I would not recommend running a heater off a UPS... will drain it way too quickly. Rather run return pumps. if there is load shedding it should not affect the temp that much. Rather try insult the room to keep in as much heat as possible. Close windows, curtains closed doors etc... even running a heater off a generator suck power. The are in no way energy effient.
Perhaps get a cheap 75W heater set at 2 degrees below the normal temp, then this will kick in if the temp drops belows this level. Run the small heater off the ups constantly in needed, it should never be needed unless you have a very very long power cut, and even then it will struggle, but it will keep the tank warm for the short term.