How to Choose a Power Bank: Capacity, Speed, and the Fine Print

Capacity: what mAh really means

Power banks advertise capacity in mAh (milliamp-hours), a bigger number means more stored energy. A rough guide: a typical phone battery is somewhere in the few-thousand-mAh range, so a 10,000mAh bank can recharge a phone roughly a couple of times, and a 20,000mAh bank more. But here's the catch: you never get the full rated capacity in practice. Energy is lost to heat and voltage conversion during charging, so real-world output is meaningfully lower than the label. Plan for fewer charges than the raw number suggests.

Why you get fewer charges than expected

This surprises many people, so it's worth understanding. The mAh rating refers to the internal cells at their voltage, but charging your phone requires converting that to a different voltage, and conversion always loses energy (often a noticeable chunk). So a 10,000mAh bank doesn't deliver 10,000mAh to your phone; expect roughly 60-70% efficiency as a rough rule. That's normal physics, not necessarily a scam, but it means you should size up: if you want two full phone charges, don't buy the bare-minimum capacity to match on paper.

Charging speed matters as much as size

Capacity is how much; speed is how fast. Speed is measured in watts (W), and modern fast charging uses standards like USB Power Delivery (USB-C PD). A higher-wattage bank refills your devices quicker, important if you're topping up in a hurry. Check both the output wattage (how fast it charges your device) and that it supports the fast-charging standard your device uses. Also note input speed, how fast the bank itself recharges, since a huge bank with slow input can take many hours to refill. Match speed to your devices and patience.

Ports, cables, and USB-C

Look at the ports. USB-C is now the standard, ideally with Power Delivery for fast charging, and increasingly banks use USB-C for both input and output. Multiple ports let you charge several devices at once (though total power is shared). Some banks include a built-in cable, handy for travel. Make sure the bank's ports and standards match your devices, a bank capable of fast charging does nothing if you use a slow cable or an incompatible port. For most people today, a USB-C PD bank is the sensible, future-proof choice.

Size, weight, and the travel trade-off

Capacity comes with bulk. A slim 5,000-10,000mAh bank slips in a pocket and tops up a phone, ideal for daily carry. A 20,000mAh+ bank charges phones several times or powers larger devices like tablets and some laptops, but it's heavier and takes longer to recharge itself. Match the size to your use: a small one for everyday backup, a large one for travel or multi-day trips. Carrying a giant bank you rarely deplete is just hauling dead weight, be honest about how much power you actually need on the go.

The airline rule you must know

One practical warning for travelers: power banks (lithium batteries) generally must go in your carry-on, never checked luggage, for safety, and airlines limit capacity. Most allow banks up to a certain watt-hour rating (commonly around 100Wh, roughly 27,000mAh, without special approval), with larger ones restricted or banned. Rules vary by airline, so check before you fly, and keep banks accessible in case security asks. Buying a monster bank that airlines won't let you carry defeats the purpose if you travel, factor this in before purchasing.

Related on Skillo

See also: USB-C and Thunderbolt explained, GaN chargers explained: smaller, cooler, faster.

Sources

• USB Power Delivery - Wikipedia
• Battery charger (power banks) - Wikipedia

Published date reflects the original event date (2026-04-06). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.