Do you want to know the ultimate speed power bank review? However, a general overview of what you might consider when reviewing power banks: Capacity: The capacity of a power bank is measured in milliampere-hours (mAh) and determines how much charge it can hold. Consider your device's battery capacity and how many times you need to recharge it before selecting a power bank with an appropriate capacity. Consider how portable you need your power bank to be and choose a size that fits your needs.