The resignation of David Crane as chief executive of NRG Energy last week illustrates the risks that leaders of U.S. electric-power companies face in a changing industry.

Crane stepped down amid a nosedive in the company’s stock, a slump that reflected investor discomfort over his efforts to transform the company from a traditional supplier of electricity to one increasingly focused on rooftop solar systems, electric-vehicle charging and home energy management. (Read more)