What the Label Actually Means
Every power bank sold in India leads with one number, printed in the largest type the box will allow: 10,000mAh, 20,000mAh, sometimes 27,000mAh. Buyers reasonably read that as a promise about their phone. If the phone holds 5,000mAh and the bank holds 10,000mAh, the bank should refill it twice, with a little spare.
It will not. In practice a healthy 10,000mAh bank returns somewhere between 6,000mAh and 6,800mAh to your phone, which is one full charge of a 5,000mAh handset and a useful part of a second. Nothing is faulty and nobody has cheated you. The number on the box is answering a question about the cells inside the bank, and you are asking a question about electricity arriving at your phone. Those are different measurements, and the gap between them is arithmetic rather than fraud.
The Arithmetic, Once
Milliamp-hours are not a measure of energy. They are a measure of charge, and charge only becomes energy when you pair it with a voltage. That is the whole source of the confusion, because the two sides of the transaction run at different voltages.
The lithium-ion cells inside a power bank have a nominal voltage of about 3.7 V. So a 10,000mAh bank stores:
10,000mAh × 3.7 V = 37 watt-hours (Wh)
The USB port, by contrast, hands out power at 5 V. If you could move that stored energy across with no losses at all, the charge available at 5 V would be:
37 Wh ÷ 5 V = 7,400mAh
That figure is the ceiling, not the outcome. Before any inefficiency, before any heat, simply by expressing the same energy at a higher voltage, the 10,000mAh on the box has already become 7,400mAh at the port. You have lost about a quarter of the headline number to a unit conversion.
Where the Rest Goes
The remaining shortfall is real loss, and it has three main homes.
- The boost converter. Stepping 3.7 V up to 5 V is done by a switching circuit, and no switching circuit is perfect. Good ones run at 85 to 93 percent efficiency. The energy that does not make it through leaves as heat, which is why a hard-working power bank is warm to the touch.
- The cable and connectors. Every metre of cable has resistance, and resistance turns current into heat. This is worse at 5 V than at higher voltages, because delivering the same power at a lower voltage requires more current, and cable loss rises with the square of the current. A thin, long, cheap cable is a measurable tax on your charge.
- The phone itself. Your handset does its own conversion, stepping the incoming 5 V down to the roughly 3.85 V its battery wants, with its own losses. Some of what leaves the bank never reaches the cell.
Stack those on top of the voltage conversion and you land at 60 to 68 percent of the label as charge delivered. Hence 6,000 to 6,800mAh from a 10,000mAh bank. Reputable manufacturers now print this second figure on the box as rated capacity or output capacity, usually in much smaller type than the headline.
What to Expect by Size
Assuming a 5,000mAh phone, which is typical of Android handsets sold in India today, and a mid-range bank:
| On the box | Stored energy | Delivered at 5 V | Full phone charges |
|---|---|---|---|
| 5,000mAh | 18.5 Wh | 3,000–3,400mAh | About two-thirds of one |
| 10,000mAh | 37 Wh | 6,000–6,800mAh | One, plus a third |
| 20,000mAh | 74 Wh | 12,000–13,600mAh | Two, plus a half |
The pattern is worth internalising: divide the headline number by roughly 1.5 to get usable charge. It is a rougher rule than the arithmetic above, but it is close enough to shop with, and it will stop you buying a 10,000mAh bank in the belief that it covers a weekend away from a socket.
If you are choosing an actual unit, our ranked picks are in the best power banks under Rs 999 guide.
Watt-Hours and Flying
There is one context where watt-hours are not merely the honest way to compare banks but the only figure that counts, and that is air travel. Airline rules on lithium batteries are written in watt-hours, not milliamp-hours, because watt-hours measure the energy available to go wrong.
The widely applied thresholds are:
- Under 100 Wh: permitted in carry-on baggage without special approval.
- 100 to 160 Wh: permitted only with the airline’s prior approval.
- Above 160 Wh: not permitted in passenger baggage.
Convert before you pack. At the 3.7 V nominal figure, 100 Wh works out to roughly 27,000mAh, which is why 20,000mAh banks at 74 Wh travel without trouble while some very large capacity bricks do not. Power banks belong in your cabin bag, never in checked luggage, and rules tighten periodically, so confirm with your airline before a long trip rather than at the security desk.
How to Compare Banks Properly
Once you know the label is measured at cell voltage, comparing banks becomes straightforward:
- Look for watt-hours. A bank that prints its Wh figure is telling you how much energy it holds in a unit that does not depend on which voltage you measure at. It is also the figure the airline wants.
- Look for rated or output capacity. Where a manufacturer prints this alongside the headline mAh, it is quoting the number you will actually experience, and the willingness to print it is itself a decent signal.
- Treat charge-count claims as marketing. “Charges your phone three times” is only meaningful next to a stated phone capacity, and it rarely is.
- Weigh the weight. Energy density is roughly fixed across the market, so a bank that claims high capacity while being suspiciously light is claiming something it cannot hold.
- Buy a decent cable. It is the cheapest efficiency upgrade available, and the one most people skip.
None of this makes power banks a bad purchase. It makes them a predictable one, which is better. A 10,000mAh bank that you know will refill your phone once and a bit is a genuinely useful object. The same bank, bought in the belief that it holds three charges, is a small disappointment waiting for a railway platform.
