[Ie: lead-acid batteries lose a _lot_ of capacity below, say, -15C]

It rarely gets below freezing here in the Pacific Northwest.

A 1/2HP water pump takes about 800W to run. About 2500W to start

Thanks, that's a figure I've been looking for.

A gas generator is much more cost-effective, and capable of extended
operation. Which is what you need to recharge a 250gal pressure tank
anyway.

It only takes about 10-15 minutes to refill our pressure tank. And, if the
power goes out and we conserve our water usage, the pressure tank can
supply all the water we need for hours. A generator seems like a hassle to
install, maintain, and operate just for running a pump for 15 minutes.

If the power is out long enough that the batteries would go dead, we'll
have other issues than just the water supply.

You'll probably have better luck with a gas generator - keep it at the
house, and feed the pump from there.

Not practical. The house is too far away from the pump and the pump has
it's own power connection UPSTREAM of the house.

Anthony