In a bold move being closely watched by utilities, environmentalists and the clean technology industry, California adopted the nation's first energy storage mandate for utilities Thursday.

State regulators with the California Public Utilities Commission, meeting in Redding, unanimously approved Commissioner Carla Peterman's groundbreaking proposal that requires PG&E, Southern California Edison and San Diego Gas & Electric to expand their capacity to store electricity, including renewable energy generated from solar and wind.

"The decision lays out an energy storage procurement policy guided by three principals: optimization of the grid, integration of renewable energy and reduction of greenhouse gas emissions," said Peterman, a rising star who was appointed to the agency by Gov. Jerry Brown in late 2012.

The state's three investor-owned utilities must collectively buy 1.3 gigawatts, or 1,325 megawatts, of energy storage capacity by the end of 2020--or roughly enough energy to supply nearly 1 million homes. The ambitious 1.3 gigawatts is a capacity target, because different storage technologies have different rates at which they can accept and discharge energy, and the mandate aims to be technology neutral.