> A running car consumes gas even when it's idle.

Bad extrapolation of analogy, but still - how convenient that you decided to focus on the lower bound. How about the high bound? Can a car go trillions of miles per gallon? Because Bitcoin block can contain trillions of transactions every 10 minutes.

> We can certainly take an average of transactions to get a reasonable number for watts/transaction.

You can, it’s just not useful.