The heater has the most power draw. Even a 100W heater would draw 2-3x as much as a HOB
(small ones runs 25-50W).
A computer UPS typically has only enough power to allow you to shut down the system. It prob. won't last too long if you plug in a heater.
Better solution is a power box. If you get one for power tools, it should have enough amps to run a tank for quite a while. <Eg. a 60 Amp-Hr box should run a 100W heater (nominal draw 1 amp) some 60 hrs.> These boxes are rather expensive, however.
It all depends on how long your power outages will be. If you live in a rural area where the power can be out for days at a time in winter, it might be better to get a generator. A light duty power box or UPS will run your pumps for a few hours if your outage is brief. For tank temp., you can decrease heat loss form the tank by wrapping it in blankets. Some people have also kept the tank warm by adding a bit of hot water every few hours. <You would want a camp stove or BBQ so you can heat water during a power out.>