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! Thats 23% longer to get a full charge on the USB. If youve 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.