To put real numbers to this: The iPad battery stores 25 watthours of energy. USB 1.0,1.1, and 2.0 devices may draw a maximum of 500mA from the 5v lines of the

To put real numbers to this: The iPad battery stores 25 watthours of energy. USB 1.0,1.1, and 2.0 devices may draw a maximum of 500mA from the 5v lines of the connector, this is 2.5 watts. USB 3.0 will up this to 900mA for 4.5 watts, but it isn't here yet. Actual current available at a port depends on topology. If I plug my keyboard in to my USB 2.0 computer, then plug my iPad into the keyboard it will only be authorized for 400mA, because the keyboard claimed 100mA. 2 watts. So, best case, charging an iPad from USB 2.0 will take 10 hours, if it is off. Notice this is suspiciously close to the runtime on battery, i.e. a running iPad consumes ~2.5 watts, your whole USB power allotment. Hence you can't charge while running on a stock USB port. So, what is with the 10 watt charger? Apple, and probably others, got tired of how long it took to charge and have deviated from the USB spec to allow larger charging currents. 1A chargers are common for iPhones, and at least some Macintosh models have higher current USB ports on board. I don't know how they negotiate the higher current, but a 10 watt charger could theoretically charge your iPad in under 3 hours.