Power banks are sold on one number, and it is the least useful one. Capacity tells you almost nothing about whether a battery will charge your laptop, how many times it will refill your phone, or whether an airline will let you carry it.
Capacity, and the number you actually get
Capacity is quoted in milliamp hours, typically 5,000 to 27,000. A modern phone battery is roughly 3,000 to 5,000mAh, so arithmetic suggests a 10,000mAh bank gives two or three full charges.
It does not. Expect around 60 to 70 percent of the rated figure to reach your device.
The loss is real and unavoidable. Cells inside run at about 3.7 volts while USB delivers 5 volts or more, and that conversion wastes energy as heat. More is lost to the charging circuit and to the bank’s own idle drain.
So a 10,000mAh bank realistically delivers around 6,500mAh: one full phone charge plus most of another. Any listing promising four charges from 10,000mAh is describing arithmetic rather than behaviour.
Watts matter more than capacity
Capacity is how much energy is stored. Power output, in watts, is how fast it comes out, and it determines what you can charge at all.
- 5W to 12W — slow phone charging. Fine overnight, frustrating in an airport.
- 18W to 30W — fast charging for phones and tablets. The sensible target.
- 45W to 65W — will charge most thin laptops, though more slowly than their own adapter.
- 100W and above — full-speed laptop charging.
A large bank with low output is a common and disappointing purchase. It holds plenty of energy and delivers it too slowly to be useful.
Look for USB-C Power Delivery
Power Delivery, usually written USB-C PD, is the open standard that lets a charger and a device negotiate voltage. It is what makes fast charging work across brands.
Buy a bank with at least one USB-C PD port. Proprietary fast-charging systems only reach full speed with matching hardware; PD works with essentially everything modern.
Check the port is input and output. Cheaper banks charge over a separate, slow micro-USB socket, which can mean six hours to refill the bank itself.
Pass-through charging, where the bank charges your device while itself plugged in, is a genuinely useful feature worth checking for.
The airline limit
This one catches people at security. Lithium batteries must travel in hand luggage, never checked, and there is a size limit expressed in watt hours.
The widely applied rule is 100Wh without approval, and 100 to 160Wh with airline permission. Above that is generally refused.
To convert, multiply capacity in amp hours by voltage, usually 3.7:
20,000 mAh = 20 Ah x 3.7 V = 74 Wh (fine)
27,000 mAh = 27 Ah x 3.7 V = 100 Wh (at the limit)Around 26,800mAh is where manufacturers stop, precisely because it sits just under the threshold. Buy a bank with the watt hour figure printed on it; several airlines now check, and an unlabelled battery is easier to refuse than to argue about.
Rules have also tightened on using and storing power banks in flight, with some carriers requiring them to stay visible rather than in overhead lockers. Check your airline before travelling.
Choosing a size
- 5,000mAh — pocketable, one phone top-up, good for a night out.
- 10,000mAh — the sweet spot. About one and a half phone charges, still light.
- 20,000mAh — a weekend without a socket, or a tablet. Noticeably heavy.
- 26,000mAh with 65W or more — laptop capable, and the largest most airlines accept.
Safety
Buy from a recognised brand. Lithium cells that fail do so energetically, and this is the wrong category for an unbranded bargain.
Retire any bank that swells, becomes hot in normal use, or has been badly dropped. A deformed case means the cell inside has expanded, and that is a battery to dispose of properly at a recycling point rather than in a bin.
Lithium cells are also the reason so many safety notices are issued, and our guide to how product recalls work covers how to check whether something you own is affected.
Join the discussion