An inspiring talk at #37c3 included this graphic. It shows how 500 developers each making 10 small optimisations of 1 cpu-second that happen 20 times a day for each of 1.5 million users, can save power equivalent to 30,000 two-person households.

Another form of that is, if you have 100 thousand users of your software, and make a 1 cpu-second optimisation that will occur 50 times a day, then you can save the power of a single two-person household.

https://events.ccc.de/congress/2023/hub/en/event/software_licensing_for_a_circular_economy/