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There's really no difference, it's all personal perception. The wall charger puts out exactly 500ma, same as a powered USB port. I just tested it with a multimeter earlier today. It's going to charge just as fast on the wall outlet as it will on your USB port. :)
The wall charger puts out 845ma where the usb port caps out at 400ish ma. So its actually twice as fast.
 
There's really no difference, it's all personal perception. The wall charger puts out exactly 500ma, same as a powered USB port. I just tested it with a multimeter earlier today. It's going to charge just as fast on the wall outlet as it will on your USB port. :)


the wall charger (at least when you have an original!) puts out 1000mA at 5V and a usb-port on a computer or a cheap usb charger puts out 500mA, if they can reach that level (usb-ports tend to get less and less power if the computer is used heavily).
The charger of my iPad is 2000mA and charges even faster then the wall-charger that came with my iPhone 4. I can't tell how much faster, but it is noticeable.
 
the battery caps the input to prevent overcharging, you could plug the thing into a car battery and it won't charge any faster or slower
 
Wanna have some real fun? Plug it in to the iPad charger. It will charge the phone noticeably faster than the iPhone wall charger and hasn't blown the phone up yet...
 
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