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.
7 months later update:
I've expanded HA quite a bit and have a decent grasp on where my electricity is being used. Suspending my desktop has saved a shitload of power: under Linux, when suspended, it draws ~10W of power at any given time vs ~100W when idle. Also, I put smart plugs on the server cluster; the total usage of the entire cluster - 4 mini PCs, 1 SFF, and 5x HDDs (NAS) - over the last 7 months so far is currently at a whopping 465 kWh. That's ~66 kWh a month. Barely 5% of my monthly electrical usage.
It's attached via USB to a 2014-era Mac Mini running OMV; it's a dedicated NAS and nothing else. Honestly not a huge fan of that hardware setup at this point, as the Proxmox cluster running all of my VMs and whatnot sees it drop out periodically for absolutely no reason. I've already tweaked the network adapter within the OS to stay powered on, because apparently Apple hardware has a mind of its own and just decides to shut various components off for "power saving" reasons.
The kicker is that I'm upgrading it to a 7th-gen based server soon. My dad gave me an old Pentium 4-powered HP Proliant DL110 last year, the case of which has 10x 3.5" drive bays, and is fully ATX compatible, so I'm gonna drop in a 7th gen mobo with Pentium G4560T (already have that on my desk), a newer PSU, and an HBA card. Don't need a ton of processing power for a dedicated NAS running OMV - just a lot of expansion capacity.