A device (anything, a phone, a light bulb) will draw only what it's designed to draw. Your house is a source of, probably, 22,000 Watts, but when you turn on a 100 Watt light bulb, it draws only 100 Watts.
The phone and the charger "negotiate" over the data lines (USB consists of power wires, data wires and a few others) to determine the maximum voltage the phone can accept, and the charger sends the phone current at (or below) that voltage. (Except in an old "non-fast-charger" - then the voltage is 5 Volts, it can't change.) An 18 Watt charger can supply 9 Volts at 2 Amps, but it can also supply 12 Volts (probably at 3 Amps - which is 15 Watts, and probably what your phone wants), and that's what will happen.
03-23-2019 12:01 PM