Exelon Corporation is an American energy generator and distributor headquartered in the Chase Tower in the Chicago Loop area of Chicago, Illinois, United States. Exelon is the largest operator of nuclear power plants in the United States. It was created in October 2000 by the merger of PECO Energy Company and Unicom, of Philadelphia and Chicago respectively. Unicom owned Commonwealth Edison. Exelon operates regulated utilities in Illinois, Pennsylvania, and Maryland.