The number on the box is real. It is just not measuring what most people assume it is measuring.
Buy a 10,000 mAh power bank, look at your phone's 4,000 mAh battery, and the arithmetic seems obvious: two and a half charges, maybe more. In practice you will get somewhere between one and a half and two. Nothing is wrong with the bank. The two numbers are simply measured at different voltages.
Where the capacity goes
A power bank's cells sit at about 3.7 V. Your phone charges over USB at 5 V or higher. Getting from one to the other requires a boost converter, and that conversion is where most of the gap appears.
Run the real numbers on a 10,000 mAh bank:
- Stored energy: 10,000 mAh × 3.7 V = 37 Wh.
- After conversion at roughly 90% efficiency: about 33 Wh reaches the cable.
- Expressed at 5 V, that is around 6,600 mAh of usable output.
Then subtract the losses at the other end — heat in the phone's own charging circuit, the screen you keep checking, background apps — and a realistic figure is 60 to 70% of the rated number. That is not a defect. It is physics, and it applies to every brand equally.
Wireless costs more than wired
Magnetic wireless charging trades a little efficiency for the convenience of no cable. Energy crossing an air gap between two coils runs warmer than energy travelling down a wire, so expect roughly 10 to 15% less delivered capacity than the same bank charging over Type-C.
Whether that matters depends entirely on the situation. Topping up while walking through an airport, the convenience is worth more than the percentage. Charging overnight from a low bank, plug the cable in.
How to compare banks honestly
| What to look at | Why it matters |
|---|---|
| Watt-hours (Wh) | The only figure that is voltage-independent, and the one airlines use |
| Output wattage | Determines charging speed, not total capacity |
| Cell chemistry | Affects cycle life and how capacity fades over years |
| Weight per Wh | The honest measure of how much you are carrying per unit of power |
Choosing a size that fits the job
Once you adjust for real-world output, capacity choices get clearer. A 5,000 mAh bank delivers roughly one full phone charge — the right size for a day out, a commute, or a flight, at a weight you will not notice. A 10,000 mAh bank delivers close to two, which covers a weekend away or a long day of shooting photos.
Beyond that, you are mostly buying weight. The extra capacity is genuine, but so are the extra grams in your pocket every single day, for a reserve you use a handful of times a year.
Common questions
Is my power bank faulty if it gives fewer charges than advertised?
Almost certainly not. A 30 to 40% gap between rated and delivered capacity is normal and expected across all brands, because of voltage conversion losses.
What does mAh actually measure?
Milliamp-hours measure charge at the cell's own voltage, usually 3.7 V. Watt-hours measure energy and are directly comparable between devices.
Does wireless charging waste a lot of power?
It is roughly 10 to 15% less efficient than charging by cable. Meaningful over a full cycle, negligible for a quick top-up.