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You can store energy in a dam. Pump it up during the day, run it through the turbines at night. Probably not too efficient but lack of storage is not a convincing reason for giving up on renewables, IMO.

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“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...

Actually, that's wrong by three orders of magnitude. One cubic km has 10^9 cubic meters. Each cubic m has 1000 kg of water, so one cubic km has 10^12 kg. If you now have 300k cubic km, that's 3*10^17 kg.

So you actually only need to use 0.1% of all lakes.

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