mWH or watt-hours is the ideal way to measure a battery's stored energy as it is voltage-independent and takes into account the total energy of the battery. So a power bank with 10000 mAH capacity actually has 10000 mAH capacity at 3.
Consider a power bank with an energy content of 37 Wh and a capacity of 10 Ah. Compared to the residential battery System A with a capacity six times as large, the energy content of the power bank is as much as 264 times smaller. This is due to the difference in internal voltage, as the power bank battery voltage is only 3.7 V.
The voltage is monitored with a voltmeter for a determined number of hours according to the power bank capacity. If the power bank battery lasts for the same number of hours as listed in the capacity, then it is the actual capacity. In reality, this capacity is less due to power losses.
A current of 1Amp or 1000mA will circulate through it as 5V is the standard USB output. The voltage is monitored with a voltmeter for a determined number of hours according to the power bank capacity. If the power bank battery lasts for the same number of hours as listed in the capacity, then it is the actual capacity.
If you are using a power bank in a high-temperature environment then the efficiency rate will drop. That means the power bank will lose more power trying to convert the voltage. It's best to use a power bank in a cool temperature area. Therefore, the real battery capacity depends on the quality of your power bank.
This difference between the battery voltage and the power bank output voltage is the reason why the capacity of a power bank at its USB output port is different from the capacity indicated on its internal battery. For example, a 10000mAh power bank would have a capacity of 7400mAh at its USB output port at a charging voltage of 5V.
But that's not all! The real capacity of the power bank is even smaller!! This is because of yet another factor that needs to be accounted for: power losses. As previously mentioned, power banks have a native 3.7V, but they actually need to supply 5V.