mAh to Wh
With capacity 20000, nominal voltage 3.7 V, load 5 W, mah to wh comes to 74.0 Wh — watt-hours. It is reached in 6 steps, the last of which is 20 * 3.7, and each one is printed on the page with its numbers filled in. The formula is the one published by IATA Lithium Battery Guidance Document, not an approximation fitted to it.
Battery capacity in mAh converted to watt-hours, which is the only figure two batteries at different voltages can be compared on — and the one airlines limit.
Formula and sources checked · How we check
Capacity given in mAh — milliamp-hours, Capacity 20000, Nominal voltage 3.7 V, Load 5 W
74.0 Wh
Watt-hours for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Amp-hours
1 * (20000 / 1000) + 0 * 20000 + 0 * (20000 / 3.7) + 0 * (20000 * 1000 / 3.7)20 Ah- Milliamp-hours
20 * 100020,000 mAh- Watt-hours
20 * 3.774 Wh- Kilowatt-hours
74 / 10000.074 kWh- Runtime at that load
74 / 514.8 h- Usable after 85% conversion loss allowance
74 * 0.8562.9 Wh
Ask about this in the chatmAh to Wh Chart
Worked example
A 20,000 mAh power bank holds 74 Wh, not the 20,000 mAh of charge people expect it to deliver at 5 V. At the phone end it is nearer 63 Wh after the boost converter takes its cut, which is why a "20,000 mAh" bank refills a 3,300 mAh phone four times rather than six.
How to work it out yourself
- 1.Enter the nominal voltage of the cells, not the output voltage. A power bank marked 20,000 mAh is counting at 3.7 V; the 5 V it hands to a phone carries proportionally fewer milliamp-hours.
- 2.Watt-hours is the figure to compare on. mAh is meaningless across different voltages — 5,000 mAh at 3.7 V and 5,000 mAh at 12 V differ by a factor of three in stored energy.
- 3.Check the watt-hour figure before flying. Under 100 Wh needs nothing, 100 to 160 Wh needs airline approval, and above 160 Wh is not carried.
- 4.The usable line allows 15% for the conversion losses between cell and device. Manufacturers quote the cell figure; what reaches the phone is smaller.
Power bank mAh to Wh at 3.7 V
| Capacity | Watt-hours | Kilowatt-hours |
|---|---|---|
| 2000 | 7.4 Wh | 0.01 kWh |
| 3000 | 11.1 Wh | 0.01 kWh |
| 5000 | 18.5 Wh | 0.02 kWh |
| 10000 | 37.0 Wh | 0.04 kWh |
| 15000 | 55.5 Wh | 0.06 kWh |
| 20000 | 74.0 Wh | 0.07 kWh |
| 26800 | 99.2 Wh | 0.1 kWh |
| 27000 | 99.9 Wh | 0.1 kWh |
| 30000 | 111.0 Wh | 0.11 kWh |
| 40000 | 148.0 Wh | 0.15 kWh |
| 50000 | 185.0 Wh | 0.19 kWh |
26,800 mAh at 3.7 V is 99.2 Wh — a hair under the 100 Wh limit airlines allow in cabin baggage without approval, which is exactly why so many power banks stop at that odd number.
The formula
- Amp-hours
1 * (20000 / 1000) + 0 * 20000 + 0 * (20000 / 3.7) + 0 * (20000 * 1000 / 3.7) - Milliamp-hours
20 * 1000 - Watt-hours
20 * 3.7 - Kilowatt-hours
74 / 1000 - Runtime at that load
74 / 5 - Usable after 85% conversion loss allowance
74 * 0.85
Source: IATA Lithium Battery Guidance Document — 100 Wh and 160 Wh cabin limits, FAA — packing safely with lithium batteries, IEC 61960 — rating and marking of secondary lithium cells and batteries
Questions people actually ask
- How do you convert mAh to Wh?
- Multiply by the voltage and divide by a thousand: Wh = mAh × V ÷ 1000. A 20,000 mAh bank at 3.7 V holds 74 Wh. The voltage is not optional — mAh is a quantity of charge and Wh is a quantity of energy, and the volts are what turns one into the other.
- Can I take a 20,000 mAh power bank on a plane?
- Yes. At 3.7 V it is 74 Wh, comfortably below the 100 Wh IATA cabin limit. The threshold falls at 27,027 mAh, which is why so many large banks are marked 26,800 mAh — it is 99.2 Wh, just under.
- Why does my 20,000 mAh bank not charge my 3,000 mAh phone six times?
- Because the bank counts at 3.7 V and the phone charges at 5 V, and the boost converter between them loses 10 to 20%. 74 Wh of cell energy is about 63 Wh delivered, against roughly 11 Wh a phone battery holds — so four full charges, not six.
- What is the difference between Ah and Wh?
- Ah counts charge, Wh counts energy. Two batteries with the same Ah store different amounts of energy if their voltages differ: a 100 Ah 12 V leisure battery is 1,200 Wh, and a 100 Ah 48 V pack is 4,800 Wh. Only the watt-hour figure is comparable.
- What voltage should I use for a car battery?
- 12.0 V nominal for a lead-acid starter battery, though a healthy one rests near 12.7 V. A 60 Ah battery is therefore about 720 Wh — of which only around half can be drawn without shortening its life, unlike lithium where most of it is usable.
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