[bobo888]

Usually a hydro power plant is used as a "battery". When the demand is low, the electricity produced by wind/solar power plants is used to pump the water uphill. When the demand is high, the hydro power plant releases that water to produce energy.

http://en.wikipedia.org/wiki/Pumped_hydro

[Luyt]

That works in Norway, but would it work all over the globe?

[jbm]

They do it in Japan, and I think they do it in Quebec. Don't quote me on the second one.

They had to have rotating power outages for a week or two in Japan after the earthquake because of the need to replenish those reserves.