Which MagSafe Power Adapter Do I Need For My MacBook?
MagSafe Power Adapters exist in three different wattages - 45W, 60W, and 85W. As of late 2011, there are a few straightforward rules to determine which wattage your computer requires.
The MacBook Air uses the 45W MagSafe Power Adapter.
The MacBook, every MacBook Pro 13 inch, and the 2.53 GHz MacBook Pro 15 inch (mid 2009) use the 60W MagSafe Power Adapter.
Other than that one MacBook Pro 15 inch model listed above, every other MacBook Pro 15 inch and every MacBook Pro 17 inch uses the 85W MagSafe Power Adapter.
You can use a higher wattage than is permitted, but you should not use a lower wattage. For instance, the MacBook Pro 13 inch uses a 60W MagSafe, but you can use an 85W MagSafe to charge or power it (although it will not charge any faster). You should not use a 45W with the MacBook Pro 13 since that is lower than its 60W requirement.