I'm in the process of getting my Home Assistant environment up and running, and decided to run a test: it turns out that my gaming PC (custom 5800X3D/7900XTX build) uses more power just sitting idle, than both of my storage freezers combined.
Background: In addition to some other things, I bought two "Eightree" brand Zigbee-compatible plugs to see how they fare. One is monitoring the power usage of both freezers on a power strip (don't worry, it's a heavy duty strip meant for this), and the other is measuring the usage of my entire desktop setup (including monitors and the HA server itself, a Lenovo M710q).
After monitoring these for a couple days, I decided that I will shut off my PC unless I'm actively using it. It's not a server, but it does have WOL capability, so if I absolutely need to get into it remotely, it won't be an issue.
Pretty fascinating stuff, and now my wife is completely on board as well; she wants to put a plug on her iMac to see what it draws, as she uses it to hold her cross-stitch files and other things.
Yeah, anything Zigbee needs a hub of some sort that interfaces with the server. Zigbee is a mesh-like network of its own - it doesn't use wifi or Bluetooth or anything.
I bought Nabu Casa's Connect ZBT-1 dongle; it's like $35 and plugs directly into the HA server. Super simple to configure as well, since HAOS detects it automatically. Plus, the smart plugs act as routers, so as long as there is a path of router-enabled devices that can see each other all the way to the dongle on the server, you shouldn't need anything else.