NOTE:

Portable Battery charger case has two regular standard of capacity , one is the battery capacity, one is the rated capacity. 
 
Why exist this difference?
 
The most important reason is the conversion rate. Because most of power bank's battery voltage is 3.7V, and power bank needs to increase the voltage to 5V when charging the mobile phone, which making the power bank's conversion efficiency only 3.75/5=74%, Also include generated heat when battery voltage boosting, and various of wire consumption, Electricity and self-consumption. these points add up to account for 30%-40% of the power of the mobile power source.
So the 5,000mAh power bank can really be used by us only about 3000-4000mAh.
 
Also, The conversion rate of the entire mobile power industry is basically in the range of 60%-70%. Please kindly noted.
 
Please search on Google if want to know the details!