6.     Property, Plant and Equipment

Property, plant and equipment, including equipment under capital lease obligations and capitalized software development costs, consists of the following:

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For the three months ended September 30, 2021 and 2020, total depreciation expense was $2.9 million and $1.6 million, respectively. For the nine months ended September 30, 2021 and 2020, total depreciation expense was $8.5 million and $4.7 million, respectively.

Property and equipment financed through capital lease obligations, consisting of computer equipment, totaled $12.3 million and $10.7 million on September 30, 2021 and December 31, 2020, respectively. As of September 30, 2021 and December 31, 2020, accumulated depreciation related to property and equipment financed through capital leases totaled $9.4 million and $7.6 million, respectively. Refer to Note 13, Commitments and Contingencies.

6. Property, Plant and Equipment, net

Property, plant and equipment, net, including equipment under capital lease obligations and capitalized software development costs, consists of the following:

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For the years ended December 31, 2020, 2019, and 2018 total depreciation expense was $6.7 million, $4.7 million and $3.0 million, respectively.

Property and equipment financed through capital lease obligations, consisting of computer equipment, totaled $10.7 million and $9.0 million as of December 31, 2020 and 2019, respectively. As of December 31, 2020 and 2019, accumulated depreciation related to property and equipment financed through capital leases totaled $7.6 million and $5.2 million, respectively, refer to Footnote 14, Commitments and Contingencies.