Given the need to maintain a ~100°F temperature differential on at least 6 days so far this year (at least here in Montana), a solar-augmented generator is probably not enough.

Our heat pump has a natural gas backup, and even it was running at about a 60-70% duty cycle on those days.

All that to say: +1 on a wood stove backup.

Too bad few backup generators have the capability of dumping their heat output into the home via coolant loops or exhaust heat capture.

Because the residential ones are mostly small enough to be air cooled.

Exhaust heat is fine in theory but HN would be screaming bloody murder if even one person died from CO as a result of a shoddy install or cracked heat exchanger so it's basically a nonstarter if you're a big juicy lawsuit target like a business. While there's nothing fundamentally different from a forced hot air furnace the novelty factor makes fending off the ambulance chasers more expensive than it's worth.

The way to do it is probably to colocate the generator with the condenser unit and use a thermostat-controlled second loop to move heat to/from the main refrigerant loop as needed. This gives some options to keep the generator from getting too cold to start as well as recover waste heat when it's up to temperature.

> The batteries would need to somehow be kept warm though.

If I were buying such a setup, I'd just put the batteries in the basement. Plenty of space, and it stays 40-50°ish year-round here.

Not that many homes are built with basements nowadays.

Water to water heat pump with pipes being directly under building's basement might've been pretty neat solution tbh, if you're digging for basement anyway might as well put some piping there for the heat pump

(Googles) I didn't realize basements were falling out of favor. Interesting!

Heatpump also needs to be kept warm, but those pumps do that themselves.

Starting a generator below a certain temperature becomes hard too (most lubricants do a nice impression of putty when sufficiently cooled).

As an aside, if you have a large free-standing propane tank you probably don't need to worry about stretching out the supply. I've known people who use propane as their primary heat source and can go a month between deliveries. Even assuming your propane heater + heat pump is 1/3 as efficient, that should give you 10 days of heat.