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ENERGY STORAGE SYSTEMS, NET - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Property, Plant and Equipment [Abstract]        
Depreciation expense $ 3.6 $ 3.4 $ 7.3 $ 5.7