Thank you for contacting Anker via the community.
We usually do not suggest customer measure the capacity or output via the USB meter unless a qualified professional equipment is tested.
To verify whether the battery outputs its rated specs, we can test it to charge a device/phone and see how long it takes to charge.
Generally speaking, the battery is normally capable of a 60%-70% charge transfer of its capacity depending on the internal resistance of the USB cable.
Approximately 30-40% of the battery capacity is unable to be used because of efficiency problems that affect all electronics devices. These include power lost from circuit heat and voltage conversions from the battery, the charging cable and your device.
For more details on the transferring rate of external batteries, you can refer to below link:
To calculate roughly how many times the battery should charge your device you can use this formula:
(Capacity of the portable charger *0.6~0.7) / Capacity of your device = Number of possible charges
If you have any problems with your PowerCore 20100, please contact us via firstname.lastname@example.org
Wish you have a nice weekend!