Two years after completing a costly upgrade to the Monticello nuclear power plant, Xcel Energy said the reactor on Wednesday began sustained operations at the new, higher power output.

The power increase results in enough electricity to serve an additional 53,000 homes, Xcel said.

“Our Monticello plant can now deliver carbon-free energy to more than a half-million customers,” Chris Clark, president of Xcel’s Minnesota regional division said in a statement.

The upgrade, which also replaced equipment to extend the plant’s life, was plagued by construction challenges from 2009 to 2013. Cost overruns drove up the price from $320 million to a final cost of $748 million, including inflation and financing-related allowances. Even after the work finished, the power boost faced testing and regulatory delays.

The Minnesota Public Utilities Commission, after an investigation of the problems, largely blamed Xcel’s “imprudent management” for the cost overruns, and didn’t allow the company to profit from the investment. As a result, Xcel wrote off $125 million, nearly half its profits in the first quarter.

The reactor has operated at 600 megawatts since 1998. Xcel has been testing the reactor at higher levels, generally operating from 640 megawatts to 656 megawatts since December, according to regulatory documents. A megawatt is 1 million watts.

On Tuesday, Xcel said, the U.S. Nuclear Energy Commission granted permission for sustained operation at its new capacity, 671 megawatts, a nearly 12 percent gain over its former output.

That opens the door for Xcel to include the cost of the power upgrade — but no return on the investment — in rates.

“This investment represents real value for customers and is a key part of Xcel Energy’s goal of delivering 63 percent carbon-free electricity to our Upper Midwest customers by 2030,” Clark added.

The plant, located 45 miles northwest of the Twin Cities, began commercial operation in 1971.