The contract will be filed under the green communities act, which requires all of the commonwealth’s investor-owned utilities to enter into contracts to purchase at least 3 percent of their electricity supply from renewable generators.

The filing actually includes two contracts; the first would enable National Grid to purchase 50% of the output of the wind farm for its customers, while the second would facilitate the purchase of the remaining 50% by another party or parties.

National Grid said that it will only be purchasing slightly more than three percent of its total electricity supply from Cape Wind.

Tom King, president of National Grid, said: “It’s truly fitting that the next milestone in our nation’s clean energy revolution is taking place in the Bay state and New England. We believe this project will provide long-term economic and environmental benefits here, throughout the region and across the nation.

“We absolutely must develop our homegrown renewable energy resources if we are to meet state and federal renewable goals, secure our energy future and seize the leadership position in the global clean energy economy.”

Jim Gordon, president of Cape Wind, said: “National Grid’s decision to move forward with this agreement helps put Massachusetts at the forefront of this emerging industry and provides their customers with secure and stable-priced renewable energy.”

Under the terms of the contract beginning in 2013, National Grid would purchase from Cape Wind 50% of the wind farm’s output including electricity, renewable energy certificates, and other market attributes for $20.7 cents per kWh. That price, which assumes existing federal tax incentives, would increase 3.5% per year during the 15-year term of the contract.