USB Power Bank Capacity explained - is it a ripoff?

Check out my deal/sale tracker for up to date deals on gear http://bit.ly/2svRpvn usb power banks @ gearbest http://bit.ly/2t2caMO Romoss Sense 6 20000mah http://bit.ly/2sisa0p Xiaomi power bank 2 - 2000mah (97% efficiency) http://bit.ly/2f00c3n Besiter 6X power bank - 20000mah (90% efficiency) http://bit.ly/2hf54mf A look at power banks, and an explanation as to why their capacity is not what the manufacturer reports. The problem lies in the lipo and li ion batteries that are 3.7v The batteries must be boosted to 5v and as a result the capacity is lower than the actual batteries themselves. for example. 20000mah battery bank can only be 14,800 mah hours. Using WH=Voltage x capacity we can learn that it is 74 watt hours. Divide that by your 5v and you will get the capacity for that voltage which is 14,800 Not all power banks are created equal. Despite this, power banks lose capacity as a result of resistance in the usb cables, and the electronics. Power banks can be anywhere from 75-97% efficient. So in the end... a crummy 20,000mah power bank may actually only provide in the neighborhood of 10,000 actual mah.