Duke Energy sought the price increase after capital investments of $4.8 billion for power plant modernization, environmental compliance, and other construction projects approved by state regulators since 2009.

Lawyers for the utilities regulator and Duke Energy told the court the impact of higher rates was considered and the company's earlier 15.2 percent increase request reduced. The lesser rate increase and concessions to help poor households approved by the Utilities Commission were negotiated by the regulator's division that advocates for consumers, the Public Staff.

Attorneys for the Public Staff and Duke Energy played the unusual role of sitting together before the Supreme Court and arguing their deal benefited all sides.

"What the attorney general is in fact doing here is seeking to obtain all the benefits to residential customers of the settlement reached with Public Staff but to slough off any burden that's associated with that settlement. It's a package," said Duke Energy's attorney, Kiran Mehta.

Since the utilities commission was created by law to balance several interests and set rates, its decisions are "by definition just and reasonable," Mahta said.

North Carolina's attorneys general have rarely challenged utility rate increases to the Supreme Court. The last time may have been 1988 involving another Duke rate case, said a spokeswoman for Cooper, who took office in 2001.

This case comes at a time that both Duke Energy and the utilities commission are under fire.

Cooper's office and the utilities commission are running separate investigations into whether Duke Energy misled regulators ahead of its takeover of Raleigh-based Progress Energy, which was finalized in July. Hours after the deal was concluded, Duke Energy's board ousted the CEO promised throughout the 18-month process of combining North Carolina's two Fortune 500 energy companies.

Duke Energy now has 7.1 million residential and business customers in North Carolina, South Carolina, Ohio, Kentucky, Indiana and Florida.

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