System Information provides additional detail about USB power on your Mac. Connect the USB device directly to your Mac, then select USB from the sidebar in System Information. For each USB device, System Information shows:

Current Available: The default power provided by the port to which your device is connected.

Current Required: The power needed by your device.

Extra Operating Current: Mac computers and Apple displays introduced after 2006, when directly connected to certain Apple devices such as iPhone or iPad, can provide extra power over one or more ports. The Mac or display must be powered on and awake to provide this extra power. A Mac started from the Windows operating system doesn't provide extra power.

Learn more about the power available under each USB specification, such as USB 2 and USB 3.