The 15-watt charger is fast charging, which is the use of measures to increase the voltage and current at the same time to achieve the effect of fast charging, a charging system composed of adapters, cables and terminals, starting from the initial charging state, to charge for 30 minutes, during which the average current entering the battery is greater than or equal to 3A or the total charge is greater than or equal to 60% of the rated capacity of the battery. So 5V3a is a fast charge.
It's a fast charge, but it's low now. Now it's 40 to 60W fast charge.
15 watts is an ordinary charger, the power is not fast, and the standard of fast charging is generally more than 18 watts.