It might make a good energy hedging strategy
Thinking about it some more and in contrast to my previous comment, this might make a good energy hedging strategy. Google has to worry about energy costs over the years-long life of a data center. A windmill has a fixed installation cost, and its maintenance costs are relatively low and predictable. I believe that the utilities in California have to buy the power based on prevailing rates. So if five years from now rates go up enormously, Google will automatically get a bigger return on its windmills to offset the higher rates it has to pay at its data centers.
Windmills must still be cheaper than solar; otherwise Google could have done the same thing with a solar plant instead. Wind and solar are also more incremental than other kinds of power generation. Google's power demands are still small enough that building a small natural gas or coal plant to offset its power needs would be much more expensive and also subject to increases in fuel costs.
Using wind or solar to hedge future power needs could actually make good sense for many businesses. The fact that it's renewable and good for the environment is not the main point, but a nice added benefit that's good PR.
