USB 2.0 is supposed to max out at 500mA, 3.0 at 900mA. The recommended power supply for a Pi2 is at least 1.8A. Incidentally, because of these standards many cheaper/older micro-usb cables don’t use thick enough wire to handle enough current for the Pi, creating a possible situation where you have a 2A wall charger but are still power starved.

This is dumb that it is this way and very difficult to figure out unless someone tells you. I wonder if there was a way to tap into the alert on a Pi2 and present a more meaningful message than a rainbow square. I almost feel as if this info should be presented when setting up Retropie.