over the years, I've only experienced two power outages that lasted almost a day here in Milpitas, both during winter, so keeping the tank temperature is the main concern, and I don't think any UPS will be capable of running a tank heater. I use a small generator, and it can barely handle the 300watt heater when it kicks in (the startup surge is high). Worse case as a last resort, if you cannot keep your tank warm, you may need to keep your car running and run cables to an inverter to power your tank heater.
The small generator is loud, but during winter, windows are closed so nobody would notice the generator sound.
For powerheads, if you use a 24v DC powerhead, you can get 2 12v SLA (sealed lead acid) batteries at frys. They go on sale from time to time. using 24v dc directly would be a lot more efficient than inverting dc to ac then plugging in the ac to dc adapter to power the dc powerhead. You can get trickle chargers for the SLA batteries to keep them ready.