I have an old ThinkPad 11e running Debian that I have repurposed into a home server. It’s only supposed to run TVheadend. I don’t need any other services for now, but later on i might add a few using docker.
Is it enough to set multiuser.target as default to disable gui and keep the system always on?
How can I disable all unnecessary services and minimize power usage?
The easiest way to disable unnecessary services is to uninstall them with aptitude, or whichever package manager you like. Try terminating services one by one, and see if anything bad happens. If nothing bad happens, you can probably uninstall it. On the other hand, if the system does get wonky a reboot should fix it. Or, you can research the services by name and decide whether to uninstall them. (avahi-daemon for example is a good idea to uninstall.)
To make the GUI not run, uninstall your display manager (gdm, xdm, nodm, or whatever) and uninstall your xorg server or wayland server. There may be GUI programs remaining after that, but they will only be consuming disk space, not RAM or CPU.
If the battery is old and holds little charge, you may save a few watts by removing it and throwing it away, instead of letting the system keep it topped off.
Get a power meter, such as a Kill-a-watt device. Then, experiment with different settings. If it’s consuming less than 30 watts, you’re probably fine. If you live in the US, one watt-year is about one US dollar (or a little more), so for every watt it consumes, that’s about how much you will pay per year for its electricity.