Not to mention, a single transaction can power an infinite amount of (wash) transactions thanks to 2nd layer stuff like the lightning network. So it'd be similarly misleading to try quote in terms of that.
So the correct unit would be power usage, per block or time unit.
I get what you mean, if you decide not to make a transaction, the energy still gets spent but the more you use bitcoin the more valuable it becomes which incentivizes miners to keep mining. Also someone elses transaction will just slot in to fill the gap left since there is a very limited amount of space for transactions which is always fully utilized.
With enough people using the second layer network, each "settlement" could mean finalising hundreds of transactions for thousands of people. This second layer network acts like a caching layer, and consolidates many transactions into a single transaction (put into very simple terms).
[1] Combinations of inputs and outputs.
If each block takes 10 minutes, and the miner network indeed draws 73 TWh each year, then that works out to ~1.4 GWh per block.