Buying a power bank: what you really need to check

Power banks all look much the same, and the number on the packaging says less than you might think. Once you understand how the capacity figure comes about, you buy far more confidently.

Why 10'000 mAh is not 10'000 mAh

The printed capacity refers to the cells inside, which work at around 3.7 volts. Your phone, however, wants 5 volts and more. Stepping the voltage up costs energy, and there is waste heat on top of that. In practice, roughly 60 to 70 percent of the printed capacity therefore arrives in the device. So 10'000 mAh becomes roughly 6'000 to 7'000 mAh, which for most smartphones means one and a half charges, not two or three.

That is not a scam, it is physics. Reputable manufacturers also state the real output in watt hours, and that is exactly the figure you should compare.

Which size suits you

• 5'000 mAh: flat, fits in a trouser pocket, gives you a good half to one phone charge. Ideal as an emergency reserve for the commute.
• 10'000 mAh: the all-rounder. A day at a festival or hiking without a socket is realistic with this.
• 20'000 mAh and above: for travelling, several people or a tablet. In return you notice the weight in your backpack.

Watts matter more than mAh

A large power bank that only delivers 10 watts fills your phone agonisingly slowly. Look for USB-C with Power Delivery and at least 20 watts of output, then you charge in a reasonable time while out and about. Just as important: how quickly does the power bank itself recharge? Models that only take 5 watts on the way in spend the whole night on the cable. You will find the basics on charging power and standards in our charger buying guide.

Practical details that count day to day

A display with a percentage readout is worth more than four LED dots, because you really know where you stand. Pass-through charging means the power bank can charge your phone while it is itself plugged into the mains, handy when there is only one free socket in the hotel. And if you want to charge wirelessly, look for a magnetic mount, otherwise the power bank slips in your bag and charges nothing.

Air travel: the 100 watt hour rule

Power banks always belong in hand luggage, never in checked baggage. Up to 100 watt hours they are allowed by most airlines without prior notice, which corresponds to roughly 27'000 mAh. Above that, approval is required. The watt hours are printed on the housing, and that is exactly why it is worth buying a model that prints this figure legibly at all. Check the rules with your airline before flying, they differ in the detail.

How long does a power bank last?

Like every lithium battery, a power bank ages too. After a few hundred charge cycles the capacity drops noticeably. If your power bank barely half fills your phone any more, it has reached the end of its life and belongs in electronic waste collection, not in household rubbish. You will find new models with us under power and cables.

Conclusion

Compare watt hours rather than mAh, look for at least 20 watts over USB-C Power Delivery and buy the size you will actually carry. The strongest power bank is no use if it stays at home because it is too heavy.