They still depend on the grid
This is more of a cost management strategy than an option towards providing electricity to their data centers. It's not like they run a power line from the windmills to their servers; they still have to hook up to the grid and if there are brownouts or other power failures in their area they will suffer the same as everybody else. At the margin their windmills might provide incremental power to the overall grid, but they won't stop a major power outage or shortage. And of course the windmills don't produce power 24/7, while their data centers require that (another reason why Google still depends on the grid).
I would be interested to know how much the windmills cost per KWH versus hooking up to the grid, and what their actual power output is.
