Cape Wind, the first offshore wind farm in the US to win regulatory approval, has a buyer for half of the electricity it will generate.

National Grid has signed a power purchase agreement with Cape Wind Associates, the project's developers, less than 10 days after Cape Wind won regulatory approval to construct 130 turbines off the coast of Cape Cod, Massachusetts.

The utility agreed to buy electricity from the farm at 20.7 cents a kilowatt hour, a roughly 8-cent premium over non-renewable electricity. That translates into about $1.59 per month for the typical customer, or about a nickel a day. The contract is for 15 years, and prices will rise at a 3.5 percent annual rate.

National Grid serves approximately 3.3 million customers in Massachusetts, New Hampshire, New York and Rhode Island. The closest market served by National Grid is the nearby island of Nantucket. Cape Cod, which gave the project its name, would not be served by this agreement.

The 130-turbine farm hopes to go online by the end of 2012, and is expected to generate an average of 182 megawatts of power.