As part of the deal, GE will also retrofit longer blades and other more efficient components to extend the lives of turbines.
"Repowering our older wind turbines brings us closer to achieving that vision in a way that provides both economic and environmental benefits to our customers and the state of Iowa," said MidAmerican Energy CEO Bill Fehrman.
The work is set to begin later this year at MidAmerican's 150MW Century, 99MW Victory and 175.5MW Intrepid wind projects with further work to be announced.
MidAmerican is also developing the 2GW Wind XI complex in Iowa, set to feature Vestas turbines.
When the repowering programme and WindXI are completed in 2021, MidAmerican expects to provide 95% of its Iowa customers' annual energy use from wind.