Exelon Corporation is an American public utility headquartered in Chicago, and incorporated in Pennsylvania. Exelon is the largest electric parent company in the United States by revenue and is the largest regulated electric utility in the United States with approximately 10 million customers. The company is ranked 187th on the Fortune 500. Exelon owns six regulated utilities: Atlantic City Electric, Commonwealth Edison, PECO Energy Company, Baltimore Gas and Electric, Delmarva Power and Light, and Pepco. Wikipedia