The commission also approved the utility's request for a 200-megawatt solar plant using concentrated solar technology that not only helps generate electricity from the sun, but also allows energy to be stored for later use.
The commission approved, too, Xcel's request to add 850 megawatts of wind energy to its system.
"That's great news," said Harriet Moyer Aptekar, development manager of Ausra, a Silicon Valley company supported by venture firm Kleiner Perkins and Sun Microsystems co-founder Vinod Khosla. "We will be very interested when Xcel seeks bids and we'll be competing for it."
Spanish company Abengoa Solar, with U.S. headquarters in Lakewood, BrightSourceEnergy and Ausra are among the solar companies that have established a presence in Colorado to tap into its growing market. Abengoa, which is building a 280-megawatt solar plant in Arizona, has indicated it will compete for Xcel's project.
The proposed solar and wind projects would catapult the utility into compliance with state laws that require larger utilities to receive 20 percent of their electricity from solar, wind or biomass sources by 2020.
Commission spokesman Terry Bote said a written decision likely will be issued by mid-September, clearing the way for Xcel to proceed with its plan.
"Based on the approved framework, Xcel Energy will then solicit bids consistent with the approved plan and submit its recommended choices to the PUC," he said. "An independent evaluator will also review the bids and make recommendations."
Xcel spokesman Mark Stutz said he couldn't say how soon the utility would seek bids for the solar and wind projects.
"We are generally pleased, however, that the commission essentially has agreed with our final plan concerning additional wind and solar power, and our plans to close two of our power plants," Stutz said. "The additional wind and solar provides benefits to the environment, and we look forward to those additions to our system by 2015.
"Gov. (Bill) Ritter last year called for a 20 percent reduction in carbon dioxide emission by 2020," he added. "The plant closures will help us reduce our carbon emissions and put us well on the way toward meeting the governor's goal."
In November, Xcel offered to shut down two coal-fired plants, the Arapahoe at 2601 S. Platte River Drive, and the Cameo in Grand Junction.
To replace the 229 megawatts the plants provide, Xcel initially said it would build a 480-megawatt natural gas-fired plant at the Arapahoe site but later withdrew its proposal in the face of criticism.
The Cameo plant is scheduled to close by December 2010, while units at the Arapahoe will be shut down in 2012.