You have an original real capacity 10,000 mAh powerbank, and that should charge your 20,000mah phone about 5 times, right?

Well, here is where the confusion begins. This powerbank has 4 3.7V 2,500mAh batteries, together that equals 10,000mAh and this is correct. Modern slim power banks have thin lithium polymer batteries so are still based on 3.7V but can be built in smaller sizes.

But USB is 5 volts! Inside powerbanks are 3.7V batteries, but the USB standard is 5V.  

Between the battery and the USB socket is a conversion circuit and this changes the 3.7 V into USB friendly 5V. When converting to a higher voltage, you must also convert the mAh into the new voltage.

How to calculate theoretical USB output?

A simple equation can be used to convert the 3.7V into 5V. ACTUAL 5V mAh = 3.7 X Advertised mAh / 5
For a 10,000mAh powerbank 3.7 X 10,000 / 5 = 7,400 mAh
So a 3.7V 10,000 mAh powerbank really only supplies 7,400 mAh output at the 5V USB connection.

So straight out of the box is a 23% reduction in the stated mAh.  This is not the actual level as there is also conversion loss.

What is conversion loss?

As you use your powerbank the circuit inside that converts 3.7V to 5V USB uses some energy and also creates heat. During this conversion, you lose some extra mAh. 

There is a wide range in conversion efficiency and most brands don’t state the losses, Cager has prized themselves on their conversion efficiency chips which are up to 93% efficient, meaning you only lose 7% off your battery power in the conversion. Some others can consume as much as 10% during conversion.