Power bank size: how big a power bank do you need

Verdict: most people need a power bank of around 10,000 mAh. It is small enough to carry daily and gives a phone a couple of charges. Bigger ones are for trips and tablets.

The size of a power bank is given in mAh, short for milliamp hours. It is a measure of stored energy. Bigger number, more energy, more weight. This page shows how to work out the power bank size you need.

The decision rule

1. Find your phone battery size. It is on the maker's spec page, in mAh.
2. Divide the power bank size by that number.
3. Multiply by about 0.6.

The 0.6 is a rough allowance for energy lost as heat during charging. Real results vary.

A worked example with round numbers

Say your phone battery is 4,000 mAh, and your power bank is 10,000 mAh.

10,000 divided by 4,000 is 2.5. Multiply by 0.6 and you get about 1.5 charges. That is enough to get a phone from empty to full and then half again.

A 20,000 mAh bank in the same test gives about three charges. A 5,000 mAh bank gives under one, so it works as an emergency top up.

These are examples. Check the numbers for your devices.

What else sets the size

• Tablets. Tablet batteries are often 7,000 to 10,000 mAh or more. A phone sized bank gives one partial charge.
• Weight. A 10,000 mAh bank is typically pocket size. A 20,000 mAh one is noticeably heavy.
• Charging speed. Fast charging needs a bank and a cable that support it. Check the watt rating on both.
• Flights. Airlines and aviation authorities set limits on power banks in carry-on bags. Check your airline before you fly. Power banks usually must go in hand luggage, not in checked bags.

Which size for which person

• Commuter or day out: 5,000 to 10,000 mAh.
• Weekend trip or long day with a tablet: 10,000 to 20,000 mAh.
• Camping or several days without a socket: 20,000 mAh or more, and think about a solar option.

Ruling: buy the smallest size that covers your longest day without a socket, plus a little spare.

Safety basics

Use a power bank from a maker that lists its safety marks and has a returns policy. Do not use one that is swollen, hot or damaged. Do not charge it under a pillow. Follow the maker's manual and local guidance on lithium batteries.

For related buying advice, see wired vs wireless earbuds and smartwatch vs fitness band, since both also need charging.

Common mistakes

• Buying the biggest one and leaving it at home because it is heavy.
• Using a long, thin cable that charges slowly.
• Forgetting to charge the bank itself.
• Ignoring the airline rules.