The Massachusetts utility NStar has agreed to buy power from the proposed Cape Wind offshore wind farm for more than double what electricity from conventional sources is projected to cost during the 15-year term of the deal.

The contract filed with state regulators on Friday totals about $1.6 billion, assuming the project obtains hoped-for tax credits. According to estimates in the contract, that’s $940 million above the market price of conventional electricity during that period.

Critics of Cape Wind, which aims to be the nation’s first offshore wind farm, have long said its power is far too costly. The NStar contract is more evidence state ratepayers will be “gouged’’ by the 130-turbine project, which would be located 5 miles off Cape Cod in Nantucket Sound, said Audra Parker, president of the anti-Cape Wind group the Alliance to Protect Nantucket Sound.

“It’s hard to imagine that the state could have forced a more expensive and burdensome agreement on Massachusetts households, municipalities and businesses,’’ she said.

State regulators still must approve the deal.

In Cape Wind’s earlier, nearly identical deal with the utility National Grid, state regulators said, and the state’s highest court agreed, that the project is worth the cost because of various benefits, such as local jobs, cleaner air and a reliable renewable power source near a busy coastline. Cape Wind spokesman Mark Rodgers said the NStar deal’s above-market price doesn’t change that.

NStar says the deal will add $1.08 to the monthly bill of the average residential customer.

NStar spokeswoman Caroline Pretyman said Cape Wind is worth the price because the utility must buy renewable power from a diverse mix of sources, including offshore wind, or it won’t meet state requirements to obtain increasing amounts of green energy.