Ian Field wrote, on Mon, 30 Nov 2015 21:40:17 +0000:

The official spec for USB says its limited to 500mA at 5V.

That's old information.
Here is newer information:
http://www.extremetech.com/computing...our-smartphone
How USB charging works, or how to avoid blowing up your smartphone

The article says there are three types of USB 3.0 ports:
1. Standard downstream port (900mA at 5VDC = 4.5Watts)
2. Charging downstream port (1.5A at 5VDC = 7.5Watts)
3. Dedicated charging port (1.5A at 5VDC = 7.5Watts)

In addition, tablet-charging USB ports typically are 2.1 Amp
minimum, and some are as high as 2.4 Amps per port like this
charger says it is.

The problem I have is that it says it's 40Watts, which it
can't be if it's also limited to 6.8 Amps per port for 5
ports.