Originally Posted by
Loren Pechtel
A charger with a higher power rating than the device should be ok. A charger with a lower rating is another matter--it should not be done.
Actually, if a device wants more power than the charger can provide, they will "agree" on the charger just providing its maximum output. This is the USB spec - when you plug something into a charger, the device identifies what it's plugged into and determines how much power to draw.
The only time an over-powered charger would matter is if it doesn't have good voltage regulation and responds to a lower draw by producing more voltage.
In other words, a really badly designed charger. Avoid the $2 "Genuine Apple Chargers" and you will be fine.