The drop in cost for natural gas and wind energy to produce electricity was a key factor in the decision to close Iowa’s only nuclear power plant in 2020, five years earlier than projected, Radio Iowa reports. Alliant Energy spokesman Justin Foss said ending Alliant’s purchase of power contract with the Duane Arnold Energy Center early will save the average customer $40 a year. Prior to announcing the plant’s early closure, Alliant and Invenergy signed contracts in mid-July for a 1,000-megawatt Iowa wind farm expansion. Alliant expects wind energy to account for approximately one-third of its Iowa total capacity by the end of 2020.