I thought this video was rather interesting, because at 12:27, the presenter crunches the numbers to find out how many years it would take for a new computer purchase to be more environmentally friendly (in regards to total CO2 expended) compared to using a less efficient used model.
Depending on the specific use case, it could take as little as 3 years to breakeven in terms of CO2 if both systems were at max power draw forever, and as long as 30 if the systems are mostly at idle.
I’ve never used Proxmox, so I can’t say. But TLP is a utility that starts at boot. I’ve used it with virtualbox running, so if Proxmox runs atop a host operating system, I don’t imagine that it would interfere with TLP.
As an additional note, I usually set the min/max thresholds at 40/80, so the battery will charge any time it’s plugged in and below 40% and stops charging when it reaches 80%.