Five cents a day. That is the best available prediction of how the Cape Wind development will add to the power bill of the average household if and when its electricity begins to flow in 2013.

The estimate comes not from the Cape Wind developers themselves, but from the transmission company which has contracted to buy power generated by the proposed 130-turbine development in Nantucket Sound.

Under the agreement finalized last Friday, National Grid committed to purchasing 50 per cent of the wind farm’s output, including electricity, renewable energy certificates and “other potential market attributes” for 20.7 cents per kilowatt hour, beginning in 2013.

The price, which assumes existing federal tax incentives, would increase 3.5 per cent per year during the 15-year term of the contract.

“Based on its forecasts of what customers will pay for electricity in 2013, National Grid projects this will translate to a total monthly bill increase of $1.59, roughly two per cent per month, for a typical residential customer who uses 500 kilowatt hours per month,” said a news release announcing the agreement.

“It is important to note that National Grid will only be purchasing slightly more than three per cent of its total electricity supply from Cape Wind,” it said.

The announcement answered perhaps the last big question about the project. Opponents had long contended that it would see huge increases in power bills.

And it is true the price is more than double that paid for power from big, traditional, fossil-fuel-fired generating plants in the state, but this week Cape Wind president Jim Gordon argued it will be seen over time as a bargain.

“America has been in a significant recession which has driven down energy prices. As we emerge from this global recession, people can safely assume coal, gas and oil prices will go up. There is the growing demand from China, India, et cetera.

“What this is, is a long-term energy contract that provides predictable energy prices over the next 15 years. A lot of folks are trying to compare it to the short-term prices of dirty fossil fuel. But it’s apples and oranges. You can’t compare a one-day spot market price or a six-month standard offer price for fossil fuel to a steady and predictable energy supply over 15 years,” he said.

And once you deduct the 6.7 cents per kilowatt hour renewable energy credit from the government, and a couple of small factors from the cost, “our energy price would be about 11.9 cents,” he said.

“That’s not too far off the nine cent basic service price today. And what’s the basic fossil fuel price going to be in 2013?”

The other half of Cape Wind’s production is not yet sold, but Mr. Gordon said it would be marketed at the same price.

The economics of the whole thing are complicated, not least because no one can predict where fuel prices are going, but according to one study trumpeted by the proponents, Cape Wind will actually save the state big money over time, by supplanting power from the most expensive of the 350 generating units in the state.

Mr. Gordon explained the mechanics of that.

“There is a New England power pool, in which units bid their prices every hour into the New England ISO and are put into what’s called a bid stack, from lowest to highest price,” he said.

“The last unit that comes on to fully satisfy the demand in that hour, that sets the clearing price for what every power producer gets paid that hour. Bring in a zero power price unit like ours, and you will displace that most expensive unit, and lower the price across the whole pool. That is why [the consultants] said this project will save $4.6 billion over the 25 year-life of the project.”

He used the numbers to hit back at the opponents of Cape Wind, particularly the Alliance to Protect Nantucket Sound and claims that Cape Wind was economically a bad deal.

“First they said we were going to kill all the whales. After the studies came in, Greenpeace, which cares a lot about whales, came out and endorsed the Cape Wind Project.

“Then we were going to kill all the birds, and after all the research was done, Massachusetts Audubon endorsed the project. They wouldn’t have done that if the things the Alliance said were true.

“The we were going to kill tourism and tourism directors from Denmark came over and did public forums here, saying their projects built off beautiful coastal communities did not kill tourism.

“They’ve been through everything, and now, last, they want to distort the energy economics of this project. They said it would dramatically increase electric bills, by $20 to $40 a month.

“Well, National Grid are experts, and all of their rate analysis shows this is going to cost $1.59 a month beginning 2013, at conservative estimates,” he said.

Mr. Gordon also noted last week’s story in the Gazette, reporting on a meeting between Vineyard fishermen and the Alliance, at which Alliance executive director Audra Parker revealed the new strategy was to blitz Cape Wind with legal actions from as many community groups as possible.

Ms. Parker told the fishermen she hoped that even if the legal challenges failed, they could hold up the development long enough that federal stimulus funds would expire, making Cape Wind economically unfeasible.

“If their strategy is to deprive the citizens of Massachusetts of these public interest benefits that have been delineated by the federal and state regulators who have exhaustively reviewed this project for nine years, that’s really unfortunate,” he said.

“If it has to go down this path, the courts are going to give great deference to the regulators’ decision. Has this been transparent, comprehensive and exhaustive? We’re confident we will prevail in court.”

And if the stimulus money dries up?

“Through all the delays and manipulation and abuse of the regulatory process over the years we did not quit. We kept on because we believed this was the right project, right place, right time.

“We will build this project with or without it. And frankly we believe the incentives are going to be extended. But we will build it either way.”

Mr. Gordon took issue with those who would condemn the project because it relies on government subsidies.

“The utilities were mandated, long before Cape Wind, by a renewable portfolio standard requiring them to buy increasing amounts of renewable energy as the years progress,” he said.

“All qualifying alternative energy projects get these tax incentives because Congress has determined that alternative energy is in the national interest and they want to facilitate its growth.

“Taxpayers in Massachusetts are paying for wind farms in Oregon, California, Iowa. We’re paying for biomass projects in Texas and Florida. Solar projects in other places. Now, finally there’s a chance to bring federal stimulus money to Massachusetts, to Cape Cod, to create new energy jobs, and keep the power price lower.

“What’s wrong with that?” he asked.

The fact is, he said, a lot of the costs of traditional, fossil-fuel generated power to the environment, to health, to national security were not factored in when people looked just at the dollar cost of alternative energy.

That point, he said, was driven home by the huge oil spill in the Gulf of Mexico, which oil companies and the government now are desperately struggling to control.

“It’s a tragedy,” he said. But what it has done is given our nation a chance to pause and reflect on what our energy choices are, what the real costs are and how we live with them,” he said.