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ENERGY STORAGE SYSTEMS, NET (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019
Property, Plant and Equipment [Line Items] |  |  |  |
Less accumulated depreciation | $ (36,758) |  |  |
Total energy storage systems, net | 119,842 |  | $ 123,703 | $ 131,569
Depreciation expense | 3,800 | $ 3,600 |  |
Energy Storage Systems |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Total energy storage systems, net |  |  | 91,593 | 98,358
Depreciation expense |  |  | 13,900 | 9,700
Energy storage systems placed into service |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Energy storage systems, gross | 145,555 |  | 144,425 | 124,789
Less accumulated depreciation |  |  | (33,389) | (20,489)
Energy storage systems not yet placed into service |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Energy storage systems, gross | $ 11,045 |  | $ 12,667 | $ 27,269

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