"You're saying that you generate more energy than you expend by pumping the water back to the top. That would mean every multistory apartment building would have a water tower with one of these generators to power the apartments and we would have endless energy. No, there is something missing here. Anyone else care to comment on this?"
They may be pumping in hours of low demand. In the Sierras in California, they make a whole industry of it. At night, they use surplus electricity to pump water up the mountain to holding basins, then feed it back through the turbines during hours of peak usage.
The energy to do that comes from hydroelectric plants that don't have that system in place. The cost to run those is minimal, so they use off-peak energy to pump water back uphill.
Very interesting stuff. They've been doing this as long as I can remember. It only works on hydro systems, though, because there are always losses in pumping water up the mountain. If you have another hydro plant, though, that runs all the time from river-fed dams, the cost is minimal.
O.k. That's a different story and I agree that is becoming common where you have a source of water and a nearby hill or cliff (or a mine shaft) where you can do this to produce peaking power. But the energy to pump it back up to the top does not come from the energy produced by its descent, but from another source.