NRG is the majority owner of the 290-megawatt Agua Caliente plant that First Solar is building near Yuma, Arizona, and the two companies previously teamed up on a 21-MW plant in Blythe, California, in 2009 and the 20-MW Road Runner plant in New Mexico last year.

The new plant, the Avra Valley solar project, will be built by First Solar and use the U.S. company's thin film panels mounted on a tracker that tilts the panels to follow the sun's arc, the companies said on Monday.

Financial details of the deal were not released.

It is expected to be completed by the end of 2012, and will sell its output to UniSource Energy Corp's Tucson Electric Power under a 20-year contract.

Shares in First Solar rose 1.7 percent in premarket trading to $27.96 per share.

(Reporting By Matt Daily; Editing by Gerald E. McCormick)

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