40 Amps (and I suspect it's actually probably more than 40 Amps-my heat pump, not a geothermal one) is no where near 7 tons, and is on a 50 Amp, 240 Volt circuit-and that does NOT account for the startup power) sounds low.

the wattage can easily be calculated:

The formula is simple: P=IE. Technically, this is the DC formula, not the AC formula, but it is a lot simpler math, and it's close, maybe 10% low.

So P (Watts) =Voltage x Current.
P=240x40
P=9600 Watts.

Now, since you need to add about 10% for the different formulas, you need roughly a 11,000 Watt or 11 KW generator. However, you have to add 50% for startup load, which brings it up to 15 KW.

You will probably want to power your fridge, a TV, and some lights as well, so you probably should look at something like a 18-20 KW generator (you don't want to run it full power, as it shortens the life of the generator).

SInce you are talking about a permanent install (in this size range) the companies who sell generators in this size range will help you with the calculations, for your specific needs.

Make sure it is correctly installed by a licensed electrician through a transfer switch. Every year, a lineman is killed by a generator that is incorrectly installed.

Wow...7 ton... I have a 3.5 ton that I drive with a 8/13.5 K (continuous-max) portable gen (using a manual transfer switch). My experience is that you have no idea how cheap electricity is until you try to make it yourself. Running thru the night my gen will burn 8 gallons of gasoline (propane/ng units are even thirstier, especially ones the size you need). Here in FL cooling is the important thing. We ended up getting a 1K portable and window shaker for longer term use during an extended outage so we could stay cool reasonably. Seems you'll have the problem in reverse...I think your woodstove idea has merit.

We have a Generac 15kw air cooled natural gas unit that handles both of our AC units in the heat of summer. One is 4 ton and the other 3 although both are very high efficiency units. The 3 ton uses a double 20A breaker and the 4 ton a double 30.

Have had it run 12 hours or more at a time and the gas bill really did not go up but by a few dollars. Yes, it costs more than utility electric but its far cheaper than gasoline and I don't have to store it on site either.

Thanks for sharing your experience and advice. We have very little need for AC this far north (but it sure is nice to have on those few hot days). Given that we will need to get a relatively huge generator to power the heat pump, I'm leaning toward firing up the woodstove if the power goes out, and keeping my 1 KW honda generator to power some lights and the refrigerator when needed.

I concur with that idea. And the good thing about the Honda, if it's an EU1000, as it probably is if it's the 1 kw model, is it's an inverter generator that produces great, clean power for your computer, TV, other sensitive electronics. Running stuff like that off the bigger generators can be a risky.

If you did get a big generator for the heat pump, I'd suggest running it for periods of time to warm the house up, and then shutting it off and using the Honda for your small loads like lights, etc. The big generators are even more ridiculously inefficient when you run them at low load because they keep hammering away at full speed even if all you are using is a few hundred watts.