Apple's got a 90, 60, and 45 watt PSU for their various notebooks (unless they have larger I'm not aware of). And...all three cost the same price. I'm just wondering-are they interchangeable? Like a system that comes with a 45, can it use a 90? Or a 60 a 90? I'm just thinking if they're all compatible, there would seem to be no reason for me to buy a 45 watt PSU for an air (or a 60 for a 13" Pro) if the 90 works, as then I might be able to use the 90 on more systems down the line? EDIT: Sorry, I should clarify that I know I shouldn't use a smaller one to power a larger one, but I was hoping it was okay to use a larger one to power a smaller one, if that makes sense. Wasn't sure if Apple used the same connector, or was doing something else weird, but it sounds like they're all the same, so you can buy a 90 for the Air or 13" Pro!