I've the same but one gen newer. i5-8500T with 16GB of RAM. I am not using RAID since I don’t want to run it over USB. I run OMV on Proxmox with two weakly RSYNC jobs for backup. Also run Plex/PiHole/Website via LXC 24/7.
15W idle: ~41€ annually
For a homeserver alone you won’t need more than one system with Proxmox. You would save quite a bit of power consumption.
50W - $3 a month
~$0.08 per kW
That’s damn cheap. I am sitting at ~15W for 0.33€ per kW