Why isn't my Mac supported?
Only a few Mac models currently support Power Nap, and those that do require a firmware update. But why don't more Mac models support Power Nap?
The answer relates to power usage, noise, and heat. Power Nap works whether your Mac is running off AC or battery power, it works even if your laptop's screen is closed, and it performs its magic silently. This means Power Nap must use minimal battery power--no one wants to put their MacBook to sleep at night with a good amount of juice remaining, only to open it up in the morning to an empty battery. It also means that Power Nap's activities must not cause your Mac's components to produce a lot of heat, as, for power-saving and noise reasons, your Mac can't turn on its cooling fan(s). Finally, your computer's components must all be able to work together seamlessly to automatically wake up, allow OS X to perform Power Nap's various functions, and go back to sleep--again, silently and efficiently. Put these requirements together, and you get a feature that depends on flash storage, cool-running processors and graphics chips, and very specific hardware combinations.
For starters, as Apple's Mountain Lion features page states, "Power Nap requires a Mac notebook with built-in flash storage." You'll notice that the text doesn't say "solid-state drive" (SSD), but rather "built-in flash storage." That's because Power Nap currently requires on-board flash storage--flash-memory modules connected directly to the logic board. Even discrete SSDs, the kind that Apple has used in several MacBook and iMac models and that you can buy to replace a 2.5-inch hard drive, apparently require too much power or produce too much heat.
The supported Mac models also all include an Intel integrated graphics chip, which uses less power than a discrete GPU and is better integrated with the CPU's energy-saving features. (The Retina-display MacBook Pro includes both the integrated Intel HD Graphics 4000 and the discrete NVIDIA GeForce GT 650M.) On these Macs, the computer can essentially wake up without waking the GPU.
Finally, the currently supported models all use Intel's Sandy Bridge or Ivy Bridge processors. What's interesting here is that, in addition to running cooler than previous processor architectures, these processors support Intel's Smart Connect Technology, which is specifically designed to support Power Nap-like features. I haven't been able to confirm that Apple is specifically using Smart Connect for Power Nap, but it's worth noting that the Late 2010 MacBook Air, which would otherwise seem to offer the right mix of hardware for Power Nap, doesn't support Smart Connect--and isn't currently Power Nap-capable.
(Note that Apple's Mountain Lion specs page originally listed the Late 2010 MacBook Air models as being Power Nap-compatible. In fact, during the developer-preview stage of Mountain Lion, the 2010 MacBook Airs actually received the necessary firmware updates for Power Nap support. However, the 2010 Airs were removed from Apple's list of supported Macs on July 25, the date of Mountain Lion's release. A discussion on MacRumors.com provides instructions for using those developer firmware updates to enable Power Nap for these models, but I recommend against doing so, as we don't know why Apple decided not to support them.)
It's possible that Apple is initially limiting Power Nap to Macs that ship with non-upgradeable flash storage and RAM so the company can test Power Nap on known, consistent hardware before extending the feature to other power-efficient Mac models. If that's the case, Apple could eventually provide firmware updates that enable Power Nap on other Mac models. We'll see. But it may very well be that Power Nap will always be limited to specific Macs that satisfy very specific hardware requirements.
Assuming your Mac qualifies, what Power Nap means is that whenever you wake your laptop, all your updated data is there, ready for use; any software updates are ready to be installed or downloaded; and your backups have been updated, so you can pack up and be on your way. In other words, Mountain Lion took a great feature from iOS and made it better for the Mac. It doesn't seem to be working flawlessly for everyone yet, but it's already a welcome addition to OS X in my testing, and I look forward to future improvements.
Dan Frakes (@danfrakes) is a Macworld senior editor.