Sizing a whole house generator for a 3 Ton Air Conditioner


Solar Addict
Grandma got spooked from the Texas cold and blackouts and wants a generator to run her furnace. Must operate autonomous which pretty much must be a whole house unit with auto transfer switch. Might as well do it right and size to run the AC unit during summer hurricanes.

Please confirm my sizing calculations for the 14A running, 77A locked compressor. It appears the absolute minimum is 7.5KW however the Generac charts suggest a 16KW which seems excessive.

Will a 10KW generator be acceptable? I can eliminate the microwave, clothes & dishwasher and other nonessential appliances from the transfer switch.

Major benefit of the generator for me: her house is nearby so will be a reliable source of backup power to run the wall charger on my Powerwheels golf cart.


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Mike Jordan

Solar Enthusiast
I switched my portable 5500kw and 2200kw generators over to tri-fuel (gasoline, propane, and natural gas) to operate under numerous zombie or liberal apocalypses scenarios. I have the manual back feed set up that would not be conducive to grandma. I have a much larger A/C, so I do not think I will get to running it with my gens. But for as small as grandmas unit is, look for a soft start device. That will drastically reduce your start up amps and let you run the A/C reliably with a much smaller gen.


Photon Sorcerer
14A, 240V is 3360W (5 HP)
Ideally, you'd have an inverter and battery capable of delivering the 18 kW starting surge.

Specs for my SI-6048 are 120V, 5750W continuous at 25 degrees C, 11 kW for 3 seconds, 180A (21.6 kW) for 60 milliseconds.
There is an off-chance one of these with 120/240V transformer could start the compressor.
A pair of them for 120/240V, no problem.

Some other inverters I've seen mentioned on the forum had 18 kW surge. It was a 6kW high-frequency inverter, with 20 seconds 18 kW surge. Only $1200

I think such an inverter with a small auto-start generator (and optional PV) would be a good way to go.