“Water in lakes has ~300k cubic kilometers ~ 3 * 10^14 kg http://ga.water.usgs.gov/edu/earthwherewater.html Potential energy is ~ 9.81weightheight Storage requirements (USA): 336 billion kWh = 1.2 10^18 J height = 1.2 10^18 / (9.81 * 3*10^14) kg ~ 408m

We would need to pump water in all lakes about 408m high to have 7 day worth of reserves for USA.”

http://physics.ucsd.edu/do-the-math/2011/08/nation-sized-bat...