Pepco employee James Tarantella climbs out of a bucket after fixing a blown fuse on a power outage callin 2012 in Rockville. (Matt McClain/For The Washington Post)

Exelon’s proposed $6.4 billion takeover of Pepco Holdings, which would create a single dominant electricity provider in the Mid-Atlantic, could lead to higher rates for millions of Pepco customers, according to a report scheduled for release Wednesday by a research organization that supports alternative energy.

The Institute for Energy Economics and Financial Analysis, a Cleveland-based think tank centered on energy and the environment, urged the D.C. Public Service Commission to reject the merger, in part because Exelon’s business model relies too heavily on an aging group of nuclear power plants.

“Exelon’s shaky financial position gives it an incentive to raise rates, as it has done four times with Baltimore Gas & Electric just since 2012,” Cathy Kunkel, an IEEFA Fellow and the lead author, wrote in an e-mail. “The merger would weaken D.C.’s control over its electric utility and jeopardize progress toward the city’s renewable energy goals.”

Pepco spokeswoman Myra Oppel said the merger will not prompt an increase in rates. “In fact, by combining our companies, we will operate more efficiently and generate cost savings that will be passed on to customers,” Oppel said in a statement.

The two companies announced April 30 that Chicago-based nuclear energy giant Exelon would be acquiring Pepco in an all-cash transaction, which is a $2.5 billion premium above the value of the Washington-based utility’s assets in April of last year.

“The $2.5 billion acquisition premium further puts Pepco customers at risk, because Exelon must earn returns high enough to justify the premium,” according to the report.

Pepco has more than 2 million customers in an arc stretching from the District and its Maryland suburbs to the Delaware shore and north to New Jersey.

“The two companies’ support for renewable energy will continue,” Oppel said in her statement.”Exelon is already the 11th-largest U.S. wind producer and has made major investments in solar, including a 230-megawatt project in California and the nation’s largest urban solar project in Chicago, and our utilities will continue to facilitate customers’ installation of solar panels on their homes and businesses.”