I Just Cant Do It Captain – I Do not Have The Power
AC Access Points are hungry beasts and require a lot of power, unfortunately the power required is in most cases more than an 802.3af PoE switch can provide. This leads to expensive upgrades as switches need to be replaced with expensive 802.3at equivalents.
However (and here is the kicker), there is a high chance that the person selling you your funky new AC Access points, is unlikely to tell you that without 802.3at power you will not get the full benefits of the faster speeds available. Now this may well be due to them not knowing this fact as most AC Access Points are advertised by the manufacturers as being 802.3af compatible.
Granted this is true, and they will run from 'af' power however what isn't made quite so clear is that fact that without 802.3at power, your APs will be running in a crippled state, and you may not even realise it.
As an example, in order to get the most from 802.11ac when running the latest Aruba access point, you need to run the 2.4GHz in a single stream mode to free up power for the 5GHz Radio.
Cisco also state on their AC module datasheet that the power draw is 18 watts and thus 802.11at power is required.
A quick look at Ubiquiti yields the same result - 'at' power is required.
In fact, the only company I know of who has stated that their APs should be 'af' compatible without having to compromise on features is Ruckus (who have yet to release their first AC Access Point).
For reference, the difference between 'at' and 'af' power is a step up from 15.4 watts to 25.5 watts.
So as you can see, when upgrading to AC, you also need to consider the expense of new switches or running all APs from PoE injectors or mains cables (which can be messy).