An Apple computer started up to Windows via BootCamp will not provide extra power. On those computers, the second or third port is enabled when an appropriate device is connected. Some Apple computer and displays may offer the ability to operate more than one USB port at 1100 mA at 5 V. The remaining ports will continue to offer 500 mA at 5 V. The port providing extra power is determined by the first Apple peripheral or device to connect to the Apple computer or display that requires power beyond 500 mA at 5 V. If your Apple computer or display is powered off, no power will be provided. If your Apple computer or display is asleep, all ports will provide a maximum of 500 mA at 5 V. 
Your Apple computer or display must be powered on and must be awake. Apple peripheral devices connected to hubs will not have access to extra power above the standard USB specification of 500 mA. An Apple peripheral device must be plugged directly into an Apple computer or display. This power is available under certain conditions: 
To meet requests for additional power from these Apple peripherals and devices, some Apple computers and displays can provide up to 1100 mA at 5 V through the port to which the Apple peripheral or device is connected. Apple has a support document on this topic.
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