15W fast charge means that the maximum power provided by the charger is 15 watts, and the output voltage of the charger is 5V, so the output current can be seen as 3A by the formula P=U×I. The 15W fast charge is equivalent to a charger with an output current of 3A. When charging, the size of the power and the size of the charger output current is directly related, therefore, 15W fast charge can provide 3A current charge, you can quickly charge the device. Under normal circumstances, the fast charging speed is several times faster than the ordinary charger.
15w fast charge represents power, power = current * voltage, generally speaking, the greater the power, the faster the charging speed.