By Laila Kearney

NEW YORK, April 8 (Reuters) - Major U.S. power provider Constellation Energy CEG.O defended its planned acquisition of Calpine to regulators on Tuesday, after consumer groups protested that the deal would give the merged company too much market power, a regulatory filing showed.

In January, Constellation announced its agreement to buy Calpine, a privately held natural gas and geothermal company, for $16.4 billion, in one of the country's biggest-ever U.S. power industry acquisitions. The companies, later that month, asked the Federal Energy Regulatory Commission to approve the transaction, which would make Constellation the largest U.S. independent power company.

Consumer advocate groups in Maryland and Pennsylvania have since filed protests with FERC against the deal, citing concerns that the merger had the potential to ultimately stifle competition and drive up power bills.

"The merger would incentivize and facilitate the surviving company's opportunities for anticompetitive conduct, such as the withholding of supply, given the fleet changes post-merger and the supply and demand conditions in the generation and capacity markets," the Maryland Office of People's Counsel said in its protest last month to FERC.

The consumer advocate for Pennsylvania echoed the sentiment in its separate protest, saying Constellation's share of competitive retail supplies to the state would grow to nearly 40% from 32% post-merger.

Environmental advocacy groups, including Earthjustice, have also asked that FERC deny the acquisition application.

Constellation, in its filing on Tuesday, said its deal with Calpine was not anticompetitive and that it passed the commission's screenings to identify mergers that were not anticompetitive.

Constellation, which asked FERC to reject the protests, said that there were numerous safeguards in place to prevent it from withholding supply and that it had no financial reasons to do so.

"Constellation applicants have both a legally binding obligation and an economic incentive to bid and sell their power generation," Constellation said in its filing with FERC.

(Reporting by Laila Kearney; Editing by David Gregorio)

((Laila.kearney@thomsonreuters.com; (917) 809-0054;))