Comment Re:flywheel (Score 4, Interesting) 245
Pumped storage costs about $200 million per GWh of electricity stored to build. It needs specific geography, high and low reservoirs close to each other to reduce losses pumping water uphill over long distances. It also needs a guaranteed supply of water, lots of it and the sunny parts of the US where large amounts of solar power are being generated are distinctly lacking in water to the point of being either deserts or often in drought conditions during the summer. Pumped storage is also lossy, typically about 65% efficient round-trip.
Mass battery technology costs about ten times as much as pumped storage ($2 million per MWh for sodium/sulfur batteries from NGK), flywheels are a bit less but still a lot more than pumped storage. Cheaper methods of energy storage like compressed air tend to be very lossy.
Grid gas, coal and nuclear generators don't need storage as they either run flat out to meet the instantaneous demand and they can throttle back in quieter times. At the moment intermittent wind and solar generators use the grid as free storage but the more intermittent power that is added to the generating mix the more that storage will be needed to deal with peak inputs and debits. Getting wind and solar farm operators to pay for this extra storage probably isn't going to happen, sadly.