Why does Apple not support Power Nap on some of the Macs that have Power Nap capability?

Solution 1:

Only the MacBook Air (Mid 2011 or newer), MacBook Pro with Retina display were originally supported supported for Power Nap. The reason for this is An SMC update is required to gain all the functionality of Power Nap, and these are the only models Apple has chosen to actively write the firmware updates to support the feature. In other words, it's not that Apple runs some exclusion script to keep other models out; it takes an active update to the firmware to make a model run Power Nap, and they've chosen to do that for two particular lines of computers.

In a subsequent firmware update, Apple included Mid 2010 MacBook Air models in Power Nap.

As far as why they've made that decision, we enter the realm of speculation, but fortunately, we have some good information here.

The idea behind Power Nap is that when the computer appears to be asleep, from time to time, it actually wakes up and performs various tasks. This waking, however, is not noticeable to the user because the computer remains dark and silent. This means no audio or video is activated. It also means, of particular relevance to this question, the fan is also kept off.

This requires a storage device that can run without a fan and a processor that can run without a fan (sorry notoriously hot G5 — I doubt there would ever be a way to support you for this).

According to John Siracusa's epic Mountain Lion review, the Dark Wake mode that Power Nap invokes does not turn on the fans. Thus, only devices Apple knows are designed to be able to access their storage without risk of overheating (as a spinning hard drive might) will take advantage of Power Nap. Siracusa suggests that third party SSDs might have a different heat profile.

While SSDs could be installed in any computer, and Apple could perhaps write firmware updates for all their Macs so that Power Nap checks to see if the computer has a SSD drive and if so, runs Power Nap, they haven't chosen to do so. Instead, they've made Power Nap work only on those models of computer that they know will have a SSD drive.

The limitation might not be drive-based; it could be the processor that can't run silently. This seems slightly less likely to me, given the supported models, but it could explain why the 2010 MacBook Air is excluded. Power Nap monitors temperature, and if the system gets too hot to run without cooling, Power Nap disengages, but if the processor in earlier MBAs was triggering such an alert too often, that may be why Apple removed them from the list of models that support Power Nap.

tl;dr: likely it's hardware; specifically, heat.

Solution 2:

The why did Apple engineers X question is always a hard one to peg, but the simple answer is that a very small subset of Mac hardware is targeted for the initial Power Nap implementation.

Specifically, the Sandy Bridge and newer Air and the retina MacBook Pro all have non-user replaceable RAM and SSD so Apple has great certainty that there will be no oddball ram timing issues or storage related timeouts. Furthermore, these models all have intel integrated graphics and intel chipsets that were designed for this sort of "operation while sleeping" to perform network operations in a low power state.

Macs have long had the ability to wake up using their internal clocks from the Energy Saver preference. Next, they were programmed to wake periodically to maintain DHCP leases and talk with Airport hardware so that the routers could proxy bonjour sharing requests while the Mac slept. Power Nap takes this ability of the network interfaces to remain alert while in a very low power state one step further. Now the CPU, storage and network stack (but not the fans or display and keyboard lights) power up on an hourly basis to see what tasks can be accomplished before returning to sleep.

If you look at what Power Nap does - it is clear that the functions are designed to use as little power as possible:

• App Store updates are only triggered weekly
• Only one time machine backup is completed while sleeping. After that, it stops trying to back up while that sleep lasts
• Software Updates (security) trigger once daily
• The rest of the items like Photo Stream, Mail, iCloud documents and synced data are an hourly refresh while sleep + nap continues
• Napping activity stops once the battery level has 30% full charge capacity remaining

Even the name Power lets you know that the primary focus is preserving power but still do the things that you will have to do at some point. Others have speculated that heat is a design issue, and in so far as heat comes from using battery power I would agree that heat is a byproduct of the primary design limitation of Power Nap.

Anyone that has used a MacBook Pro in clamshell mode or had one wake up while shut in a bag is aware that these machines run just fine at temperatures that most people are alarmed and worry about light burns / pain while holding.

Only time (or some public statement, lawsuit deposition or leak) will show us which other models gain Power Nap functionality going forward and whether my reasoning is on the mark about why three models only support Power Nap at launch.