NRG Yield, a yieldco subsidiary of NRG Energy, also took on the $1.6 billion project financing cost and associated land leases.
The Alta Wind I, II, III, IV, V, X and XI phases are included as part of the deal.
Following the acquisition, NRG Yield adjusted its projections for 2014 full year earnings to $455 million, from $410 million.
Alta Wind is a cluster of projects constructed near Los Angeles with over 1GW of capacity. It includes GE and Vestas turbines across a number of different phases brought online over recent years.
US energy firms such as NextEra Energy and Warren Buffett's Berkshire Hathaway Energy, formerly MidAmerican Energy, own other projects at the facility.
The project has long-term power purchase agreements from Southern California Edison, with 21 years of remaining contract life for phases I-V and 22 years, beginning in 2016, for phases X and XI.