A California law that requires utilities to get 33 percent of their electricity from renewable sources like solar and wind is widely credited with accelerating the state's cleantech economy. Now state regulators are poised to compel utilities to invest in "energy storage," which could jump-start technology long considered the holy grail of the electricity industry.

On Thursday, the California Public Utilities Commission is expected to vote on a groundbreaking proposal that would require PG&E, Southern California Edison and San Diego Gas & Electric to collectively buy more than 1.3 gigawatts of energy storage by 2020 -- roughly enough electricity to supply nearly 994,000 homes.

The first-in-the-nation mandate is expected to spur innovation in emerging storage technologies, from batteries to flywheels. Once large quantities of energy can be stored, the electric grid can make better use of solar, wind and other technologies that generate energy sporadically rather than in a steady flow, and can better manage disruptions from unpredictable events such as storms and wildfires.