I would still urge the OP to swap his current air handler for one with a propane furnace

Two 4 ton heat pumps are my ONLY heat source. Both must be able to operate.

You might want to consider doing what my brother in law did for emergency heat. He has a cottage where the power occasionally fails for several days. The cottage was originally heated by electric baseboard heaters. That was not satisfactory during a power failure so he installed a direct vent propane heater which uses a millivolt thermostat that does not require AC power. When he is out of the cottage he uses the propane heater. The heaters were not expensive and can provide a lot of heat rather than using your heat pumps during a power failure. Consider one or more units like this one:

»www.amazon.com/Home-Comfort-DV21···01AH8K68

However, the air handlers will not blow cold air. There is a plugboard where a minimum temperature from the heat pump can be set when in heat pump mode, if the heat pump doesn't provide sufficient heat, the air handler will turn off until it does.

What is the name plate amps on the condenser and air handler of your heat pumps. Your heat strips already draw about 18 Amps at 240 Volts. Each condenser will probably draw about 25 Amps and each air handler exclusive of your heat strips 5 Amps. When in the defrost mode your current requirement should be around 48 Amps per heat pump. Contact your HVAC contractor and confirm.

The last thing you will want to do is purchase a generator that cannot carry the offered load. I would not recommend guessing because the information is already on the specification plates on your air handler and condenser. My 3.5 ton condenser requires 20 Amps my air handler requires 5 amps and my 4.8 KW heat strip requires 20 Amps for a total of 45 Amps while in the defrost mode. My 2.5 ton condenser requires 40 Amps while in the defrost mode.

We just went through this in another recent thread, and the OP tends to believe that the defrost heaters aren't necessary even though I tried to explain otherwise:

»Anyone have a suggestion for standby whole home generator?

As to defrost, I tend to doubt that much power is required.

No chimney, no flue, no way to vent propane at this time from inside the house (other than cooktops).

I would expect that the power required by both of your heat pumps running in the defrost mode simultaneously would be 24 KW or approximately 100 Amps or 50 Amps each. The maximum continuous power from a 20 KW generator is 83 Amps. If you want to be able to use most of your appliances at the same time, you should consider at least a 36 KW Generator, which will provide 150 Amps @ 240 Volts. Since you are having a 400 Amp service installed, for the difference in price I would consider a 48 KW generator.

I agree with Cowboyro.

You'd be much better off using the propane to heat the house directly.

Wasn't there just another thread about this same thing? You already have the Generac ATS, so you need to look at Guardian, Generac, Siemens, or Centurion.

Some folks here have generators with inverter outputs. They say they're much quieter, are more efficient, and produce cleaner power.

Looks like $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

It appears to be a nice unit. Did you get a price?

I just have to ask, why do you need one so large? Is it for a business? I can run everything except the central AC on my 7000 watts gas genny.

You may want to look at Diesel units since you are looking at a 20 KW. I googled it and there are some that run at 1200 rpm.

Here's what I googled "20 KW Diesel emergency generator"

You may want to look at Diesel units since you are looking at a 20 KW. I googled it and there are some that run at 1200 rpm.

Here's what I googled "20 KW Diesel emergency generator"

I just have to ask, why do you need one so large? Is it for a business? I can run everything except the central AC on my 7000 watts gas genny.

Last year I was looking at a Kohler 20REOZJC to locate next to my fuel oil tank but never acted on it.