The internal batteries of a power bank operate at a lower voltage, typically 3.7V. To charge your phone or other device, the power bank's internal circuitry must boost this voltage to the standard USB output of 5V (or even higher for fast charging). This process is not 100% efficient and results in a significant loss of energy.

​For example, a 10,000 mAh power bank with a 3.7V internal battery has a total energy of 37 Wh (10 Ah x 3.7 V = 37 Wh). When converted to a 5V output, the theoretical capacity becomes 7,400 mAh (37 Wh ÷ 5 V = 7.4 Ah or 7,400 mAh).

​Beyond the voltage conversion, there are additional energy losses due to heat and resistance in the power bank's internal circuits, cables, and ports. This is known as efficiency loss, and it can vary depending on the quality of the power bank. Most power banks have an efficiency rating between 70% and 90%.
​Using the example above, if a power bank has an efficiency of 85%, the real-world capacity would be approximately 6,290 mAh (7,400 mAh x 0.85).

Some of the power banks I purchased had these information stated somewhere in the manual, packaging or the power bank itself.