The correct answer is 15W, as 5Vx2A and 9Vx1.67A are available as charging modes. I own the device myself.

W=VxA
Depending on the voltage (V), the power (W) varies between 10
and 15 watts. Adaptive adapter!

5V x 2A = 10W, charging time of less than 2 hours for a 2018 smartphone