Up close with Mountain Lion: Power Nap

More than a few Mac users worry that OS X is becoming too much like iOS, thanks to the former gaining features obviously inspired by the latter. But even the most anti-iOS Mac user has to admit that sometimes this is a good thing. To whit: With our iPhones and iPads, we've come to expect that even when the device has been asleep, waking it will immediately present us with our latest email messages, events, reminders, changes to contacts, and more. These devices will even back up to iCloud and sync with iTunes when unattended. Under Mountain Lion (OS X 10.8), if you've got a compatible Mac laptop, you'll get many of the same benefits thanks to a new feature called Power Nap.

Which Macs are compatible? Currently only the Mid 2012 MacBook Pro with Retina Display and the Mid 2011 and Mid 2012 MacBook Air models. Each of these models requires a SMC firmware update (Mid 2011 Air, Mid 2012 Air, Mid 2012 Pro Retina) to support Power Nap.

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How do I use Power Nap?

There are two places you enable Power Nap's features on a supported Mac, both in the Energy Saver pane of System Preferences. In the Power Adapter tab is the option, enabled by default, to Enable Power Nap While Plugged Into A Power Adapter. In the Battery tab is an option, disabled by default, to Enable Power Nap While On Battery Power. The settings are independent of one another--you can choose to enable both, disable both, or enable only one or the other. (I cover each setting below.)

Once you configure those two Power Nap settings, you don't have to do anything for Power Nap to do its job, and your laptop sleeps normally whenever you close the lid or use the Apple Menu's Sleep command. But assuming the computer is connected to a power source or has sufficient battery charge, it periodically--once per hour--wakes up and connects to your wireless or wired network to perform a variety of tasks that will let you get right to work when you get back to your Mac.

Even better--both for energy conservation and because many people keep their laptop in the room where they sleep--Power Nap works without turning on the screen or fan(s), without producing any system or application audio, and, according to Apple, without significantly affecting your laptop's battery life. (In my use of the feature, I haven't observed a noticeable drop in battery life due to Power Nap performing its tasks.) In addition, if your battery level is 30 percent or less, Power Nap suspends its tasks until you connect your laptop to its power adapter; similarly, Apple says that if your Mac's internal temperature gets too hot, Power Nap is suspended until the computer cools down.

Note that your Mac's USB, Thunderbolt, and FireWire buses provide power during Power Nap wakes, so if you've enabled battery-power Power Nap and you want to ensure your Mac uses as little battery charge as possible, you should disconnect external devices--except for your Time Machine drive, of course--before putting your Mac to sleep.

What tasks does Power Nap perform?

When your MacBook is running off battery power, Power Nap is limited to only a few minutes of activity per wake, so the list of tasks it performs includes things that don't require a lot of juice or extended wake time:

  • Checks for new email and Messages messages (hourly). This feature appears to require that Mail and Messages, respectively, be running.
  • Updates contacts, reminders, notes, and iCloud documents with any changes you've made on other devices (hourly).
  • Updates your calendars with any changes you've made on other devices or any new invitations you've received (hourly).
  • Updates your Photo Stream with any newly added photos (hourly).
  • Checks for any communications from Find My Mac (hourly). According to Apple, the Play Sound and Send Message features of Find My Mac will not make a sound during Power Nap wakes.
  • Checks for configuration-profile updates (hourly)--for Macs in a managed environment.

Apple says that Power Nap supports VPN on demand, letting your Mac perform these updates even if your company requires you to connect through its VPN. (I wasn't able to test this feature.) However, this option is restricted to VPN connections that authenticate using a certificate; Power Nap won't automatically connect to a VPN that requires you to enter a password when connecting.

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