When we consider the power of a Power bank, we must take into account the Real capacity of their batteries, for instance, if a portable USB charger with a 10000mAh (Real Capacity) claims to charge your Samsung S5 with a battery capacity of 2800 mAh up to 5 times, then it is not a justified claim because considering an average conversion rate of 80% of the its battery, if we apply this simple formula we get to know that in reality the device with its real capacity of 10000mAh can only charge the phone up to 2 times.
REAL CAPACITY * conversion rate * phone battery health / phone capacity = total charge times
EXAMPLE:10000 x 0.8 x 0.8 / 2800= 2.28(approx 2 times) or
20000 x 0.8 x0.8/ 2800 = 4.57(approx 4 times)
So check your phone, then decide what size battery pack you want because in the long run, you'll be happier!