I actually did a lot of research on this before I got one for work, anything under 8,000Mah is probably a waste of time and maybe get 1-2 full charges out of it before you need to charge it again. Seemed a waste of time to me so I went for a 20,000Mah Aukey one and have fully charged my phone everyday for a week and got a good charge out of my other phone on top of that before power bank needed charging again.
Granted it is a bit chunky but really is no problem putting it in my backpack, i now dont charge my phone off a socket anymore just charge this up once a week, best bit of tech I ever bought has lighting charge both ways/fast charge. A lot of these dont seem to put out the power they advertise so better to get a higher capacity one to start with
https://www.amazon.c...B/dp/B0183K0GZC