Apple - Retina macbook max USB power output

With the System Profiler you'll be able to know how much power does the USB delivers, and how much is required for each device connected to a USB. Under the Hardware tab, you'll find, listed, all the USB ports, and, within, all the devices connected to them. Just select one of those to get detailed information.

To open System Profiler just choose Apple Menu / About this Mac and clicking More Info, or go to Applications / Utilities.

Macs with USB 2.0 from the last few years can generally provide 1.0A via USB. The USB 3.0 standard specifies 900mA; I suspect that USB3 Macs still provide 1A or even a little more. Note however, that in order for a device to actually draw that much, it must specifically request it from the bus. Apple's iDevices certainly do this, as you can see in System Profiler (as mentioned in the other answer). For a USB 2.0 device to request more than 500mA, it must implement a special protocol extension, which few devices follow. The reason those devices might still draw 1A from a pure charger is because they detect that the data lines are not connected in the usual way.

Likewise, the iPad only draws 1.0A when connected to a Mac (or even 500mA from most PCs), but 2.0A when connected to the special iPad charger. The charger has its data pins pulled to special indicator voltages via a potential divider, and the iPad only draws that much current if it detects this configuration to avoid damaging whatever it's plugged into.

With a MacBook Pro (Retina, 15-inch, Early 2013)

I got this when charging an iPad, which is 2100mA in total.