I noticed that my laptop "burns through the battery" much faster when I use SO, as opposed to other browser-based applications. The battery lasts approximately as long as it does when I do development work using an IDE and a compiler, or roughly roughly one-half the time that it lasts when I'm browsing or using a text editor (4 hours vs. 8 hours). Same goes when I browse the full version of the site from my iPod, except the battery does not last that long.
I think that the aggressive WYSIWYG and spell checking contribute to the drain of the battery at least partially. Automatic requests of the current reputation may be responsible too. I am sure that there are other things of which I am not aware in the site scripts that provide a better user experience in exchange for some CPU cycles. I love these features, but I would gladly trade some or all of them in exchange for, say, an extra hour or two of my laptop's battery life.
It would be nice if I could control "power settings" on the SO sites, similar to the way they are controlled in the operating systems. The application could then replace some of the features with battery-friendly ones: say, instead of auto-refreshing WYSIWYG or checking spelling every time that I type, it could give me a button to refresh the preview or to check spelling. Of course switching the settings to "outlet power" should bring back the enhanced user experience.