I do something similar. Fanless PC (but I do have a heppa filter pointed at it for hot days) running CentOS 7 and El-Repo 5.2.x kernel. I enable power managem

I do something similar. Fanless PC (but I do have a heppa filter pointed at it for hot days) running CentOS 7 and El-Repo 5.2.x kernel. I enable power management on everything except for the network cards and disable most other devices on it. On UPS it can run about 3 hours. I have unbound DNS and iptables to intercept all DNS requests destin for the internet, so I can block some ads. I null route the DoH servers and some hostile countries. Works great.