PROVIDENCE — National Grid has agreed to purchase electricity from Deepwater Wind’s proposed wind farm off Block Island in a critical step forward for the offshore wind developer’s plans to bring clean energy to Rhode Island.

National Grid, the state’s largest electric utility, filed the agreement with the Public Utilities Commission late Wednesday. The deal is subject to the approval of the commission, which will review its terms and vote before a Jan. 31 deadline set by the General Assembly.

National Grid will pay Deepwater 24.4 cents per kilowatt-hour of electricity starting in 2013 when the eight-turbine wind farm three miles from Block Island is expected to go on line. The price will then rise by 3.5 percent annually over the 20-year agreement.

National Grid estimates that the typical Rhode Island household’s annual electric bill — which currently stands at about $957 — will see an increase of $16.20 in the first year of the contract. That includes a 2.75-percent markup on electricity generated from renewable sources that National Grid is allowed by state law. It also includes the cost of a power cable from Block Island to the mainland that would be required for the project. The wind farm would supply power to Block Island, but any excess would be fed to the rest of the state.

National Grid’s estimate of the additional cost for its customers differs significantly from Deepwater’s calculation, which is about $7. The discrepancy can be explained, in part, because Deepwater’s number does not account for the cost of the cable. Deepwater did not factor in that cost because it’s still unclear who would install it.

Jerry Elmer, staff attorney for the Conservation Law Foundation, which has been monitoring the talks closely, said the premium that customers will pay for clean energy is reasonable, especially considering the detrimental effects on the environment from cheaper sources of power.

“We think it is very, very important to get new renewable-energy projects up and running,” he said in an interview. “It’s worth paying a small premium to do that. It’s important to account for wind generation that has zero carbon emissions. When you internalize the cost of carbon, that raises the price of electricity from fossil fuels.”

Governor Carcieri’s office announced the agreement minutes after the filing was made, heralding it as “a significant milestone in Rhode Island’s path toward developing the nation’s first offshore wind farm and establishing a green-collar industry.”

“This is a tremendous accomplishment and ensures that Rhode Island remains the leader in developing the nation’s first offshore wind farm,” Carcieri said in a statement. “I commend National Grid and Deepwater Wind for recognizing the importance of this agreement both to the ratepayer and the Rhode Island economy. Investing in renewable energy reduces our exposure to the volatility of energy prices on the world market, and this agreement establishes confidence in the financial feasibility of the project.”

The accord comes after months of negotiations and two rejections by National Grid of earlier offers from Deepwater.

It follows the announcement last week that National Grid was preparing to initiate negotiations with the developer of the Cape Wind project, a 130-turbine wind farm proposed in Nantucket Sound in Massachusetts.

If the agreement in Rhode Island is approved by state regulators, Deepwater would become only the second offshore wind developer in the nation to tie up a power-purchase agreement with a utility. In Delaware, Bluewater Wind signed an agreement last year with Delmarva Power, that state’s largest electric utility, for the sale of power from a proposed 70-turbine wind farm.

The Block Island project’s cost is estimated at $200 million. Deepwater plans to spend another $1.3 billion on a second, 100-turbine wind farm that would be installed after the smaller one off Block Island. Developers of such projects typically borrow about 70 percent of the money to finance them.

At an offshore wind-energy conference in Boston last week, an executive from a leading European bank that finances wind-energy projects said that a long-term contract is the most important incentive for lenders. Such a contract ensures pricing stability and guarantees a return on investment, said Jerome Guillet, head of energy in Dexia Credit Local’s Structured Finance group.

In an interview, William Moore, chief executive officer of Deepwater, said the agreement is critically important to his company in securing financing for the project.

“A project like this would not happen without a PPA [power-purchase agreement],” he said. In a letter filed with the signed agreement, National Grid attorney Ronald T. Gerwatowski said power from the wind farm will be more expensive than from conventional sources, which average about 9.2 cents per kilowatt hour.

“Having said this, however, there are valid policy reasons to move forward,” he wrote. “Specifically, if the State of Rhode Island desires to meet climate-change objectives through the development of offshore wind, this small demonstration project is a reasonable place to start.”

TIMELINE How the deal happened

June 26: Governor Carcieri signs law requiring National Grid to purchase renewable energy.

Aug. 31: Deepwater Wind is the only developer to submit a proposal by National Grid’s deadline.

Oct. 15: National Grid rejects Deepwater’s first offer, estimated at 30.7 cents per kilowatt hour.

Nov. 18: National Grid rejects a second offer from Deepwater of 25.3 cents per kilowatt hour.

Dec. 9: An agreement is reached at 24.4 cents per kilowatt hour.

akuffner@projo.com