My ASRock B450 Pro4 draws about 3W from the wall if WOL is enabled, which in ASRock's firmware seems to keep all PCI devices (and attached USB devices) powered. 30W seems impossible though, anything not specifically designed to be fanless will almost certainly need to turn on the fan at that power usage (and even fanless might have difficulty dissipating a constant 30W).

Monitors can dissipate a huge amount of power without a fan. The OP blames mostly them, and their stand-by power consumption usually isn't even available to a buyer.

Surely the display panel itself is switched off when on standby, though? Is this a case of not including a separate standby power supply circuit, so that it's the "main" one inside there running at a fraction of its capacity, where it is least efficient?

The specific case I'm thinking of was an Advantech AIMB-274 with a Mini-Box PSU (I don't remember which one). I was evaluating this setup for inclusion in a battery powered mobile robot; in particular we needed to decide whether the computer could be hooked up to the battery full time or if there needed to be a relay in between them in order to preserve the battery when the robot was "off". The answer turned out to be that we very much needed to disconnect the computer.

Admittedly this is not a consumer PC or a "modern" gold/bronze-designated power supply, but I still thought it was pretty significant.

Mine is <100W now. Including a big monitor.

30W with high end CPU and graphic card in desktop and tons of plugged pheripals and big screen? They have good power management these days but not that good.

Plus many people use electronics build 5-10 years ago which are generally bigger offenders.

It doesn't matter if it is a computer with multiple GPUs for cryptomining. If it's 'off', PCI should be off, etc.