About 40 percent of the 700 megawatts of large-scale battery storage capacity installed in the U.S. is found in the grid operated by PJM Interconnection LLC thanks to favorable market conditions, according to a May 21 report from the U.S. Energy Information Administration. PJM created a frequency regulation market in 2012 to provide better payments for fast-ramping generators including energy storage, although recent rule changes could undermine the profitability of the market. Installations in the grid run by California Independent System Operator made up about 20 percent of existing U.S. large-scale battery storage power capacity last year. In 2013, the California Public Utility Commission set a mandate for the state’s investor-owned utilities to procure 1,325 MW of energy storage by 2020.