Utility and power grid managers in the U.S. are learning that the best way to cut carbon emissions and improve efficiency is the easiest: Just change your lightbulbs. The nation’s largest grid, serving more than 61 million customers from Washington to Chicago, is revising its demand forecasts after recognizing that better lighting has undercut its projections. Swapping all of Thomas Edison’s incandescent lightbulbs with lamps containing light emitting diodes, or LEDs, would save enough electricity to power 20 million American homes, according to the Energy Department.