As people have said, you can add Jellyfin as a service to start with windows regardless of users being logged in.
No one seems to have said how to do this.
The easiest way is to use the NSSM open source tool - it stands for “Non Sucking Service Manager” and it gives a GUI route to create services, as well as some useful reliability and fall back functions.
It can also be used from the command line if you prefer but regardless it’s probably the easiest way without faffing around with powershell or command line and in built windows tools (which do suck).
Edit. The official website is NSSM.cc and it includes guidance on how to use it. There are also plenty of guides online if you search “how to create a windows service”.
Edit2: the easiest way is to use the Jellyfin windows installer itself but the documentation is pretty vague on that and gives a warning about ffmpeg config. It should work but using NSSM will give you more direct control. I think the installer uses NSSM anyway.
For electricity generation: Solar across the UK was about 5% in last year, while Wind was about 29% and Nuclear 13.9%, and hydro 1.3% - so 49.2% of electricity generation over the last 12 months was carbon neutral.
That’s a huge success story - still a long way to go, particularly as that does not include Gas burned in homes, but the UK is moving in the right direction. And Scotland is a huge source of Wind & Hydro power for the whole country.
So even if the barriers to solar in your home are still high, the grid is getting cleaner and cleaner every year. There are also community projects installing wind generators which you can join/invest in if you do want to try and get a slice of cleaner energy and solar is not realistic.
Edit: Source on UK electricity generation: https://www.energydashboard.co.uk/historical Good data on UK electricity generation