Power Banks: Capacity, Wattage and Ports Explained

mAh, watt-hours and real usable capacity

A power bank's mAh figure is measured at the cell voltage of about 3.7 V, not at the 5 V a phone charges at, so usable output is lower once conversion losses are counted. Multiplying mAh by 3.7 and dividing by 1000 gives watt-hours: 10,000 mAh is about 37 Wh, and 20,000 mAh is about 74 Wh. A phone battery of 4,000 to 5,000 mAh therefore gets between one and two full charges from a 10,000 mAh bank.

Ports, fast-charge wattage and what you can carry on a plane

Match the output to what you charge. A USB-C port with Power Delivery, such as a 45 W model, can fast-charge phones and some laptops, while a basic 5 V USB-A port is slow. Count the ports: two or three outputs let several devices charge at once but share the total wattage. Built-in cables are handy but cannot be replaced if they fail. Airlines usually limit power banks to about 100 Wh and require them in cabin baggage, which rules out the largest capacities.

Questions about Power Banks: Capacity, Wattage and Ports Explained

Can a power bank charge a laptop?

Only if it has a USB-C Power Delivery output with enough wattage for the laptop, and the laptop accepts USB-C charging. A 45 W bank suits many thin laptops, while gaming laptops need much more.

Can I take a power bank on a plane?

Yes, in carry-on baggage only, never in checked bags. Most airlines allow banks up to 100 Wh, which covers 20,000 mAh models. Larger ones need airline approval or are not allowed, so check the rating on the label.

Why does my power bank charge slowly?

Slow charging usually comes from a basic 5 V port, a thin or worn cable, or a phone that is capped at a lower rate. Use a USB-C Power Delivery output with a cable rated for the wattage, and avoid charging several devices at once.