Take a look at this chart from When Will Apple comparing the amount of time it takes to charge an iPhone 4 by USB versus wall outlet.
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. If you’ve ever had a completely drained iPhone, you know it takes a bit of time before it turns back on. Plugged into an outlet, it took 3 minutes 11 seconds to come back to life. On USB it took almost 2 minutes longer, taking 4 minutes 49 seconds.
A USB port can supply 500mA charge current.
The wall outlet charger is 1A charge current.
So obviously the wall outlet charge is capable of charging the iPhone twice as fast, that is if the charge circuitry and battery is capable.
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