Of course, 20 watts charge fast. Watt is a unit of electrical power, a physical quantity that indicates how fast or slow an electric current does work. 15 watts means that the current does 15 joules per second, and 20 watts means that the current does 20 joules per second. The power is 20 watts. The current works fast. The faster the current shorts, the less time it takes when the amount of energy the panel can store is fixed. So 20w charge fast.
It's got to be 20W the more power the faster