I’m really sorry to hear about your loss 😦 Unfortunately, other than a generator, I wouldn’t know… But maybe there are other options out there, or small generators.
Anyone have ideas?
Insulation!
Put Styrofoam around the back and sides of the tank, like the material coolers are made from. Heck, cut up one of those coolers they mail things in.
There are probably battery-powered heaters somewhere? There are definitely backup batteries. You plug the heater into the backup and the backup into the wall, and the backup runs electricity up to the heater but also stores electricity until it’s full. When the power goes out, the backup runs the heater off of its stored electricity. I have one for all my filters.