Charging an iPhone, difference between USB port and power outlet
Here is a page where you have the result of the comparison of charging an iPhone4 from USB or power outlet.
Here is their conclusion:


  Stating the obvious, the iPhone 4 takes longer to get a full charge on USB than an outlet. What you may not have realized is how big the difference was, an extra 30 minutes! That’s 23% longer to get a full charge on the USB.