Table:
|  | 2019 | 2018 |
| --- | --- | --- |
| Land | $14,241 | $14,382 |
| Buildings and improvements . | 664,266 | 567,605 |
| Machinery and equipment . | 2,436,997 | 1,826,434 |
| Office equipment and furniture . | 159,848 | 178,011 |
| Leasehold improvements | 48,772 | 49,055 |
| Construction in progress | 243,107 | 405,581 |
| Property, plant and equipment, gross | 3,567,231 | 3,041,068 |
| Accumulated depreciation . | (1,386,082) | (1,284,857) |
| Property, plant and equipment, net | $2,181,149 | $1,756,211 |
Paragraphs:
Property, plant and equipment, net
Property, plant and equipment, net consisted of the following at December 31, 2019 and 2018 (in thousands):
We periodically assess the estimated useful lives of our property, plant and equipment whenever applicable facts and circumstances indicate a change in the estimated useful life of an asset may have occurred. During the year ended December 31, 2019, we revised the estimated useful lives of certain core Series 6 manufacturing equipment from 10 years to 15 years. Such revision was primarily due to the validation of certain aspects of our Series 6 module technology, including the nature of the manufacturing process, the operating and maintenance cost profile of the manufacturing equipment, and the technology’s compatibility with our long-term module technology roadmap. We expect the revised useful lives to reduce depreciation by approximately $15.0 million per year. Depreciation of property, plant and equipment was $176.4 million, $109.1 million, and $91.4 million for the years ended December 31, 2019, 2018, and 2017, respectively.
Question:
What were the reasons for the revision of manufacturing equipment in 2019?
Answer: Such revision was primarily due to the validation of certain aspects of our Series 6 module technology, including the nature of the manufacturing process, the operating and maintenance cost profile of the manufacturing equipment, and the technology’s compatibility with our long-term module technology roadmap.