Do I need a power bank and which one?

Power banks are bought on the headline number, which is the one specification that does not translate directly into charges.

Safety

Step by step

1. Work out what you actually need.A day out with a topped-up phone needs 5,000mAh. A weekend or a festival needs 20,000mAh. A laptop needs 20,000mAh and USB-C Power Delivery. Buying larger than you need means carrying a brick.
2. Understand why the capacity overstates it.The printed figure is the cell capacity at 3.7V. Converting to 5V for USB loses energy, so expect around 60 to 70% of the number to reach your phone. A 10,000mAh bank realistically delivers about 6,500mAh.
3. Check the output wattage, not just capacity.Capacity is how much; wattage is how fast. A large bank with a 10W output charges a modern phone slowly. 20W or more for phones, 45W or more for laptops.
4. Match the charging standard.USB-C Power Delivery is the common one and what most phones and laptops now use. Some Android phones use their own fast-charge standards and only reach full speed with a matching bank.
5. Count the ports and check the shared output.Many banks split their total output between ports, so charging two devices halves the speed of each. Check the per-port figure rather than the headline.
6. Check how the bank itself recharges.A 20,000mAh bank with a slow input can take most of a day to refill. USB-C input with a decent wattage matters more than people expect.
7. Consider pass-through charging.Charging the bank and a device at the same time from one socket. Useful when sockets are scarce, and not all banks support it.
8. Weigh the weight.Capacity is roughly proportional to weight. A 20,000mAh bank is noticeably heavy in a pocket, and the one you leave at home charges nothing.
9. Know the flight rules.Hand luggage only, never in the hold. Most airlines allow up to about 100Wh without asking, which covers roughly a 27,000mAh bank, and 100 to 160Wh with approval.
10. Buy a recognised brand.Cell quality and protection circuitry are what stop a lithium battery failing dangerously, and they are exactly what the cheapest unbranded banks omit.
11. Look for the safety features.Over-charge, over-discharge, short-circuit and temperature protection. Reputable manufacturers list them; anonymous ones do not.
12. Look after it.Do not leave it fully flat for months or in a hot car. Charge it to around half if storing it, and top it up a few times a year.

Tips

• Assume you get about two-thirds of the printed capacity. That single adjustment makes the comparison honest.
• Check output wattage as carefully as capacity. A big bank with a weak output is a slow bank.
• Never put a power bank in checked luggage. It is a hand-luggage item on every airline.

Common mistakes

• Buying on capacity alone — A large bank with low output wattage charges slowly, and conversion losses mean the headline figure is roughly a third higher than what you get.
• Buying an unbranded bank on price — Protection circuitry and cell quality are where cheap ones cut cost, and those are precisely what prevent a lithium fire.

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