Self-hosting

That's already a very low power solution. If you have external storage connected there might be still some saving potential, 3.5" HDDs use quite a bit of power, so do high end NVMe drives.

Software wise... maybe check the crons and reduce the frequency? Like your RSS feed reader probably doesn't need to check every 5 minutes or so for new entries.

I'm currently using an "old" mobile (Samsung A20) with Termux (standard ROM, not rooted) and works like a charm! I'm hosting the *arr suite, Kavita, Podgrab, Homeassistant, ntfy and there's still room left (in terms of CPU and RAM). In terms of consumption we're in the order of a few (<5W) Watts per day. The only issue is the storage, I've added a 128GB SD card and every once in a while I consolidate the content in an external HDD. And the nice thing is that you breath a new life in old hardware.

I guess it depends on your definition of "self hosting" but I'm in the process of migrating a lot of my services to a remote vps on vultr. It doesn't make much sense to have a big, hot server running at home that needs capacity to cope with peaks but isn't used 99% of the time.

Sharing server resources with other virtual servers is the most significant least pain to benefit ratio action I can think of.

All that will really be left at home is a torrent client and gerbera (upnp) instance which can happily run on a NUC with an nvme. gerbera won't do any transcoding so the load is negligible.

It seems like you wouldn't have asked that question if your network worked flawlessly. Saving bandwidth on local network seems like a weird saving on its own.

So I imagine that you might be suffering from a more fundamental problem: like having a low end router that you push to self-host and cater to a family of wireless devices at the same time.

Am just speculating of course. Could you tell us why are you chasing to reduce power of 5-20 watt device?

For power, using a wired network instead of WiFi is going to help.