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So far as I understood the Power on with keyboard isn't standby, the PC is as turned off as it would be when normally shut down.
I'd guess it'd be similar to Wake on LAN. So it'll draw a tiny amount of power when shut down, but a watt or two at most. Similar to a TV needing to be awake enough to listen for you pressing the on button on the remote.
It wouldn't be on standby sitting there getting warm.
I configure the BIOS to power on the machine when power is applied. Then I turn it on at the plug. I have an extension lead on the desk for this purpose. Then I turn off completely once the machine is shutdown. That way it's not sitting on standby getting warm.