From mAh to watt-hours
Battery capacity is usually printed in milliamp-hours (mAh), but that number alone does not tell you how much energy is stored: it depends on the voltage. The useful unit is the watt-hour (Wh):
Wh = mAh × V ÷ 1,000
A 20,000 mAh power bank at 3.7 V stores 74 Wh. A 12 V car battery rated 60 Ah stores 720 Wh, ten times more, although its “Ah” number looks smaller.
Runtime
- Runtime (hours) = Wh × efficiency ÷ device power (W)
- Number of recharges = Wh × efficiency ÷ capacity of the battery to recharge (Wh)
The efficiency accounts for losses in the voltage converter and in the charging circuit: around 80 to 90% for a power bank, which is why a 74 Wh bank never delivers 74 Wh to your phone.
Worked example: a 20,000 mAh power bank
- Energy: 20,000 × 3.7 ÷ 1,000 = 74 Wh, below the 100 Wh airline limit.
- Powering a 10 W device at 85% efficiency: 74 × 0.85 ÷ 10 = 6.29 h, or 6 h 17 min.
- Recharging a phone with a 15 Wh battery: 74 × 0.85 ÷ 15 = 4.2 full charges.
- At 10 W, the bank supplies 2.70 A at its 3.7 V cell voltage.
Runtime for other loads (74 Wh, 85%)
| Device power | Runtime |
|---|---|
| 1 W (Bluetooth speaker) | 62 h 54 min |
| 5 W (LED lamp) | 12 h 35 min |
| 10 W | 6 h 17 min |
| 20 W (tablet charging) | 3 h 9 min |
| 45 W (small laptop) | 1 h 24 min |
| 65 W (laptop) | 58 min |
Flying with batteries
FAA and airline rules follow the IATA limits for lithium-ion batteries:
| Capacity | Rule |
|---|---|
| Up to 100 Wh | allowed in carry-on, no approval needed |
| 100 to 160 Wh | airline approval required, two spares at most |
| Over 160 Wh | forbidden on passenger aircraft |
Spare batteries and power banks must always travel in carry-on bags, never in checked luggage. If the Wh rating is not printed, compute it from mAh and voltage as above.
Typical capacities
| Device | Capacity |
|---|---|
| Smartphone | 12 to 20 Wh |
| Tablet | 25 to 45 Wh |
| Laptop | 40 to 100 Wh |
| E-bike battery | 400 to 750 Wh |
| Car starter battery (12 V, 60 Ah) | 720 Wh |
| Home battery | 5 to 15 kWh |
Why real runtime is shorter
Batteries lose capacity with age (about 20% after 500 full cycles for many lithium-ion cells), deliver less in the cold and less at high discharge rates. Devices also rarely draw a constant power. Treat the result as an upper estimate and keep a 10 to 20% margin.