Xcel Energy, the state’s largest utility and a key backer of Colorado’s aggressive renewable energy standard (RES), reacted with skepticism to Monday’s lawsuit seeking to overturn a state law mandating 30 percent of Xcel’s electricity be produced by renewable sources by 2020. “... We understand that [the complaint] was made by a Washington, D.C.-based advocacy organization,” Xcel spokesman Mark Stutz told the Colorado Independent. “We would be surprised if a federal court would overturn Colorado's legislatively approved Renewable Energy Standard.”