For the past decade, California has been a leader in the clean-energy revolution. Groundbreaking state laws require our major utilities to purchase 33 percent of their power from renewable sources by 2020. And some green-energy experts expect that mandate to rise to 50 percent or more in the following decade. To date, the rapid growth of solar and wind power has fueled the move to renewables. But for California to fulfill its green-energy future, it must solve an important problem: how to deliver electricity to consumers when the sun isn't shining and the wind isn't blowing.