You can follow Engineer Detlef Thoms, who does relevant back-of-the-envelope calculations in the "Sustainable Programming" course (see 04:20–06:10).
Calculate the energy savings, make certain assumptions about use (e.g., how often the savings apply), and multiply out. Given the global nature of software, the numbers scale up very quickly.
https://open.hpi.de/courses/cleanit2021/items/5DHsS3tJsXAqfUE4q4F82Z