The motherboard itself does not use power however, to supply the cpu with constant power the motherboard downgrades and converts the power not used directly into heat. that means that even if the cpu is idle the motherboard is still drawing power to run it. So to answer your question the average(prebuilt system non gaming) motherboard draws 25-40 watts, the average "gaming" motherboard draws 45-80 watts.
The mobo has many other components (e.g. a NIC, DC-DC) with different efficiency. For example, I am looking at the Gigavyte X470 mobos, they all have "gaming" in their name, as are many of the X370. I want to select an efficient board in ATX format (so more sockets). No need for extra speed stripes of LEDs... This is a 24/7 server which does little most of the time.
So, how do I know which boards draw 25w and which 40w?
I will use a 2700 (probably) or 2700X: I hear both use the same power normally but the 2700X can be pushed further when called for.