Pumped storage efficiency is about 80% which is on par with Lead Acid batteries. At $106 to $200 per kwh, pumped hydro beats battery storage which costs about $400 to $500 per kwh. This seems like an excellent idea for large scale storage of energy, the small difference in efficiency will be offset hugely by the capital costs. Especially if you have grid scale batteries to manage frequencies while turbines come up to speed.<p>Looking forward to seeing how this goes. It would be a wicked great way of transporting water over mountains too. They already do something similar for the Colorado river aqueduct. <a href="https://en.wikipedia.org/wiki/Colorado_River_Aqueduct" rel="nofollow">https://en.wikipedia.org/wiki/Colorado_River_Aqueduct</a>