Somewhere around 8 watts idle would be typical running windows with a screen attached etc [1]

Of course, there are a wealth of hardware options out there, so it varies. I've heard as low as 4 watts.

[1] https://www.youtube.com/watch?v=veRja-F4ZMQ

A ThinkCenter usually is just a laptop with no screen or keyboard. I have one.

btop says CPU takes 1W when "idle" (seeding some torrents), no idea what the rest of the board and NVMe could take.

The wattage on labels on PSUs are just safe max power draw, above which the supply has the right to shut itself down. It's not 1:1 representative of idle draw.

They still use only a few watts at idle.