How to choose a power bank

Capacity: what the mAh number means

Capacity is shown in milliamp-hours (mAh), but the rating is for the internal battery cells at about 3.7 volts. Converting to watt-hours (Wh) gives a fairer comparison: Wh = mAh x 3.7 / 1000. A 10,000 mAh power bank is about 37 Wh.

You never get the full rated capacity, because some energy is lost when the voltage is converted. Plan on roughly 60 to 70 percent. That means a 10,000 mAh power bank typically gives a modern phone about one to two full charges.

Charging speed

Look at the output in watts, not just the capacity. USB Power Delivery (PD) is the common fast-charging standard for phones, tablets and some laptops. Your device only charges quickly if the power bank, the cable and the device all support it.

• Check the power bank's output is at least what your device needs.
• Use a cable rated for the wattage; thin cables can limit speed.
• Check the input speed too, so the power bank itself does not take all night to recharge.

Ports and size

USB-C is the most useful port, ideally for both input and output. Extra ports are handy if you charge several devices, but remember that the output is shared. Bigger capacity means more weight, so think about whether you will carry it every day.

Safety

• Buy from a brand with a track record and a warranty.
• Look for protection against overcharging, over-current, short circuits and overheating, plus any safety certifications required in your country.
• Stop using a power bank that is swollen, very hot, damaged or smells odd.

Taking a power bank on a plane

Most airlines allow power banks of up to 100 Wh in carry-on bags, and require them to stay out of checked luggage. Between 100 and 160 Wh usually needs airline approval. Some airlines also limit how many you can bring or restrict using them during the flight, and rules change, so check your airline's current policy before you fly.

Spotting unrealistic claims

Cells have weight. A genuine 20,000 mAh power bank typically weighs a few hundred grams, so a very light one advertising a huge capacity at a tiny price is a red flag. Compare weight and price against well-known brands, and read reviews from buyers who have tested the real capacity.