Reply #1: There are two things to be concerned about.

[trotsky]

1) How long the battery will last.
2) How much water the battery-operated pump can move.

When that water starts coming in, how often does your regular pump cycle? Is it working hard to keep up? If so a backup pump might get overwhelmed.

I just hung a large (3000VA) computer UPS on my regular pump (my previous company had several of them to get rid of), even though our power is pretty reliable. I also always replace my pump the year after the warranty expires, even if it's working fine, just for peace of mind.

Another solution is the water-powered sump pump. If you have a municipal water supply (i.e., you don't rely on an electrical pump to get water out of a well), you might want to check this out:

http://www.radonseal.com/pumps/water-powered-pumps.htm