MacBook – Is it safe to swap the “duckhead” connector from an old power supply to a new MagSafe 2

When measuring the usage of power adapters, it is important to keep in mind the efficiency of the PSU when making calculations.

For PC builders, it's important to ensure that you choose a suitable PSU for the components you intend to use, to ensure it's big enough etc. As such, the rated capacity is shown as what the PSU is capable of delivering, rather than what it uses. So, for example, a 600w rated PC power supply for a desktop computer that is rated at 80% (considered the minimum in order to get the "80 plus" certification) will actually use up 750w at the wall. 750W * 80% = 600W.

For laptops, the maths is often considered the other way around. As you are unlikely to be putting extra drives, graphics cards, PCI cards etc in your laptop, its internal power delivery capability should be of little interest to your average user, but the external wall-draw might be. Given that a laptop power adapter will likely be more efficient (as it is smaller, uses less power, will therefore run cooler, and require less active cooling in terms of fans etc etc), then it wouldn't surprise me to see it up around the 90% efficiency mark. Add to that the fact that your average laptop user should only really need to know the wattage rating when using the laptop somewhere unusual, say a plane, or some other mobile/portable power source, it may well be more useful to know the exterior power rating rather than the internal delivery rating. Therefore, 85W at the wall * 90% efficiency is 76.5W in the laptop, and likely explains the discrepancy.

You can safely use the 85W adapter on any Mac, it will only draw as much power as it needs. The 45W adapter from the MacBook Air may just about manage to power your MacBook Pro but it won't charge it. It will not damage your Mac, but you may notice the battery may still deplete when using it.

Here is the information, straight from Apple: