PowerNap is a neat new feature that was just introduced in OS X Mountain Lion. In fact, the list of things this feature can update is longer than most of us expected, with things such as Calendars, Notes, Contacts, and Reminders being synced while your Mac is only on battery power. When plugged in, PowerNap can update software and make Time Capsule backups. The full list of PowerNap’s capabilities can be found here.
PowerNap works on MacBook Air from 2011 onward, and the new Retina MacBook Pro (coming soon), due to their solid state storage. This features will undoubtedly proliferate across Apple’s line of computers as more and more of them are transitioned to solid state storage as it becomes economically feasible to do so. At any rate, Apple has released a firmware update for 2011 MacBook Airs, bringing compatibility with this feature to those customers.
I happen to own one of the 2011 MacBook Airs, and installing the firmware is as easy as checking software update in the Apple menu, downloading the incredibly small file, and restarting your machine. From there, you can turn PowerNap on and off in System Preferences > Energy Saver. So far, I haven’t had any new updates to test this feature with.