Another thought:

Forget trying to react to a loss of power, use an alternate power
source from the git go for the sump pump and the furnace.

A couple of solar panels, some T-105, 6VDC golf cart batteries wired
in series to provide either 12VDC or 24VDC and a sine wave inverter to
run the sump pump on a full time basis.

The more batteries the better within reason,

6-8 would be manageable, IMHO.

Standard stuff on a crusing sailboat, and definitely less costly than
a SCADA.

Lew