I used to use my 10 year old old netbook (intel atom n270 2gb ram - ubuntu server) as a server for Plex, calibre, pihole, ssftp.
Now I am using a Raspberry Pi 4 8GB Ram, as it consumes less electricity. Old laptops are consuming (except HDDs/SSDs) 10-30 watt. Raspberry Pi in indle consumes 2watt and when i am using it at mac power with an external hdd consumes 12watt.
It's hard to get a hold of the raspberry pi model 4 where I am unfortunately. I had wanted to use it to host some hobby projects locally and maybe as a low powered game sever, though i doubt it could handle it. It might be a fun project to try run an older laptop off solar l, I must look into it anyone has tried that.
I am using my Raspberry Pi 4 server for Jellyfin(movies), Kavita(ebooks), home assistant, ssh, sftp, nextcloud, pihole,pivpn dns recursive server. All of these services are running with containers and Caddy as the reverse proxy for the local's dns address. I am streaming even VR content to my Oculus Quest 2 and it works without any problems!! Also, I am using it as an extra 5ghz access point hahah.
Yep... The only reason I bought it was that it consume less electricity... Otherwise, you can buy a refurbished ThinkPad for exactly the same price, witch core i5 6nd gen...