Depending on individual needs and requirements, there are several general criteria to consider when selecting a power bank:
For example if your phone battery is 2000mAh, a power bank nearly 3000mAh can charge your phone 1 time. Because battery cell is 3.7V. And phone need 5V. 3.7V/5V=0.74. Also need consider conversion rates. In the market,noramally 60-70% compare with real capacity.
b) Number of output
1 output to charge 1 device, 2 outputs can charge 2 devices.
c) Output specification
1A-1.5A output is generally for smart-phones, 1.5A-2.0A output is generally for tablets.
d) Support wireless/quick charge or not
Many new mobile phone can support wireless/quick charger.