Run pump and heater off computer battery backup

I tried running my main pump and heater off a computer battery backup today using the CyberPower OfficePower 825AVR. The heater was a 150 watts and the pump was around 300+ gph. I unplugged the battery backup from the wall and it all ran for only 18 minutes. The heater came on as soon as I unplugged it and stayed on until I plugged it back in.
It is a good idea to run off a battery backup, but by the looks of it, you will need one much larger in the case of a proper power outage. Tempting to get the vortech pumps because they have inbuilt batter backup and speed control.
Battery back ups don't provide a "true" AC voltage. I would guess that is why the heater turned on as soon as you unplugged the battery back up. Not sure what kind of heater you are using, but maybe the control circuitry didn't like the simulated AC wave form.