Following an eight-month review, state utility regulators on Monday approved NStar’s 15-year contract to buy power from Cape Wind, a deal that is expected to moderately increase the average residential customer’s monthly bill for electricity.

The Boston-based utility will purchase 27.5 percent of the power generated by the offshore renewable energy project for a starting price of 18.7 cents per kilowatt hour in 2013. The cost paid by the utility will rise by 3.5 percent each subsequent year.