I hesitate to comment as I really have no facts/numbers or direct experience to offer, but rather an observation.

I don't think think you can assume such a system would be more cost-effective than simply charging a bank of 12 volt lead-acid batteries and then pulling the juice out after dark. I seems to me to be installation/scenario/design specific. At one end, if you just want to store enough power to get through the night, the pump up, drain down system seems very inefficient, but OTOH, if you can store a really large amount of water (big pond or lake size), you could have power for days or weeks, and maybe get to some sort of steady state of reliable power, daily power.

So I think the devil would be in the details of the specific installation/design/scenario, with the pump-up system having the ability to store larger amounts of potential power (water) cheaper if you have the space and water available.

Were you thinking of a specific scenario/application?