This file photo shows wind turbines outside Boulder, Colo.

(AP Photo/Brennan Linsley)

Colorado's renewable energy mandate has been upheld by a federal appeals court.

The mandate, which voters passed in 2004 and expanded in 2010, was challenged by the free-market advocacy group Energy and Environment Legal Institute. The group argued that the renewable energy requirements violate the U.S. Constitution.

The lawsuit claimed that the requirement that large utilities such as Xcel Energy get 20 percent of their electricity from renewable sources violates constitutional protections for interstate commerce.

The plaintiffs argued that because electricity can go anywhere on the grid and come from anywhere on the grid, Colorado mandate illegally harms out-of-state companies.

The 10th Circuit Court of Appeals in Denver disagreed. The three-judge panel ruled that the mandate does not wrongly burden out-of-state coal producers. The judges also pointed out that Colorado voters approved the mandate.

Earthjustice and Western Resource Advocates defended the mandate on behalf of a bevy of other environmental groups including the Sierra Club.

“Colorado is a national leader on clean energy, thanks in large part to its ambitious renewable energy standard,” Earthjustice attorney Michael Hiatt said in a press release. “We’re happy that the Court has rejected this attack by supporters of dirty fossil fuels.”