A conversation at lunch today inspired me to work on a quick estimate of the amount of power wasted on animation and Flash-laden MySpace profiles. My conservative estimate is somewhere in the region of 320 kiloWatts, which is much less than I'd have expected. This is probably skewed by me carrying out the test on a laptop - I'd guess that it'll be worse on desktops, but I need to do some more testing to be sure (ideally involving a Watts-up and a serial connection so it can be done without a ridiculous amount of pain). By changing a couple of assumptions it's quite easy to get it up into the megaWatt range, but we're still talking "That's quite a pile of money" rather than "That's a sizable proportion of the US electricity output".
Interestingly, the vast majority of the extra CPU usage (and therefore electricity) seems to be down to Flash. Improving its performance would be a pretty big step in reducing overall power consumption, especially given the number of websites running flash adverts. Want to save the world? Turning off Flash might be a surprisingly good step.
I've written up my methodology and results here - I'd be interested in any feedback.