Shawn Blanc about the MagSafe Power Adapter:
Though it never stopped me, I always thought using a higher-wattage power adapter for my laptop was a bad idea. Turns out, according to this Apple knowledge base article, it’s no big deal at all.
Sometimes you might even want to use a lower wattage power adapter than the one that ships with your MacBook (Pro). For example when traveling on an airplane that has power plugs that only support a limited wattage.
This way your MacBook might take ages to charge completely, but it will at the very least keep it running.