The eia reports on battery storage capacity in their post Utility-scale battery storage capacity continues its upward tend in 2018 by Alex Mey, Vikram Linga, & Patricia Hutchins (8/10/2020). Their main chart is copied here.
By the end of 2018, the United States had 125 operational battery storage systems, providing a total of 869 MW of installed power capacity and 1,236 MWh of energy capacity.
There are two other graphs in the post including which regions have the most storage capacity (can you guess before you look?). There are also links to data.