Warren Buffett's MidAmerican Energy Company plans to invest $3.6 billion in what would become the nation's largest wind energy facility. At 2,000 megawatts, the proposed Wind XI project would overshadow California's 1,548-megawatt Alta Wind Energy Center -- currently the largest wind facility in the U.S. and second largest in the world. MidAmerican Energy, which serves 752,000 electric customers in four midwestern states, said Thursday that the massive investment would provide Iowa with a cleaner energy future and be a huge step toward the company's 100 percent renewable energy goal.