It is easy to find that “Input” and “Output” are printed on a wireless mobile charger or its package as below, some manufacturers prefer to mark them in terms of “V/A”, and some manufacturers prefer to mark them in terms of “W”, W=V * A
Here is a simple picture to help you understand.
“Input” usually can be easily tested via a USB device, but it is difficult to test “Output”. As we wrote before that the conversion ratio of wireless charging keeps at 60-75%, the higher the better, it means that Output=Input * (60-75)%.
Seen from the picture below, we can find that the “Input” is 9.14V / 0.56A, equals to 5.12W, so finally we can speculate that the “Output” is 3-3.8W, exact value is related to the smart phone. “Input” is changing at every moment, so is the same for “Output”.
“Input” and “Output” printed on a wireless mobile charger or its package are not exact values. Some manufacturers claim that their wireless chargers are 10W, 10W is just the maximum value of “Output”.