Using Power banks is normal in the world of smartphones, but in the laptop-space they are not seen that often. One of the reasons is that only a few battery packs have enough power to charge a laptop. Hoping to profit from accessory sales, the PC manufacturer Lenovo has now released a power bank made explicitly for its ThinkPad and IdeaPad laptops.
https://www.notebookcheck.net/Lenovo-releases-a-USB-C-power-bank-for-ThinkPads-IdeaPads.306164.0.html
"The most important specification of a power bank is of course its capacity."
I may be in the minority, but I disagree. Charging speed is the most important specification (i.e. quick-charging specification compatibility).
Too slow, and many devices simply will not charge. Slowly charging leads to throttled performance e.g. in my XPS 15 9560, and it just takes longer.
There are very few 100W USB type-C chargers available yet despite our devices and cables widely supporting it. AFAIK, there are no vanilla chargers or power banks that support this speed, only docking stations.