Essager 20000mAh/3.7V is the real capacity of the powerbank. But when charge to your phone, the output is 5V-12V, it is impossible this powerbank can provide all its power to phone battery. Taking into account losses in the system as well as the cables and connectors, this model can give about 13000mAh of output.
Product name: Essager 18W 20000mAh PD + QC Quick Charge Power Bank
Battery capacity:20000mAh/3.7V 74Wh
Battery type: Ploymer lithium battery
Input interface: PD usb * 1 + Micro usb * 1
Output interface: QC3.0 usb * 2 + PD usb * 1
Input : 5V/3A ; 9V/2A ; 12V/1.5A Max 18W
Ouput: 5V/3A ; 9V/2A ; 12V/1.5A Max 18W
Weight : 365g
Size: 137.1 * 71.4 * 25.8mm (5.4 * 2.8 * 1.01 Inch)
Package include 1 x Power Bank
1 x USB Cable
Low power output:Yes
Note: Low power output is to charge small device such as miband, bluetooth headset, etc.