mAh (milliamp-hour) measures how much electrical charge a battery can store, but on its own it does not represent total energy; you also need to know the voltage to calculate that. This is why two batteries with the same mAh rating at different voltages can store very different amounts of actual energy.
The formula to find real energy in watt-hours (Wh) is straightforward: mAh × voltage / 1000. A 20,000mAh power bank, at a typical 3.7V cell voltage, works out to roughly 74Wh of energy.
There is also loss during transfer from the power bank to the phone; DC-DC converters typically run at 80-90% efficiency. That is why a 20,000mAh power bank cannot fully charge a 4,000mAh phone battery five times over as the raw math might suggest, more realistically it is closer to 3.5-4 times.
Understanding this math gives a concrete answer to the common question of 'why doesn't my power bank charge as many times as advertised,' and it sets more realistic expectations. When choosing capacity, it makes sense to factor in this loss and pick something a bit above your actual minimum need.
Charjett lists both mAh and real output power clearly on its power bank product pages, which makes it easier for shoppers to run this calculation themselves.