How Many Times Will a Power Bank Charge My Phone? (The Real Maths)
Why a 10,000 mAh power bank does not give two full charges to a 5,000 mAh phone, how to calculate real charges in watt-hours, and how to pick the right size for travel.
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Why the label overstates it
Power bank capacity is quoted at the internal cell voltage, usually 3.6–3.85 V. Your phone's battery is rated at a similar voltage, so it looks like a straight comparison. But energy has to travel a longer route:
- The bank boosts its cell voltage up to 5 V, 9 V or more for USB.
- The cable loses a little as heat.
- The phone steps it back down to charge its battery, and also powers itself while charging.
Each step loses energy. Overall efficiency of 60–80% is normal; fast charging and warm conditions sit at the lower end.
The calculation in watt-hours
Convert both batteries to watt-hours (Wh) so voltages do not confuse things: Wh = mAh × V ÷ 1000.
| Power bank | Phone | |
|---|---|---|
| Rated | 10,000 mAh × 3.7 V = 37 Wh | 5,000 mAh × 3.85 V ≈ 19.3 Wh |
| After ~70% efficiency | ≈ 26 Wh delivered | — |
| Full charges | 26 ÷ 19.3 ≈ 1.3–1.4 charges | |
What size should you buy?
| Capacity | Typical phone charges | Best for |
|---|---|---|
| 5,000 mAh | About 0.7–1 | A top-up in a pocket or small bag |
| 10,000 mAh | About 1.3–2 | Daily carry, weekend trips |
| 20,000 mAh | About 3–4 | Travel, festivals, two devices |
| 25,000+ mAh with 60–100 W USB-C | Many phone charges, or a laptop top-up | Laptops and long trips |
Flying with a power bank
Airlines generally require power banks in carry-on luggage, not checked bags. Under widely used IATA guidance, banks up to 100 Wh are allowed without approval (that covers most banks up to about 27,000 mAh at 3.7 V); 100–160 Wh usually needs airline approval. Some airlines have added extra rules, such as banning in-flight use, so check yours before travelling.
Get more out of every charge
- Use a short, good-quality USB-C cable.
- Charge the phone while it is idle or switched to airplane mode.
- Keep the bank cool; heat lowers efficiency and ages its cells.
- Recharge the bank every few months if you store it, ideally around half full.
More battery advice: 15 tips to make your phone battery last longer.
Quick answers
Does a higher-watt power bank charge more times?
No. Watts decide speed; watt-hours (or mAh) decide how much energy it holds.
Why is my power bank empty after one charge?
It may be smaller in real capacity than claimed (common with unbranded banks), worn out, or losing a lot to heat during fast charging.