If you have a MacBook or MacBook Pro, you can use a power adapter of higher wattage than the adapter that came with the computer without problem. On the other hand, you CAN’T use one of a lower wattage because it won’t provide enough power.
Here’s what Apple has to say:
Power adapters for Intel-based Apple notebooks are available in 45W, 60W, and 85W varieties. Although you should always use the proper wattage adapter for your Apple notebook, you can use an adapter of a higher wattage without issue. For instance If you have a MacBook (13-inch Late 2009) that normally uses a 60W adapter, you can also use an 85W adapter with that computer. You would not use a 45W adapter with that computer; it would not provide enough power for that MacBook.
Using an adapter of higher wattage than the adapter that came with the computer will not cause the computer to charge more quickly or otherwise operate any differently than using the adapter that came with the computer.
Read the full support document at http://support.apple.com/kb/HT2346. It also lists which power adapter you should ideally be using with which laptop.
And remember, we lend 85w MacBook adapters in the Media Lab for up to 4 hours at a time. http://wic.library.upenn.edu/wicfacilities/lending.html