Google said today that it will buy power from a planned 100 megawatt wind farm in Oklahoma located near a data center now being built, another step in the company's goal to be carbon neutral.

The power purchase agreement to buy power from Minco II wind farm in Mayes County for 20 years is similar to one Google signed last year with the project developer, NextEra Energy Resources, for a wind farm in Iowa.

Through a subsidiary called Google Energy, Google will purchase the power from the wind farm and, as an energy provider, sell the power to the local grid at the wholesale price. Structuring the power deal that way, rather than purchasing renewable-energy credits generated by a pool of projects, gives the project developer capital to continue building projects and ensures that more renewable energy sources are added to the grid, Google explained in a white paper published today.

In a blog post, Gary Demasi from Google's infrastructure team said Google has been able to reduce its energy usage by more than 50 percent with efficient data centers. But investing in renewable-energy projects directly and purchasing offsets is the only way to reach the corporate goal of being carbon neutral, he said.

"As a company we hope that purchases like these, plus the additional $350 million we've invested in renewable-energy projects, support the market and drive down the cost of clean energy. This will enable even more companies to invest in sustainable energy solutions," Demasi wrote.

The deal follows Google's investment in two large renewable-energy projects this month, including a huge 825 megawatt wind farm in Oregon and the Ivanpah solar thermal power plant in Southern California, both of which are under construction. Those investments were equity stakes in the actual projects, rather than agreements to buy the power output from those plants.

Google said its power purchase agreement with the Oklahoma wind farm will result in the project being built. Google's power to its Mayes County, Okla., data center is expected to be operating later this year.