A power bank's capacity is measured in milliamp-hours (mAh). This is the maximum current that the power bank can provide. Essentially, the more milliamp hours a power bank has, the longer it can charge your device. On the other hand, the higher the watt hours, the faster a power bank can charge your device.A Power Bank is a portable charger designed to recharge your electronic devices when you're on the move. Ranging in size from slim, pocket-sized devices up to larger, higher-capacity Power Banks – they can be used to charge smartphones, tablets, earphones, smartwatches and other mobile devices.