I would get a little genset that you can dedicate to it, not that big of an investment compared to all the other ways you could attempt it. I lived on a farm most of my life before moving to town.
We had a generator that we could just wheel over by the pressure tank and chlorinator out in the one building next to the well, the wiring from the pressure switch didn't go direct into the panel it went to a plug and then there was a 220/240v outlet next to the panel, so you just unplugged the pump from the outlet and plugged it into the generator and away you went til the power came back. Simple and easy.. It wasn't that big of a generator either, 4000w maybe if i recall it was small and fairly lightweight. I'm pretty sure the pump was on a 20a breaker so the generator was actually a little under that with i think 15a twist lock, but it ran it fine many a winter until we got rural water piped in.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....