The deal gives Terraform, a yieldco set up by SunEdison to hold operating renewable energy plants, stakes ranging from 12.5% to 100% in five operating projects in Idaho and Oklahoma.
The projects have power purchase agreements with utilities, with a weighted-average remaining life of 18 years.
Terraform went public last year and has been actively acquiring assets from its parent company and other sellers.
Atlantic Power said it sold the portfolio to enhance its financial strength and flexibility. Atlantic CEO James Moore said: "Together with our board, we considered a wide range of options for asset divestitures over the last six months as well as options for refinancing the balance sheet without asset sales."
Atlantic Power had hired financial advisors to look into a sale or merger of the company last year, but decided it wouldn't be "in the best interests of the company or its stakeholders at this time."