Even if you exceed 20W it doesn’t matter. The only real thing that matters is the average draw and the total duration of a session. If you spend most of the time drawing 10W and peek at 50W for a minute then you can run off a 20W powersupply forever.

But a faster power supply does offer you more benefits. You can play unplugged until the battery is low, plug it in and continue. It will charge sooner letting you unplug it again sooner. If the wires bother you then that can be a big advantage.