Explainer
mAh vs Wh: Why Your 20,000 mAh Power Bank Gives Less
Short answer: mAh tells you how much charge the cells inside hold at their own voltage (about 3.7 V). Your phone or laptop is charged at 5–20 V, so the number on the box can’t be compared directly with your phone’s battery. Watt-hours (Wh) can.
The one formula you need
Wh = mAh × V ÷ 1000Power banks are rated at the voltage of their lithium cells, usually 3.6–3.85 V. Most print the Wh figure on the case in small type because airlines require it.
| Rated capacity | At 3.7 V | Roughly enough for |
|---|---|---|
| 5,000 mAh | 18.5 Wh | A top-up for one phone |
| 10,000 mAh | 37 Wh | About one and a half to two phone charges |
| 20,000 mAh | 74 Wh | Three or so phone charges, or one small laptop top-up |
| 27,000 mAh | ~100 Wh | Right at the airline no-approval limit |
The “roughly enough for” column is a rule of thumb; your phone’s battery size decides the real number.
Where the missing third goes
Two conversions lose energy on the way from the pack to your phone:
- Boosting the voltage. The pack steps 3.7 V up to 5 V, 9 V or 20 V. Good circuits lose 10–15% here; cheap ones lose more.
- Charging your device. Your phone’s own charging circuit and the heat from charging cost another 10–20%.
Put together, about 60–70% of the rated energy typically ends up in your phone’s battery. A 74 Wh pack delivering about 63 Wh at the port, then about 50 Wh into the phone, is normal, not a defect.
How to compare power banks properly
- Compare Wh, not mAh. It’s on the label of every pack sold for air travel.
- Look for measured output. A pack that delivers 85% of its rated Wh beats a “bigger” one that delivers 65%. That’s what we measure; see how we test.
- Match the output to the device. A laptop needs a pack that holds 45–100 W under load; a phone needs 20–30 W.
- Remember the airline line. Above 100 Wh you need airline approval. See our power bank airline rules.
Frequently asked questions
How do I convert mAh to Wh?
Multiply mAh by the voltage and divide by 1,000. Power banks are rated at their cell voltage, usually 3.6–3.85 V, so a 20,000 mAh pack at 3.7 V holds 74 Wh.
Why does my power bank charge my phone fewer times than the math says?
The pack has to boost its 3.7 V cells up to 5–20 V, and your phone loses more energy turning it back into battery charge. Expect roughly 60–70% of the rated capacity to reach your phone.
Is a 20,000 mAh power bank allowed on a plane?
Usually yes. At 3.7 V it is about 74 Wh, below the 100 Wh limit that needs no airline approval. Check the Wh figure printed on the case and your airline's own rules.