I was thinking in a similar direction. From a biological perspective, computation seems to be a costly activity --- if you just think of the metabolic demand the brain puts on the human being. I assumed that it is very different with computer, however. I thought that the main cost of computation for computers, nowadays, is in size, rather than energy. I might be wrong, but I assumed that even with laptops the monitor is a significant battery drainer in comparison to the actual computer. (sorry, mainly thinking out loud. I better read this and related posts more carefully. I'm glad to see the restriction on computations per amount of time, which I thought was unbounded here).