The Massachusetts Department of Public Utilities announced its decision to approve the 15-year deal Monday.
NStar’s agreement to buy 27.5 percent of the wind farm’s power was nearly identical to the utility National Grid’s previously-approved agreement with Cape Wind to buy half its power.
Critics say Cape Wind’s power is too costly, and charged NStar agreed to the purchase only under pressure from Gov. Deval Patrick’s administration, which strongly backs Cape Wind.
But NStar said it bought Cape Wind’s power to ensure it had a diverse mix of renewable power sources. And the DPU said Cape Wind is worth the price because of numerous benefits, including greenhouse gas reduction and improved grid reliability.