I got two of the Canadian Tire Noma brand battery backup units, one for each tank. I got the small 400w ones, and have them set up to run just my return pumps. I figure they should give me around four hours of operation. For that length of time I wopuldn't expect to lose enough heat to make a big issue. If there is a longer outage than that, I may be in trouble. I'll be rushing out to get a generator before they sell out.
Generally though, we have pretty reliable service here in the city, it would be an extreme circumstance to have an extended blackout (he says with his fingers crossed).