MacBook – Can i use the old MacBook charger on the new Mac

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.

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.