The latest approval by the Iowa Utilities Board adds to 500MW of new wind capacity it granted Alliant last year.
The utility plans to invest $1.8 billion and commission the full 1GW by 2020, it stated.
Doug Kopp, president of Alliant Energy’s Iowan energy company, said: "Wind energy is a major part of our transition to a clean energy future."
Construction recently started at Alliant’s 300MW Upland Prairie site in Clay and Dickinson counties in the north-west of the state.
Work is expected to start at its 170MW English Farms project in Poweshiek county in central Iowa later this year, and at the 200MW Whispering Willow North wind farm in Franklin county in north-central Iowa next year. The company expects to build additional wind farms which are yet to be announced, a spokesman told Windpower Monthly.
These three projects will be the first to be built as part of the 1GW expansion program. The 1GW capacity will be built before 21 December 2020 "to qualify for production tax credits", an Alliant spokesman told Windpower Monthly.
"Through this expansion, combined with existing wind farms and market purchases, the company expends wind to be approximately one-third of its Iowa total capacity by the end of 2020," the spokesman added.
Alliant owns and operates 568MW of wind power capacity in Iowa, Minnesota and Wisconsin, and partly owns a 225MW project in Oklahoma, according to its website.