The external battery has two regular capacity standards, one is the battery capacity, one is the nominal capacity. The 20,000mah external battery, being labeled, the core capacity of 10000mah / 3.7v becomes a nominal capacity of 12400mah / 5v.
Why does this difference exist? The most important reason is the conversion rate. Because most of the power bank battery voltage is 3.7V, and the power bank needs to increase the voltage to 5V when charging the mobile phone, which makes the conversion efficiency of the power bank only 3.75 / 5 = 74%, also includes heat generated when the battery voltage is boosted and various cable, electricity and self-consumption.