MINNEAPOLIS — Xcel Energy Inc. on Friday defended cost overruns during the five-year upgrade of its nuclear power plant in Monticello.

The Minneapolis-based utility completed the $665 million project in July, and acknowledged that replacing pumps, filters and other equipment cost more than double the original $320 million estimate.

Now, in a state filing, Xcel Energy says the costs were prudent. The company must prove that to Minnesota utility regulators in order to fully recoup the investment by raising electric rates.

Dave Sparby, CEO for Xcel’s Minnesota regional operation, told the Star Tribune (http://bit.ly/19RUsc9 ) in an interview Friday that nuclear power is “safe, reliable, carbon-free and cost-effective.”

At its peak, the construction project at Minnesota’s oldest nuclear power plant, located about 40 miles northwest of Minneapolis, employed 3,000 workers. The upgrade replaced old equipment to keep the plant running another 20 years, and boosting its power output by 12 percent.

Xcel filed the report in response to the Minnesota Public Utilities Commission’s pledge in August to investigate the Monticello investment. The company said that even with the cost overruns, the project benefits customers and would save an estimated $174 million through the remaining 16 years of its license.

However, that cost-benefit number relies on a “social cost” comparison between keeping the nuclear plant, which emits no greenhouses gases, and generating electricity from a plant that does emit them. State law says utility regulators should consider the cost of greenhouse gas emissions, although they’re not currently regulated. Without carbon-emissions savings, the Monticello upgrade would cost customers $303 million extra over its life, according to Xcel Energy’s filing.

Xcel executives defended the investment, saying they would make the same decision today, even though the energy landscape has changed since 2008, when the project was launched. Natural gas, now a favored fuel for power plants, is low priced thanks to the fracking boom. Meanwhile, electricity demand has lagged since the recession, lowering the need for new plants.

“If we didn’t have our nuclear plants, we would be taking a big step backward in terms of our CO2 (carbon dioxide) accomplishments,” Laura McCarten, an Xcel regional vice president, said.

McCarten said Monticello and Xcel Energy’s other nuclear plant, Prairie Island in Red Wing, supply 30 percent of power used by the utility’s 1.2 million electric customers in Minnesota. Nuclear is the utility’s second-biggest generating source, behind coal, for Minnesota.

Xcel Energy is preparing to ask to again raise Minnesota electric rates. Company officials declined to talk about it except to say the remaining Monticello costs will be included.