As the turbine fleet ages, older wind turbines are being upgraded with new technological advancements, adding greater capacity at those sites. According to the U.S. Energy Information Administration, about 12 percent of the wind turbines in the United States were installed before 2000, but these turbines make up only 2 percent of the installed wind electricity generating capacity. Federal production tax credits provide an incentive to increase electricity generation from existing wind turbines, which allows many owners to repower existing turbines without completely replacing them. According to General Electric, the largest wind turbine installer in the United States, repowering wind turbines can increase the fleet output by 25 percent and can add 20 years to turbine life from the time of the repower. MidAmerican Energy recently awarded a contract to GE Renewable Energy to repower as many as 706 older turbines at several wind farms in Iowa. After repowering, each turbine is expected to generate between 19 percent and 28 percent more electricity.