Power bank mAh vs Wh: how many phone charges will it really give?
The box says 10,000 mAh, your phone battery is 4,500 mAh, so you expect two full charges. You get one and a bit. The bank isn't faulty: the two numbers are measured at different voltages, and some energy is lost on the way.
Why mAh on its own misleads
Milliamp-hours (mAh) measure charge, not energy. Energy is charge multiplied by voltage. Power bank makers quote mAh at the voltage of the lithium-ion cells inside, usually 3.6 or 3.7 V. Phones quote their battery at a slightly higher nominal voltage, often 3.85 V, and laptops at 11–15 V. So “mAh” from two different products are not the same unit of energy, and a laptop's 4,000 mAh can hold more than a phone's 5,000 mAh.
Watt-hours fix that: Wh = mAh × V ÷ 1000.
| Label | Energy | Typical phone charges* |
|---|---|---|
| 5,000 mAh (3.7 V) | 18.5 Wh | about 0.7 |
| 10,000 mAh | 37 Wh | about 1.4 |
| 20,000 mAh | 74 Wh | about 2.8 |
| 26,800 mAh | 99.2 Wh | about 3.7 |
*0–100% charges of a 4,500 mAh / 3.85 V phone (17.3 Wh) at 65% real-world efficiency. Topping up from 20% to 80% stretches each figure by about two thirds.
Where the missing energy goes
- Boosting to USB voltage. The bank's 3.7 V cells are converted up to 5 V (or 9 V and more for fast charging). Good converters lose roughly 10–15% here.
- Back down inside the phone. The phone's own charging circuit converts it back to battery voltage and loses a bit more.
- Heat and the cable. Fast charging, thin cables and a warm bag all waste a little more. Using the phone while it charges also “spends” some energy before it reaches the battery.
- Age and cold. An old power bank holds less than when new, and a cold one delivers less (see why phones die in the cold).
Together that typically leaves 60–70% of the label's energy in your phone. Cheap banks with optimistic labels can do noticeably worse.
The second, smaller number on the label
Many banks print something like “10,000 mAh 3.7 V 37 Wh” and also a “rated capacity” of around 6,000 mAh at 5 V. That's the same energy, counted at the higher USB voltage after conversion losses. It's actually the more honest number for what comes out of the port.
Taking a power bank on a plane
Under the common IATA rules that most airlines follow:
- 100 Wh or less (about 27,000 mAh at 3.7 V): allowed in hand luggage. This is why 26,800 mAh banks exist: they sit just under 100 Wh.
- Over 100 Wh up to 160 Wh: only with the airline's approval, usually at most two per person.
- Over 160 Wh: not allowed on passenger aircraft.
- Never in checked luggage, whatever the size, because a battery fire in the hold can't be reached.
If the label doesn't show Wh, airport staff may refuse it, so check it's printed clearly. A growing number of airlines also ban using or charging power banks during the flight, so check your airline's own page before you fly.
Choosing a size
Work out how many charges you want, multiply by your device's battery Wh, divide by about 0.65, and pick a bank at least that big. Two full phone charges need about 53 Wh, which means a 15,000–20,000 mAh bank. Bigger is heavier and slower to refill: a 20,000 mAh bank on an 18 W charger takes around five hours.
Related
Power bank won't charge my phone, or is slow · Laptop plugged in but not charging · Is it bad to charge a phone in the cold? · Battery percentage jumps or drops suddenly · Power banks on planes: the 100 Wh rule · Interactive: chill a pretend tablet and watch it switch off
Words explained on Wirebook: watt-hour · mAh · USB Power Delivery
Questions
How many times will a 10,000 mAh power bank charge my phone?
About 1.4 full charges of a typical 4,500 mAh phone, or about 2 for an older 3,000 mAh phone, because only around 60-70% of its 37 Wh reaches the phone.
How do I convert mAh to Wh?
Multiply mAh by the voltage and divide by 1000. 10,000 mAh at 3.7 V is 37 Wh.
Can I take a 20,000 mAh power bank on a plane?
Usually yes, in hand luggage only. At 3.7 V it is 74 Wh, under the common 100 Wh limit. Never pack it in checked bags, and check your airline's rules on using it in flight.
Why does my power bank charge my phone less than the mAh suggests?
The bank's mAh is counted at about 3.7 V, energy is lost boosting it to USB voltage and back, and some becomes heat. Compare watt-hours and expect about two thirds of them to arrive.
What is the biggest power bank allowed on a plane without approval?
100 Wh, which is about 27,000 mAh at 3.7 V. That is why many large banks are sold as 26,800 mAh.
Published .