An Iowa utility owned by Warren Buffet-led Berkshire Hathaway said Tuesday it would stop burning coal at five electric generators in a settlement that ends the threat of litigation by the Sierra Club over their smokestack emissions.

MidAmerican Energy Co., that state's largest electric company, also agreed to install a solar array at the Iowa State Fairgrounds and to complete pollution-control upgrades on two other Iowa coal-burning generating units by the end of next year.

The company said it will cease burning coal in the five Iowa generating units by April 2016 and hasn't decided whether to convert them to burn natural gas. The units, ranging in age from 40 to 63 years old, are at three power plants in Sergeant Bluff and Council Bluffs on Iowa's western side and Bettendorf on the eastern side.

MidAmerican's decision comes as utilities across the nation are weighing whether to upgrade or retire aging coal-burning power plants in the face of deadlines to control mercury and other pollutants. In Minnesota, six older generators originally fired by coal in Rochester and Burnsville are being retired, and decisions on several others are expected this year.

MidAmerican spokeswoman Tina Potthoff said during an interview that the company had been considering an end to coal at the five units, and that Tuesday's settlement made it official. She said upgrades to two coal units in Sergeant Bluff had been approved by regulators in December.

The Sierra Club, as part of its "Beyond Coal" campaign, last year notified MidAmerican that it intended to file a federal lawsuit alleging violations of the U.S. Clean Air Act at the Iowa power plants. The company denied that it has violated the law, but decided not to go to court. A proposed consent decree was filed Tuesday with the U.S. District Court for Iowa's southern district.

"The retirement of these plants means our campaign has achieved an important milestone: we have helped retire more than 50,000 megawatts of coal power," said New York Mayor Michael Bloomberg, whose Bloomberg Philanthropies has contributed $50 million to the Sierra Club's Beyond Coal campaign. In a statement, Bloomberg said Iowans join those "who are helping to end our nation's dependency on coal and move the U.S. toward a cleaner energy future."

Berkshire Hathaway, based in Omaha, acquired MidAmerican in 2000 and, since then, the utility has become the U.S. leader in company-owned wind power. The company serves 732,000 electric customers and 714,000 natural gas customers in Iowa, Illinois, Nebraska and South Dakota.

"More and more it is making economic sense for utilities to be retiring these units," Emily Rosenwasser, a Chicago-based spokeswoman for the Sierra Club, said of MidAmerican's decision. "They were already considering it, and they looked at the numbers and realized coal was not the path forward for them."

David Shaffer • 612-673-7090 • @ShafferStrib