padraigdoran mAh = milliAmp-hours. V = Volts. Wh = Watt-hours The value of the capacity expressed in milliamp-hour depends on what voltage it is represented at. 20,000mAh @ 3.7V = 74Wh [So, 74Wh is theoretical max capacity] 74Wh @ 5V = 14,800mAh [but this is with no conversion losses] Factor in conversion losses, they made it 85% efficient at conversion, then: 14,800 * 0.85 = 12600mAh @ 5V So, basically the 20,000mAh is what the battery inside the power bank is rated for. Most people are interested in what the 5V rating is but the 3.7V rating sounds bigger for marketing purposes. Convert the 3.7V rating to 5V rating (i.e. standard USB) and factor in power losses and you have your rated capacity. Final note, I believe the 85% efficiency is a bit cautious, it's probably better than that with Xiaomi, more like 92%