If you're not doing any useful work, you can't claim to be efficient. End of story.
As for your uncontroversial proof of understanding, it dictates that if the Joules leaving the system per time unit equals the Joules entering the system per time unit, efficiency is 100%. Any less than 100% efficiency, and less Joules leave the system than enter it.
That's pretty amazing, and proves Pitbull's point rather nicely.
