Short Story: Get at least a 30W (20V at 1.5A) USB-PD power bank. Most of them have a maximum 45W or 65W rating (20V at 2.25 or 3.25A), but make sure it have 20V output. Capacity is at least 10Ah and up to 30Ah (at 3.7V). Max capacity (allowed by aviation regulation) is 100WH, which is about 27Ah at 3.7V.
In 2013, Dell ship their 13 inch XPS with a 47WH battery. In 2018, this same 47 WH have upgraded to a 65WH. Battery technology improvement. Also, it’s a 7.4V, 4-cell battery (2series of 2 parallel), which is quite interesting because most other laptops run a high-voltage solution (e.g. 4 in series at 14.8V) or a large-cell 3-cell 11.1V approach.
The other half of the story, is that framework laptop battery is indeed quite small. Because they need to make room for all the slots for all the modules (external connectors included), which takes a toll on the amount of space the battery have.
I’m not sure how many WH is the battery, but a good guess is that it’s around 50WH rather than 60WH. And typically, a 99WH battery (limit is 100WH) can get you past 12 hours of office, which is way more than needed.
Tricks to reduce power consumption include dimming the display, sleeping the computer when not needed, turning off the display, closing unnecessary apps (and stop unnecessary background services), reduce memory speed (if possible), slowing the fans, make sure the CPU temp is low, etc.