The number one thing you can do, by orders of magnitude, is to start with power-friendly hardware.

For example, my previous server was an old gaming machine. It’s lowest idle power consumption was 80 watts. That was with running an OS that permitted heavy power reduction control, and enabling every power saving feature in the BIOS.

Compare that to my 2019 Dell Optiplex Small-Form-Factor desktop I’m running as a server. The power supply is rated for 80 watts, MAX. It idles at 20w, peaks at about 70w when converting multiple videos simultaneously. This with an 8 TB enterprise drive for data.

So 1/4 the power draw when idle, where it spends perhaps 90%+ of its time. Even things like Resilio Sync and Syncthing don’t significantly raise CPU time.

There’s nothing in heavier hardware you could tune to get down to 20w.