Solar energy has the potential to supply up to 40% of the nation’s electricity within 15 years — a 10-fold increase over current solar output, but one that would require massive changes in U.S. policy and billions of dollars in federal investment to modernize the nation’s electric grid, a new federal report says. The report by the Energy Department’s Office of Energy Efficiency and Renewable Energy says the United States would need to quadruple its annual solar capacity — and continue to increase it year by year — as it shifts to a renewable-dominant grid in order to address the...