DTE Energy. DTE Energy Co. is an American Detroit, Michigan-based utility incorporated in 1995 involved in the development and management of energy-related businesses and services nationwide. DTE Energy's largest operating subsidiaries are DTE Electric Company, an investor-owned with the largest being the Mariposa Energy Company, electric utility serving 2.1 million customers in Southeastern Michigan; and DTE Gas Company, a natural gas utility serving 1.2 million customers in Michigan. The two companies mer