There's other things to consider as well. The temperature being a main concern. Don't think any battery backup would keep a tanks temp. stable for long. A chiller in the summer or heater in the winter really drain these backup systems. Personally, I'd be screwed if my power were to go out for much more than an hour during the heat of the summer. As much as I would like to have a backup generator, it wouldn't do me any good unless I was home when the power went out. Im away fm home a lot and the auto generators r way out of the question. Way expensive