That one was the 15kw portable.

[fredhargis]

Generac seems to be the major brand in the uUS, they also seem to make many of the others. I'll be getting a Honeywell, which is a Generac with a Honeywell label.

[TDKPE]

I suspect the claims about running two 4-ton systems are a bit, well, optimistic.  My 3-1/2 ton system has a total running amp rating of 17.9A for the compressor, plus 1.5A for the fan, which is 4500 volt-amps*.  And that doesn't count the indoor blower, which is another 3/4 hp fan, or around 6A at 230V for 1400 VA.  Locked-rotor current (LRA) is 112A for the compressor (almost 26kVA if voltage doesn't sag), which is the same thing as initial start-up, since the rotor isn't moving when it first gets power.  But it does start nearly instantly, though not quite that fast when the current the generator can deliver is limited, and the voltage is very reduced when doing it.

It probably can run one system that size, but they don't even advertise intermittent (or surge) load capability, which I find odd.  It may be able to run another, but start-up may not happen with one already running.  It just sounds like the marketing department is using questionable best-case-on-paper type reasoning.

Personally, I'd stick with Generac or Kohler, or maybe Onan/Cummins, if local service is available.

*Generators (at least large ones) are normally rated in both kVA and kW, because with motor and other reactive loads like transformers, there's actual power load on the generator and the engine driving it, plus 'extra' current flowing that's out of phase with the voltage (power factor) that isn't doing any actual work.  kVA is always higher than kW, and with motor loads, the generator windings handle more current than they would with a purely resistive load, like resistance heaters, for the same engine output power, so they're rated for both max current times voltage (kVA) which is a function of what the generator unit can handle, and max electrical power (kW) which is a function of what the engine can output.

[Roly]

I'll agree two 4 ton units is a lot of load.    I think the running of 2 ACs was in relation to the transfer switch as it states it has a air conditioner control module.     Not sure how this one operates but some can prioritize loads in relation to generator capacity.    So if you have AC 1 as a first priority then when it shuts off AC 2 will start if there is enough capacity.   Some have more than 2 priority modes.   Roly

[Phil Thien]

You do know that Generac is in Waukesha county right?

Yeah, but I don't have room for a monkey coffin shaped generator.

That is what appealed to me about the Rheem.  That small footprint would be able to sit near my AC (the only space I have).

The Generac line that is compact seems more output limited than the Rheem.  But I'll look into it.

Edit to add: All I can find at Generac.com are monkey coffins. A compact Generac I saw is apparently NLA.

[blackhat]

How about a metal stand and have it above the AC condenser. 18" of separation would be lots.

[fredhargis]

Phil, one more data point (?), I see in the local Lowes ad the 11KW Generac is being advertised at $2900; which might be 1/2 the cost of an installed system. I know that isn't the unit you want, but the pricing may be of some help.

[Phil Thien]

How about a metal stand and have it above the AC condenser. 18" of separation would be lots.

I'm not sure the pros of the Generac are significant enough for me to go to that extreme.

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Phil, one more data point (?), I see in the local Lowes ad the 11KW Generac is being advertised at $2900; which might be 1/2 the cost of an installed system. I know that isn't the unit you want, but the pricing may be of some help.

Good to know, thanks!