Monticello nuclear plant in Minnesota has begun operating at increased power two years after a costly upgrade was completed, Xcel Energy said.

The 570MWe boiling water reactor, which began operation in 1971, has generally been operating from 640-656MWe since December, according to regulatory documents. The US Nuclear Regulatory Commission has now granted permission for sustained operation at a new capacity of 671MWe.The upgrade also replaced equipment to extend the plant's life.

As a result of construction problems from 2009 to 2013, cost overruns pushed up the price from $320m to $748m, including inflation and financing-related allowances. The Minnesota Public Utilities Commission, after an investigation, 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 $125m, which amounted to almost half its profits in the first quarter.