The answer to this question is: YES, you can use a higher rated power adapter with your MBA (or whatever) with no ill effect.
the power rating of the adapter is simply the maximum power that it can supply. the voltage of these adapters is all the same (I think around 18.5volts?), so it simply means the higher rated adapters are *capable* of providing higher current if necessary. some of the models require the higher current to charge their batteries, therefore those machines ship with higher wattage chargers. Your MB Air will just draw the same amount of current whichever adapter is used, as long as the minimum wattage is provided.
in the reverse case, if you use too low a wattage adapter (e.g. 45W one on a MB Pro), there won’t be enough current to charge the battery, although you can run it off AC power while plugged in.
Summary: Learn which power adapter, power cord, and plug are appropriate for each Intel-based Apple portable. Remember that size, shape, and color of replacement adapters and cords may vary.
Products Affected: MacBook Pro, MacBook Air, MacBook
Power adapters for Intel-based Apple portables are available in 45W, 60W, and 85W varieties. Although you should always use the proper wattage adapter for your Apple portable, 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.
Note: If you are looking for a PowerPC-based power adapter please use the article PowerPC-based Apple Portables: Identifying the right power adapter and power cord.
Every MagSafe adapter consists of three pieces, two of which can be in use at a time: the Adapter, AC cord, and the AC plug or “Duckhead”.