Question on @Quora: Power Banks: How many times can a 5000mAh rated power bank charge an iPhone 6?

Most power banks comes with li-ion or polymer cells with a rated voltage of 3.7V.
The standard input voltage of smart devices is 5V so power banks need an output of 5V to charge smart devices.   

If I want to figure out how many times a 5000mAh rated power bank can charge my iphone 6 (3.82 V 6.91 Wh (1,810 mAh) ) what calculation do I need to make?
Do I need to account for the voltage difference even though the original specs for the power bank and the iphone 6 are done at almost the same voltage (3,7 and 3.8) ?

To make the calculation simple lets assume there is 100% power conversion though I realize this number is usually around 85-90%.

For example do I use the following calculation
5000mAh*3.7V=18.5Wh
The power of 18.5W keeps unchanged when power bank output raise to 5V.
18.5Wh/5V=3.7Ah=3700mAh
so 3700mAh/1810mAh = 1.7 times

or can I just do
5000mAh/1810mAh = 2.5 times

Power Banks: How many times can a 5000mAh rated power bank charge an iPhone 6?

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