Summary: Power banks have a battery with a nominal voltage of 3.7V, but they output at 5V to charge your devices. When converting from 3.7V to 5V, some energy is lost due to the conversion process, which typically runs at about 85% efficiency. This means that a 10,000mAh power bank will give you around 6,290mAh of usable power after accounting for these losses. So, while your power bank might say 10,000mAh, the actual amount of energy you can use is a bit less.