Depreciation is calculated using the straight-line method over the estimated depreciable lives of the respective assets as follows:
Depreciable Lives
Energy Server systems
15-21 years
Computers, software and hardware
3-5 years
Vehicles, machinery and equipment
5-10 years
Furniture and fixtures
3-5 years
Leasehold improvements
1-10 years
Buildings*
* Lesser of 35 years or the term of the underlying land lease.
Property, plant and equipment, net, consists of the following (in thousands):
December 31,
 20252024
   
Vehicles, machinery and equipment$203,731 $200,004 
Energy Server systems165,629 165,629 
Leasehold improvements129,665 122,413 
Construction-in-progress83,067 86,731 
Buildings53,156 53,221 
Computers, software and hardware34,761 33,910 
Furniture and fixtures11,090 10,943 
681,099 672,851 
Less: accumulated depreciation(282,592)(269,376)
$398,507 $403,475 

About PP&E Disclosures

The PP&E disclosure details a company's physical asset base — land, buildings, machinery, and equipment — along with the depreciation methods and useful life assumptions that determine how these costs flow through the income statement. Capitalization policy thresholds reveal management's judgment on the boundary between expense and asset, directly affecting both reported earnings and asset values.

Key signals: changes in estimated useful lives or depreciation methods can materially shift reported earnings without any operational change. Compare capital expenditures against depreciation expense — when capex consistently trails depreciation, the asset base may be aging and underinvested. Watch for large asset impairments or write-downs that signal overvalued carrying amounts. Asset retirement obligations reveal future environmental or decommissioning costs that are often underappreciated. Compare PP&E intensity (PP&E-to-revenue) against industry peers to assess capital efficiency and competitive positioning.