Currently Being ModeratedDec 13, 2012 5:47 PM (in response to Boat Guy)
The Thunderbolt Display's USB ports are 5 Volts and have a maximum current available of 500 mA (which equals 2.5 Watts maximum). This does not mean you cannot charge your iPad from the USB, it will just be much slower and require you to limit use on the iPad while charging.
Currently Being ModeratedDec 14, 2012 9:44 AM (in response to Boat Guy)
USB is nominally 5 Volts. The maximum available current I looked up in system information (/Applications/Utilities/System Information.app) for my Thunderbolt Display which is given as 500mA. Power equals voltage multiplied by current which results in 2.5W.
Currently Being ModeratedDec 15, 2012 8:57 AM (in response to JMAbeshaus)
Yes, I understand P = IE, but didn't know I could find the available current data in the Sys Info.
At only 2.5w, a full charge of the 42w/hr battery via the Mac or Thunderbolt monitor would probably take 18-24hs given the non-linear charge curve. An overnight charge might put back half the capacity.
Since the included iPad 4 charger puts out 12w, it is clearly the charger of choice!