How much of a difference in battery life you see also depends on your usage. If you're a power user, lowering the brightness setting can get you a good two hours of extra battery life, but if you don't use your phone that much you won't really see a big difference. If you don't feel like futzing about with your phone's brightness settings, you can automate the process on Android with Tasker, or on a jailbroken iPhone with SBSettings. An even easier solution is to use darker wallpapers and themes. While we've known for a while that lowering the brightness on your devices extends the battery life, it's nice to know just how much it can save you over time. Head over to Wired for the full breakdown and mathematics of it all.