What's the deal with using unreliable green energy in datacenters? [closed]

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http://serverfault.com – The news say that Google now uses 48 MW of electricity produced by wind turbines to power one of its datacenters. I've always heard that "green" energy is great, but it is unreliable - wind doesn't blow constantly, sometimes there's no wind and so no energy. How can unreliable wind energy be used to power a datacenter? (HowTos)