Power Bank capacity refers to how much charge is inside a power bank battery. It is measured in mAh or milliamps per hour. When a power bank is charging a phone, some of the charges are lost in the transfer of power. In simple terms, it takes energy to transfer energy. So, the real capacity is about 70-75% of the battery capacity listed on the outside of a Power Bank has a high-quality battery. For example, a Power Bank with a quality battery at 2500mAh has a real capacity of 1750-1875mAh. Around 750mAh is lost in the transfer of energy.
The result you achieve charging your phone using the power bank can be different each time, and it is pertinent to note that this is subject to your phone/battery being of good quality and optimal performance. Most of the power banks claim an efficiency rating of over 80%. In reality, climatic conditions affect these products, and you typically end up with about 70-75% power conversion efficiency rate. It is best to err on the side of caution and expect battery loss of 25% due to general climatic conditions.
A more accurate calculation must account for this viz = power bank capacity (mAh) × 75% efficiency (due to estimated battery loss of 25%) ÷ device capacity (mAh).
Example calculation for Samsung Galaxy J5 with removable Li-Ion 2600 mAh battery and 2500mAh power bank
= (2500mAh x 75% ) / 2600mAh = it means the power bank will charge the above device from 0 to about 72% taking into account the best conditions.