That's a good question.... I'm interested as well. I originally set up a large computer type UPS but i don't think it had the capacity to power much past 15 minutes. The only option that I know of for aquarium use is the Ecotech Vortech Battery backups but I think its an expensive option. And I think they only advertise them operating one of their pumps for something like 30 hours.... no mention of powering canister filters, etc.
I'm in a fairly stable hydro grid but regardless....sheet happens so the one thing that I have always thought about purchasing was a battery operated air pump..... should really get one.
I also think a good practice is to have at least one sponge filter in a tank, as if the power dies, then you can unplug your canister filters so that they dont automatically start when power resumes and get an ammonium dump. Yet the sponge filter will maintain a stable amount of BB. So with a battery powered air pump and sponge filter, the tank should be good for awhile. I dont think there is any power source, other than a generator, that would supply enough juice for heaters. I could be wrong.
If you are worried about power flucuations then there are good surge suppressor/UPS combos out there.... APC is a good brand.