On June 30, 2005, the
Federal Energy Regulatory Commission approved the merger of Exelon and
Public Service Enterprise Group Inc., a
New Jersey utility. Under this merger, Exelon would have become the largest utility in the
 The two companies later broke off the agreement
 due to pressure put on the NJ Board of Public Utilities by public interest groups, including New Jersey Citizen Action.
 The merger sat pending in front of the NJBPU for nineteen months before Exelon concluded that they were fighting a losing battle.
 On April 28, 2011, Exelon announced a merger with
Constellation Energy for $7.9 billion with the combined company owning more than 34 gigawatts of power generation (55 percent nuclear, 24 percent natural gas, 8 percent renewable including hydro, 7 percent oil and 6 percent coal).
 The merger was completed on March 12, 2012.
In 2012, when announcing the cancellation of new nuclear construction for
Victoria County Station, Texas, Exelon stated that economic and market conditions, especially low natural gas prices, made the "construction of new merchant nuclear power plants in competitive markets uneconomical now and for the foreseeable future".
Exelon announced the proposed purchase of
Pepco Holdings, Inc on April 30, 2014, for $6.8 billion in an all-cash transaction. The merger was rejected by the
District of Columbia Public Service Commission in August 2015, though it was approved by other federal and state regulators. The companies appealled the decision.
 On March 23, 2016, the merger was approved by the Washington DC Public Service Commission, under a revised set of terms. The merger was completed later that day making Exelon the largest regulated utility in the United States by customer count and total revenue.
PECO Energy, one of Exelon's local power companies, has been involved in two controversies with activists. In the 1970s, activists delayed the opening of nuclear power plants.
 In 2015, Earth Quaker Action Team began a campaign to pressure PECO to expand the solar power it purchases, and to purchase it locally to create jobs.
Exelon's merger with
Pepco faced stiff opposition from community groups and the D.C. Mayor