johno1234:

gzt: It's essentially stored energy. Maybe the same can be done with generation assets but I'm guessing there are several aspects that would not be efficient that way.

In a lossless scenario the energy required to pump it up hill is the same as the energy captured letting it flow down again. Then take away the losses in both directions and you have a significant negative energy loss?

The one benefit is the ability to store and generate to a planned schedule. 

It all sounds like a very expensive way to waste energy?