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The use of energy-storage systems is surging in the U.S. as power companies show increasing interest in the technology designed to backstop wind and solar farms and smooth out electricity flowing to the grid.

Companies deployed 40.7 megawatts of storage capacity in the second quarter, the most in more than two-and-a-half years and a ninefold increase over the same period a year earlier, according to a study released Wednesday by Boston-based GTM Research and the Energy Storage Association.

PJM Interconnection LLC, the nation’s largest power grid, and California are the top two markets for storage systems, according to the report. States including Maryland, Oregon and Washington are also pushing programs that may drive demand, the report said.

“It is promising to see that outside of PJM and California, 10 states had significant activity related to energy storage policies and programs in the last three months,” Ravi Manghani, a GTM analyst, said in a statement.

Utilities are starting to implement storage systems in part to better manage the intermittent power supplies that come from wind and solar systems. The technology is also becoming popular with commercial customers such as building owners and factories that use stored power during peak periods to lower their utility bills.