"Kenneth" wrote

Beyond that, the standard "battery backup" units for the
sump pump seem to have a run time on the order of 15-20
hours and I recall an ice storm in which we lost power for
more than 96 hours.

To borrow from the endless threads on rebuilding battery packs for tools,
why couldn't you extend the on time of the sump pump with additional battery
packs?

In fact I knew somebody who used deep discharge marine batteries for
something like this. He got the batteries from a charter fishing service who
used them for trolling motors.

Just an idea. I am not sure how practical it would be in your situation.