i understand the usb2.0 standard is 500ma. i cannot test how much my pc usb output is. i guess it is not 500ma as both pcs i tried has 8usb ports. i guess it's more like usb 'gate' issue for pcs not the ipads.
i asked around that ipad2 can be charged by pc usbs. i guess the bigger battery in ipad3 makes the difference...
The quickest way (and really the only way) to charge your iPad is with the included 10W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter (5W). When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge very slowly (but iPad indicates not charging). Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
Apple recommends that once a month you let the iPad fully discharge & then recharge to 100%.
At this link http://www.tomshardware.com/reviews/galaxy-tab-android-tablet,3014-11.html , tests show that the iPad 2 battery will charge to 90% in 3 hours 1 minute. It will charge to 100% in 4 hours 2 minutes. The new iPad has a larger capacity battery, so using the 10W charger will obviously take longer. If you are using your iPad while charging, it will take even longer. It's best to turn your new iPad OFF and charge over night.
So further research is here ---
per this document, i plugged the new ipad into the macbook, and run the system information
the numbers are here
Current available -- 500mA
Current required -- 500mA
Extra opertating Current -- 500mA
Thus i assume the minimum requirement is 1A.
So the total is 1A from the laptop.
The 10W charger is 5V at 2.1A
the 5W charger is 5V at 1A
I assume most after market ones cannot come close to 5V 1A as standard USB is 500mA. Luckily one of my car cigarette charge provides some power near 1A so it charges.
Let the search for cheap 3rd party 5V2A chargers (as backup) begin!!!!