Think very very carefully before purchasing and installing a water-powered backup sump pump. Every one I have seen uses the venturi effect. When the float gets up to the start level, a valve opens, shooting city water through a venturi, and out through the discharge hose. The flow through the venturi creates the suction that pulls up the sump water, and throws it out the discharge hose too. So the discharge water is about 50% sump water and 50% city supply water. This is going to put a lot of water outside!
But that's not the real problem! If the discharge hose gets plugged or frozen, or the check valve in the discharge path gets stuck or whatever, ALL the city water will end up getting dumped into the sump pit! And because the float is up, calling for it to run, it will continue to dump city water into your basement. So not only is it not pumping rain water out, it's filling your basement with city water! And it ain't never gonna stop.
I don't have to worry about basements any more, but when I did, besides the submersible AC operated pump, I used a 12 volt battery powered backup pump. It had a circuit board that recharged the battery when needed. I also modified a set of car jumper cables adding on some 12 gauge wire, so if AC power was out a long while, I could drive a car up near and run the pump off of the car battery. Plan was to let the hood down on a piece of 2x4 to leave a space for cable to exit from underhood. Would run car for a while before it's battery would get too far down.