First, water movement is just as important as a heater. You need to expect to keep at least a powerhead or two operating if not your main pump. This is easily accomplished with a generator or a power inverter. Powerheads dont' require much current.
Lots of people swear by UPS units used to back up computers. I do not recommend them, but larger ones can keep a powerhead going for a short while.
Heaters, on the other hand, are generally only needed if the power outage is extended, and a UPS will not last nearly long enough to operate a heater.
What's worse, if your house heat is not working, a heater is going to really struggle to keep an aquarium heated to temperature when the house temperature drops. So, you really need an extra heater or two as part of your backup arsenal.
Heaters draw a quite a bit of current, and need to run for long periods. In this case, there are really only two options: A generator, or a power inverter that is hooked up to a car, where the engine can keep the battery recharged and the inverter running.
I myself have 3 extra heaters standing by, and a large enough generator to run my entire tank. Backing that up, I also have a very large 1500w power inverter standing by should the generator fail.