Power bank buying guide: capacity, speed and safety

2 min read · Updated 2026-10-26

How to read mAh and watt ratings, what real capacity to expect, how to spot exaggerated claims and what to know before flying.

A power bank is simple in principle, yet the numbers on listings are often confusing or exaggerated. This guide shows how to compare them sensibly.

Capacity: mAh and watt-hours

Capacity is usually shown in milliamp-hours (mAh) at the cell's internal voltage of about 3.7 V. To compare with the energy your phone or laptop needs, convert to watt-hours: multiply mAh by 3.7 and divide by 1000. A 20,000 mAh power bank holds roughly 74 Wh.

Expect less than the label

Charging a device from a power bank loses some energy as heat and voltage conversion, so the usable capacity is often only around two thirds of the rated figure. A 10,000 mAh power bank therefore tends to charge a typical phone about two times, not three.

Output power and fast charging

Check the output in watts and the fast-charging standards supported, such as USB Power Delivery (PD) or Quick Charge. Your phone and cable must support the same standard to charge quickly. Laptops need considerably more power, so look for models with 45 W or more on a USB-C port.

Ports and cables

USB-C that works for both input and output is the most convenient. A weak cable can limit charging speed, so use a good one rated for the power you need.

Spotting exaggerated claims

Battery cells have a physical energy density limit. A genuine 20,000 mAh power bank usually weighs several hundred grams. A very small, light unit that claims 30,000 mAh is almost certainly overstated. Reviews that include independent capacity tests are valuable.

Safety

Prefer units that list safety protections (overcharge, short-circuit and over-temperature), and buy from sellers with many positive reviews. Stop using a power bank that swells, becomes unusually hot or is physically damaged.

Travelling with a power bank

Airlines generally limit power banks by watt-hours, often allowing up to 100 Wh in carry-on baggage and requiring approval for larger ones, and they are normally not allowed in checked baggage. Rules differ between airlines and countries, so check the current policy before you fly.

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