I have seen a lot of people argue you must use a 45W output power bank, probably because it’s the charger output Valve recommends for continuous play.
I have yet to pass 30W during play (admittedly I play with 100℅ brightness, not 120%). It will charge at a miserably slow rate, but if I start a session at full battery, I also end the session with a full battery.
If you own a 30W power bank already, it is not some strict requirement to upgrade it for the frame, just make sure to use the right output port.
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.