How to convert battery capacity mAh to watt-hour? Many USB battery chargers will state it as 5000 mAh or 20100 mAh. Some of these batteries can charge a Macbook 12 inch notebook.  On the other hand, Macbook 12 inch battery capacity is stated as 

Built-in 41.4-watt-hour lithium-polymer battery

How can this mAh be converted into watt-hour: or, if the battery capacity is 20100 mAh, then how many times can it charge the Macbook for?
 A: To convert a 'power bank' rating of a number of mAh (milliampere-hours) to
an energy (watt-hours) rating requires one to know the voltage at which the power bank operates.   Some power bank mAh ratings refer to the charge that their internal power source (a 3.7V battery) delivers, but an honest rating would be the
charge delivered at the cable terminals (after upconverting the raw battery power).   For a "USB" charger, the standard for USB delivered voltage is 5V.
Or, it's USUALLY 5V; there is a new "Power Delivery" standard that allows up to 20V, intended to allow some devices (perhaps a new Macbook?) to charge quickly.
A Macbook power adapter might use that new standard to charge rapidly, but
a general purpose 'power bank' likely would not.
A pessimistic reading of a power bank 20100 mAh would estimate its total power output to be 
3.7V * 20.100 Ah = 74 Wh (watt-hours)
before allowing for efficiency of conversion of 3.7V to 5V  (for USB) and internal conversion of 5V to the laptop battery's terminal voltage.  Losses of 10% in each conversion would lower that to 60 Wh.   With perfect efficiency and the
power bank honestly rated at its 5V delivery terminals, it would give 100Wh.
