Natural gas has replaced coal as the dominant fuel for US electricity generation. However, utilities in regulated US states have retired coal more slowly than others. We build a structural model of rate-of-return regulation during an energy transition where utilities face trade-offs between lowering costs and maintaining and using legacy capacity. A regulated utility facing carbon taxes lowers short-run coal generation 48 percent as much as a cost minimizer would. Thirty years after a sudden energy transition, a cost minimizer has retired 71 percent more coal capacity than the regulated utility. Alternative regulations may jeopardize affordability and reliability goals during energy transitions.