Some good looking stats for the Anker 737 powerbank 'powercore 24k'
This is playing BeamNG drive on Italy with traffic and mods enabled at medium graphics/detail settings. A very demanding game (Deck screen 50%, volume 50%)
1h 48m remaining on BeamNG drive - 86% Deck battery
Connected the 737. 98% 737 battery ~2 hours until 737 battery drained. 24mins projected Deck charge time.
This is pulling 40W.
I just played for ~15 mins and the Deck is now at 93% with the 737 battery at 88%
So I guestimate it provides 2 - 2.5 hours additional play time on AAA games. ~10% 737 battery per 15 mins play time / 1hr for 40%
Maybe more as the Deck would reach 100% or close to 100% and battery optimisation would kick in and 737 wattage pull would reduce right down to whatever it needs to keep the game running and deck not losing power.
A bit more testing
11:06
80% battery
98% deck - 15 mins until charged
11:23
70% battery
99% - 9 mins until charged
10 mins - 17%