It's funny, I had a similar experience taking a nap in the office. I listen to the home of the spinny hard drives all around me, and suddenly my mind zoomed out, and I became aware of how many computers there were around me, and try to imagine how many there were in a 10-mile radius.

That radius kept expanding and I became painfully aware that most of them were just sitting there, not doing anything useful at all.

Not a very original idea — I was familiar with SETI@Home — but it was weird to me how little it had caught on.

Most computers are just spinning their wheels most of the time. That's a lot of power just sitting there.

The comic doesn't answer the question "how many energy slaves is 100 horsepower?" but based on the assumptions therein, the answer is 4,706 energy slaves.