May 30 (Reuters) - MidAmerican Energy Co will become the first U.S. investor-owned utility to source 100 percent of its customers' electricity needs from renewable energy when it completes a $922 million wind farm in 2020, the company said on Wednesday.

The utility owned by Warren Buffett's Berkshire Hathaway Inc began investing in wind power about 15 years ago to both hedge against volatile fuel prices and generate more pollutant-free power, its chief executive said, adding that federal tax credits for renewable projects have kept costs low.

"We have not had to raise customers' rates, and that's a big part of the way we evaluate these projects," MidAmerican CEO Adam Wright said in an interview. "We're not building wind just for the sake of building wind."

The company, based in Des Moines, Iowa, serves 770,000 electric customers.

With the completion of the utility's twelfth wind project, MidAmerican will generate enough renewable energy to equal the amount consumed by customers in its Iowa service territory.

Because of the intermittency of electricity generated by wind, the utility will continue to use its existing natural gas, nuclear and coal-fired power plants, MidAmerican said, adding that it is seeking sites to add more wind farms.

Wind power accounts for more than 35 percent of the electricity generated in Iowa, more than any other U.S. state.

MidAmerican's 591-megawatt Wind XII project is subject to approval by the Iowa Utilities Board. (Reporting by Nichola Groom Editing by Marguerita Choy)