The current output is 335.5 megawatts (MW), which is enough to power more than 100,000 homes.
Upon completion of all four phases, expected in summer 2009, Roscoe will be the world's largest windfarm with 627 turbines and a total output of 781.5 MW.
CEO Frank Mastiaux said E.ON has managed to grow world wind power capacity fourfold to 1,800 MW within 15 months and is well on its path to move the renewables business to an industrial scale.
E.ON Climate & Renewables operates six windfarms in the U.S., five of which are in Texas. The total output is currently 727 MW.
E.ON plans to invest 6 billion euro ($8.8 billion) into renewable energy through 2010 alone and aims to reach a capacity of at least 10,000 MW by 2015. E.ON aims to cut CO2 emissions by at least 50% by 2030 and E.ON Climate & Renewables' projects in North America and Europe will play a major role in that target.