One of the cable tabs on my MagSafe charger for my MacBook came undone, so I took this as a sign to retire the charger to home use and to purchase another for travel. When I went to my local Best Buy, they had two chargers for sale, 60W and 85W, that looked the same size, sold for the same price, etc. The only difference was that the 85W supported the MacBook Pro, while the 60W only supported the MacBook.
Even though I am a MacBook owner, I figured I might, sometime in the future, purchase a MacBook Pro and I went with the 85W charger. When I opened it up I found it was slightly larger than the 60W charger it was replacing, but for all intents and purposes there's no real difference between the two. So why does Apple sell a 60W replacement (or even manufacture the 60W at all)? Are there enough people that want to save 25W?
Best Answer
Most switching power supplies are most efficient when operating at 50% capacity. And they're definitely pretty inefficient at less than 20% capacity.
Since the MacBook draws less power than a MacBook Pro, I would suspect that an 85w power supply would work just fine, but the larger power supply for the MBP probably would draw more power from the wall socket than is necessary when connected to a MB because it is operating in a suboptimal capacity range.
Here's an example: Take for example the 45w power supply for the MacBook Air. Sure you could hook up the 85w power supply, but since the MBA might only draw 20 watts, the power supply is operating at less than 25% capacity. At this capacity, the power supply may only be 60% efficient (an educated guess), that means the power supply has to draw 33 watts of AC power just to supply the laptop with 20 watts of DC power.
Long story, short: It's a power conversion efficiency thing. I'd say buy the one that is designed for your laptop so that you save energy.