According to the USB standard for charging power the maximum allowable amperage for a 5 volt connection is 3 Amps or 15 Watts.
Please remember that's not how much a charger must
make available to a peripheral, it's the maximum
possible configuration a peripheral can expect, IF
the charger is capable of providing it. "Just because that cup comes in small, medium, large, and extra large, doesn't mean ALL drinks served are available in extra large".
Drop resistors on the data lines are supposed to inform the peripheral the maximum amount of current they're permitted to draw. If they choose to draw more, (since current is supplied on demand
, the peripheral doing the demanding) the only recourse the supplier has is to shut off the power, and sometimes pop up a warning. Cheaper chargers will "sag" the voltage to keep up with the current demand because they're limited to total power(watts) delivered (watts = volts x amps) but that violates USB specifications.
I have a USB charge bar in my truck that has a total capacity of 50 watts. It has two 2 amp (10w) ports, and five 1 amp (5w) ports. So total draw shouldn't exceed 45w. That does NOT mean you can draw 3 amps (15w) from any
of the ports.