Which MagSafe Power Adapter Do I Need For My MacBook?
MagSafe Power Adapters exist in four different wattages - 29W USB-C, 45W, 60W, and 85W. There are a few straightforward rules to determine which wattage your computer requires.
The 12-inch MacBook (first introduced in March 2015) uses a 29W USB-C adapter. As of this writing, that adapter can only be used with the 12-inch MacBook.
The MacBook Air uses a 45W MagSafe Power Adapter.
The MacBook (pre-2011 version), 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.