Edison International has found what every electric utility wants in this time of sputtering demand: a new source of revenue. By adding battery storage to a quick-start natural gas turbine, General Electric Co. made a hybrid power plant that allows Edison to collect payments for keeping the generator ready to instantly supply electricity when California’s grid needs it, 24 hours a day. That’s a market that peaker plants, which are needed only when demand surges in hot weather, haven’t been able to access unless they continuously burned gas to keep the turbine spinning. The hybrid units can provide Edison’s Southern...