Welcome to the boards.
You have to look at the power or amps the heater uses. Then you would have to spec. out the Battery back up for the use. Heaters are killers of amps. I think you will realize that spending $100-$200 can keep you running an hour or two. This is only a guess because you would have to know how many times the heater turns on, how long it runs, and what power it uses in 1 hour. This is all depends on how fast the room cools, temperature of the room, and temperature drop of the tank. This would get worse as time goes on. This is just a quick estimate.
Computer battery back-ups are to let you save your work and shut-down properly not to run the system without power. That is the job of a generator.
When you talk to a big company they will say the UPS runs for 7-10 Min. to allow the generator to start and warm up.