All iBooks and 12" PowerBooks use the 45W adapter. All 15 and 17" AlBooks use the 65W adapter. Some TiBooks use 65, others 45W. PowerBook G3 and earlier models use 45W or less adapters.
Did you ever get your adapter replaced under warranty, or did you buy a spare? You might have gotten a 65W one that way. (It won't hurt anything, of course.)
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