[epistasis]

It's highly dependent on geography, you need two separate, very large, reservoirs at very different elevations. It takes a lot of water across a big height differential to store electricity.

It kind of reminds me of people that think that we should use the exercise bikes in gyms to power the lighting; yes, brilliant, but actually run the numbers and you realize that humans output very very little energy even at peak exercise. The amount of waste on generators would be phenomenal.

There are some cases where pumped hydro makes sense, but there's limited scalability.