MacBook – use 45/60/85W MagSafe era MacBook chargers interchangeably

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.

According to Apple knowledge base entry HT2346 the 85W power adapter is the right one. However, there seem to be issue with "T" style (the old) vs. "L" style (the new): see this thread. It seems that for some people, a workaround does the trick:

Turn off your computer, flip over the laptop and remove the battery, hold down the circlular button on the battery for 10 seconds, the green light should go out. Re-install the battery and turn your computer on. Your new charger should work fine.

If that doesn't work, you need to call Apple support to arrange for your power adapter to be replaced with a "T" style adapter:

Apple is aware of the issue with the pins being at a different depth in the new chargers with the "L" shaped connection. After having two new ones not work, I contacted tech support. They sent a new version of the old charger with the square mag safe connector. It works fine.

Edit: Apparently the MacBook SMC Firmware Update 1.4 should fix the "L" style power adapter issue on older MacBooks.