Xiaomi 20000mAh is the real capacity of the powerbank. But it is impossible this powerbank can provide all it's power to phone battery, taking into account losses in the system as well as the cables and connectors. As stated by Xiaomi company, this model can give around 13000mAh of output.
Product name :20000mAh Xiaomi Power Bank 3 Model number :PLM07ZM Battery capacity :20000mAh 74Wh 3.7V Battery type :Li-polymer Battery Input interface :USB-C Output interface USB-A/USB-C Input :5.0V/3.0A ; 9.0V/3.0A ; 12V/3A ; 15V/3A ; 20V/2.25A Ouput : 45W MAX USB-A (single port): 5V2.4A / 9V2A / 12V1.5A USB-A (dual port): 5V3A USB-C: 5V3A / 9V3A / 12V3A / 15V3A / 20V2A Three ports: 5V5.4A / 9V3A / 12V3A / 15V3A / 20V2A Weight Approx. 400g Color :Black Low power output :Yes Size : 153.5x73.5x27.5mm
Package include : 1 x Power Bank 1 x USB Cable 1 x instructions (in Chinese)
Note Low power output is to charge small device such as miband, bluetooth headset, etc.