I used the portapow premium usb meter and after several runs, I confirmed the amount of energy required to fully charge my S3 is ~15Wh. It does get warm which I don’t think is normal so perhaps the charging circuity is damaged somehow. It draws 0.98A for 2.5 hours and then steadily decreases as it enters CV. Using a AC watt-meter would not give the correct numbers because the AC charger is quite inefficient (gets very hot). Also 15Wh can’t be from measuring the battery because it is just 2100mAh.
These are part of my “collection” of several brands of power bank, some of them are samples sent to me by the dealer (I work in a cellphone shop), even though they are mostly chinese brands there are few units that are really really good. Some have a sticker and marked with the amount of Wh (I just wrote W), measured by giving an iPad3 a full charge. The one with highest efficiency is the one on top left, 10,400mAh with 4x purple samsung cells. It delivers 34.7Wh when using 1A port and 29Wh when using 2A port.
In the bottom right is the tester used for all measurements. The ENB, ruinovo and evertones (unbranded white square) unit you see are from fasttech and GB, they use user replaceable cells but I found they discharge the batteries one by one and not at the same time, lowering the voltage and effiiciency. When the power bank is depleted the batteries are not all discharged equal, they vary from 2.7-3.4V, strange behavior. I’ve been using and handling all sorts of power banks pretty much since they started to massively appear on the market, many of them even those from the “big brands” often use the same circuits and components from cheaper units, they just charge extra because of better advertising. Paying Premium does not always give you a better product!