One of the many new features in OS X Mavericks announced today at WWDC was a little battery-saving function called Appnap. Basically, it hibernates apps that aren't currently visible. We saw in the live demo that while a browser might make some big CPU demands when displaying high-resolution, rich video in the web browser, but if you launch iTunes and have the window open over top, the CPU demand goes down significantly. This can have some significant improvements on battery life. There are a few other battery optimizations worth checking out in OS X Mavericks, too.
More to come from the WWDC keynote - check out our liveblog!