I've had a lot of experience with power banks, cycle camping in france, trying diffrent one, This power bank I use will give you it's true value of power, most unbranded power banks will only give 55% of what it advertises at best, also the unbranded loose its ability to hold power in there battery's quit quickly, it's charge cycle as claimed may be 2000 or more times but in reality it will be 600 to 800 before you get rid, so value for money you are far better off buying a branded power bank.
I'm no expert on these it's only because I've wasted my money to get to this point,
My power bank I us is the romoss 20000mah 22.5w,
The tec side I have no idea, what it gives me is, I can ride all day on my heated jacket and at the end of the day still able to charge my phone fully,
the only thing it doesn't do what it says it can, recharge in 2 to 3 hours hours, it takes 4 to 5 that's because it gets to 90% in 2 hours then goes onto slow charge, I think that's to give greater life to the battery cells not over charging. After 2 hours of charging you still have 90% charge
Romoss do the same power bank, same size but with a 18w instead of the 22.5w but I like the fact I can fully charge my phone in one hour, where the 18w takes a little longer, not sure if that would make the units charge last longer on a heated jacket, probably not.
It's not small but will just fit in the keis battery pocket.
Hope you find this useful, if you already have a power bank us that to see if it works for you before spending £100 on a decent one