Power banks explained: what the mAh number really tells you

A 20,000mAh power bank won't charge your phone five times over. Here's the maths, and what else to look for.

Power banks are sold almost entirely on one number: mAh, or milliamp-hours. Bigger sounds better, and a 20,000mAh model next to a phone with a 5,000mAh battery suggests four full charges. In practice you'll usually get closer to two and a half or three. Here's why, and how to choose one that suits you.

Why the mAh number flatters

The mAh figure on the box describes the cells inside the power bank, measured at their own internal voltage - typically around 3.7 volts. To charge your phone, the power bank has to convert that up to the USB output voltage, and then your phone converts it back down again to charge its own battery. Each conversion loses energy as heat.

Across the whole process, it's normal for a power bank to deliver somewhere between 60% and 70% of its rated capacity to your phone. So a 10,000mAh power bank realistically gives a typical modern phone about one and a half full charges, and a 20,000mAh model about three.

Watt-hours are the more honest figure

Many power banks also print a capacity in watt-hours (Wh). This combines capacity and voltage, so it's easier to compare between devices - and it's the number airlines care about.

UK airlines generally allow power banks up to 100Wh in your hand luggage without asking, and between 100Wh and 160Wh with the airline's approval. Power banks are not allowed in checked luggage. Most phone-sized power banks are well under the 100Wh limit, but laptop-sized ones can be close, so check before you fly.

Charging speed matters as much as size

A huge power bank that charges your phone slowly can be more frustrating than a smaller, faster one. Look for:

• USB-C Power Delivery (PD): the standard most recent phones, tablets and many laptops use for fast charging.
• The output in watts: 20W is plenty for most phones; 45W or more is needed if you want to charge a laptop.
• Input speed: a large power bank with a slow input can take most of a day to recharge itself.
• Number of ports: handy if you're charging a phone and earbuds at the same time, though speed is often shared between ports.

Which size should you buy?

• 5,000mAh: slim enough for a pocket; a top-up for a long day out.
• 10,000mAh: the sweet spot for most people - about one and a half phone charges in a size that still fits a jacket pocket.
• 20,000mAh: a weekend away or a festival, or keeping two devices going. Noticeably heavier.
• 25,000mAh and up: mainly for laptops and long trips. Check the watt-hour rating if you're flying.

Look after it

Lithium batteries prefer being kept between roughly 20% and 80% and don't like heat. Avoid leaving a power bank in a hot car, store it partly charged if you won't use it for a while, and top it up every few months. Buy from reputable retailers and avoid unbranded units with no safety markings - a cheap power bank is not a bargain if it fails.

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