Power bank specifications are unusually misleading, not because manufacturers lie but because the headline number describes the cells inside rather than the energy you get out. Here is how to read the label.
mAh is not the number you think
A 10,000mAh power bank does not put 10,000mAh into your 4,000mAh phone two and a half times. It will manage roughly one and a half to two charges.
The capacity is measured at the cell voltage, typically 3.7V. Your phone charges at 5V or higher, so the bank must step the voltage up, and conversion is not free. Add the energy lost as heat and the power your phone uses while charging, and 60–70 per cent of the rated capacity actually reaching the phone is a normal, honest result.
The comparable figure is watt-hours (Wh), which some banks print and airlines require. To convert: mAh × 3.7 ÷ 1000 = Wh. A 10,000mAh bank is about 37Wh.
Watts: how fast, not how much
Capacity is the size of the tank. Wattage is the width of the pipe.
Watts = volts × amps. A charger described as 5V/2A is 10W. One described as 20W will be doing it at a higher voltage, negotiated between the bank and the device.
Two figures are worth checking and they are different:
- Output — how fast it charges your device.
- Input — how fast the bank itself refills. A 10,000mAh bank with 5W input takes most of a day to recharge; the same bank with 20W input takes a couple of hours.
Input is the specification people forget and then resent.
USB Power Delivery and the alphabet soup
USB Power Delivery (PD) is the open standard. The device and the charger negotiate a voltage, and it is what phones, tablets and most laptops use for fast charging. If you buy one thing, buy PD.
Quick Charge (QC) is Qualcomm's older standard, found on some Android phones. Many banks support both.
Proprietary fast-charging systems generally fall back to a slow, safe speed with anything else, which is why a phone can charge quickly from its own charger and slowly from a bank rated higher.
PPS is a PD extension that allows fine voltage adjustment. Some phones need it for their top charging speed. If your phone advertises a speed you are not reaching, PPS is often the missing piece.
Ports, and the shared budget
A bank advertising "65W" with three ports rarely gives 65W to each. The figure is usually the total, split when more than one port is in use — often 45W and 20W, or three ways at lower rates. The port table on the box is worth reading; the headline is not.
A single USB-C PD port is more useful than several slow USB-A ports for almost everyone now.
Taking one on a plane
Airlines regulate lithium batteries by watt-hours, and the common limits are:
- Under 100Wh — carry-on, no approval needed. This covers anything up to about 27,000mAh.
- 100–160Wh — usually needs airline approval.
- Over 160Wh — generally not permitted.
Power banks must go in cabin baggage, never checked. If the bank does not print its Wh figure, do the conversion above and expect to be asked.
Size, weight, and being honest about use
A 20,000mAh bank weighs around 400g. That is a sensible thing to keep in a bag for a weekend and an unpleasant thing to carry in a pocket every day.
For a day out, 5,000mAh is one full phone charge and fits in a pocket. For travel, 10,000mAh is the usual sweet spot. Above 20,000mAh you are buying for a laptop or several people, and the weight starts to be the point.
What to ignore
Solar panels on power banks are, at the sizes that fit on a power bank, close to decorative — expect days rather than hours to fill one. LED capacity displays showing four bars tell you much less than a percentage readout. And "fast charge" printed on a box without a watt figure means nothing at all.
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