BOSTON – NSTAR Inc., the gas and electricity utility that serves some customers in Bristol County, Mass., on Thursday reported a small increase in third-quarter earnings despite lower revenue.

NSTAR posted a profit of $87.6 million, or 82 cents a share, in the three months ended Sept. 30, up from net income of $85.8 million, or 80 cents a share, in the same period a year earlier. Revenue fell 13.5 percent to $771.48 million.

The utility said electricity sales declined 3.5 percent for the quarter because of unusually cool temperatures, with the number of cooling days more than 10 percent below normal.

“We remain on track for another good year for our customers and shareholders,” Thomas J. May, NSTAR’s chairman, president and CEO, said in a statement. “This is a tremendous accomplishment given the current economic challenges. We have been very successful in our efforts to reduce costs across the company while maintaining our service quality performance at very high levels.”

The company also reiterated its earnings guidance for 2009, forecasting a full-year profit of between $2.33 and $2.43 a share.

NSTAR Inc. is an investor-owned natural gas and electric utility with some non-utility operations. It serves about 1.1 million electric and 300,000 natural gas customers in New Bedford, Dartmouth, Fairhaven and other parts of Eastern and Central Massachusetts. Additional information is available at nstar.com.