The Durham advocacy group NC WARN said Wednesday it will appeal the Duke Energy-Progress Energy merger before the N.C. Court of Appeals.

WARN, a green-energy advocate that was the most persistent critic of the merger, claims the $32 billion deal the N.C. Utilities Commission approved last summer places a financial burden on Duke’s customers.

The Utilities Commission, which concluded a separate investigation into Duke’s leadership changes in December, has refused to reopen the case to hear those claims. Many are without substance, it ruled last month, or were raised too late to reconsider.

Under the merger’s terms and a settlement of the commission’s investigation, Duke has guaranteed to pass $675 million in savings to Carolinas customers over five to six years.

WARN argues those savings will be wiped out by Duke’s spending on former Progress nuclear plants, including a crippled plant in Florida, on new power plants and by confidential settlements with large customers before the merger was approved.

WARN said it filed notice Wednesday with the Court of Appeals that it will try to overturn or modify the merger.

Duke said the appeal lacks merit.

“The six-month-old merger continues to make good sense for Duke Energy’s customers and the communities it serves. Since the merger was consummated in July 2012, Duke Energy’s North Carolina and South Carolina customers have been benefiting from lower power plant fuel costs and a larger, combined fleet of power plants that ensures operational efficiency,” it said in a statement.

Henderson: 704-358-5051 Twitter: @bhender