UPDATE 2-RiskMetrics: NRG holders should reject Exelon slate

* Proxy adviser recommends against Exelon directors

* NRG shares close up 1.7 pct, Exelon up 2.5 pct (Adds analyst quote, Exelon comment, background, byline)

By Michael Erman

NEW YORK, July 13 (Reuters) - RiskMetrics Group, which advises shareholders on votes, recommended that NRG Energy Inc (NRG.N) shareholders vote against the slate of directors Exelon Corp (EXC.N) has put up for election, saying that Exelon's takeover bid was not high enough to justify support.

As part of its hostile offer for NRG, Exelon has proposed to expand NRG's board to 19 seats from 14 and elect nine new directors into those seats. In a report obtained by Reuters on Monday, RiskMetrics said shareholders should vote against both the expansion of the board and the Exelon directors.

"We conclude that Exelon's current bid is not compelling enough to support splitting the board with nine dissident nominees," the report said.

Exelon, the largest U.S. nuclear power company, made its initial hostile bid for the independent power producer in October.

The company, which raised its bid for NRG earlier this month, is currently offering 0.545 of a share -- or about $26.99 at Monday's close -- for each NRG share.

NRG shares closed at $23.63 on the New York Stock Exchange, well below the Exelon bid, suggesting investor skepticism that the deal will close.