Steps to repurpose old laptop as home server with Debian
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?
Depends. Usually it is still good as a UPS for a few minutes, and some laptops have a bios option to limit full charge which lowers the risk even further.
And how much need is there for a UPS in this scenario - realistically.
Some of the people here take their admin-LARPing a tad too seriously. Most households have reliable enough electricity, and even if there's an outage once every quarter, would a dead battery even help?
I advocate for being realistic with one's own needs. Don't build a five-nines datacenter for a glorified weather station or VCR.
That's typically a feature for servers or business desktops. Maybe your laptop has it, just look into the BIOS.
As I wrote in my other comment: try to be realistic about your needs. Chances are, pressing the power button every few months (if at all) is perfectly fine for your use case (and most others here).
I usually do the expert install and don't install a graphical environment in the first place. But your solution should be fine, too. I think you can show running services with systemctl and then disable unneeded ones. For example systemctl disable gdm but there shouldn't be that much running on a plain Debian anyways.
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.
A4-6210 with build in GPU has TDP of 15W. There is no point to optimize anything it is seeping power already. may be try to use tlp to limit max charge level of the battery ( i'm not sure is you laptop is supported). You can play with governors too, but I personaly will not bother. You obviously need multiuser.target but not GUI.
I install KDE Plasma on devices I intend to use as a server. If the PC is easily reachable you can use the GUI to quickly manage files or use the terminal. Disable search indexing if you install GNOME or Plasma.
I never used Debian but I assume the base system is as clean as it can be.