Duke Energy Corp , the largest power company in the United States, posted a better-than-expected quarterly profit on Thursday due to benefits from its summer acquisition of rival Progress Energy.

For the third quarter, the company posted net income of $594 million, up from $472 million a year earlier. Earnings per share fell to 85 cents from $1.06 as shares outstanding increased 57 percent, due in part to the Progress deal.

Excluding charges to integrate Progress and other one-time items, Duke earned $1.47 per share. By that measure, analysts expected $1.45, according to Thomson Reuters I/B/E/S.

Chief Executive Jim Rogers said Duke is "on track" to hit its 2012 earnings forecast of $4.20 to $4.35 per share.

Shortly after the $18 billion Progress Energy deal closed, the combined company's new board said Progress Chief Executive Bill Johnson had resigned and would not be taking the top job at the combined company as had been planned.

Rogers, Duke's CEO as the merger was negotiated, was asked to stay at the helm.

Charlotte, North Carolina-based Duke is the largest U.S. power company, with 57,000 megawatts of generating capacity and 7.1 million electricity customers in North Carolina, South Carolina, Florida, Indiana, Kentucky and Ohio.

Duke is also the second-largest U.S. operator of nuclear power plants.

Shares of Duke closed Wednesday at $62.94, down nearly 5 percent this year.

(Reporting By Ernest Scheyder; Editing by Gerald E. McCormick and John Wallace)