My backup sump pump is water powered. It connects to the house water line. It uses about 1 gallon of water to pump 2 gallons from the crock. No backup battery to worry about with long power outages.
My home has such a system. But if you aren’t on a public water system they aren’t much help. They may not be legal in certain areas, from what I am told. (Mine was installed by the previous owner)
You are correct, you need to be on public water systems. IT would run a well dry pretty quickly.
I was not aware that they were illegal in some areas but thinking about it, it makes sense. Mine was installed by the builders. It’s saved me several times.