Backup power
Amp hours to watt hours calculator
Watt hours = amp hours × volts, and amp hours = watt hours ÷ volts. A 100 Ah, 12.8 V lithium (LiFePO4) battery holds 1,280 Wh; a 100 Ah, 12 V lead-acid battery holds 1,200 Wh, but you should only use about half of it.
1,280 Whfrom 100 Ah at 12.8 V
- Energy
- 1,280 Wh = 1.28 kWh
- Capacity
- 100.0 Ah at 12.8 V
- Usable at 100% depth of discharge
- 1,280 Wh
- Runs a 100 W AC load for about
- 11 h
Amp hours to watt hours formula
Wh = Ah × V Ah = Wh ÷ V kWh = Wh ÷ 1,000 Wh from mAh = mAh ÷ 1,000 × V
An amp hour counts charge: one amp flowing for one hour. It only becomes energy once you multiply by the voltage it flows at. That is why Ah numbers can't be compared across batteries of different voltages: a 100 Ah battery at 48 V stores four times the energy of a 100 Ah battery at 12 V. Watt hours are comparable across all of them, which is why portable power stations, home batteries and airlines use Wh.
Amp hours to watt hours chart
Nominal voltages: 12 V for lead-acid and AGM; 12.8 V, 25.6 V and 51.2 V for 4-, 8- and 16-cell lithium iron phosphate (LiFePO4) packs.
| Capacity | 12 V | 12.8 V | 25.6 V | 51.2 V |
|---|---|---|---|---|
| 7 Ah | 84 | 90 | 179 | 358 |
| 20 Ah | 240 | 256 | 512 | 1,024 |
| 50 Ah | 600 | 640 | 1,280 | 2,560 |
| 100 Ah | 1,200 | 1,280 | 2,560 | 5,120 |
| 200 Ah | 2,400 | 2,560 | 5,120 | 10,240 |
| 280 Ah | 3,360 | 3,584 | 7,168 | 14,336 |
| 300 Ah | 3,600 | 3,840 | 7,680 | 15,360 |
Rated vs usable watt hours
A lead-acid or AGM battery lasts far longer if you only use about half its rated capacity, so a 100 Ah, 12 V AGM gives you about 600 Wh in practice. A 100 Ah LiFePO4 battery can deliver most of its 1,280 Wh and still last thousands of cycles. Through an inverter, take off another 10–15% for AC loads; the runtime calculator does that step.
Power banks in mAh
Phone and laptop power banks are rated in milliamp hours at the cell voltage, 3.6–3.7 V. The FAA allows lithium batteries up to 100 Wh in carry-on bags without airline approval.
| Power bank | Watt hours at 3.7 V |
|---|---|
| Phone power bank, 10,000 mAh | 37 |
| Laptop power bank, 20,000 mAh | 74 |
| Largest common carry-on bank, 26,800 mAh | 99 |
Common questions
How many watt hours is a 100 Ah battery?
1,200 Wh at 12 V, 1,280 Wh for a 12.8 V LiFePO4 battery, and 5,120 Wh for a 51.2 V server-rack battery.
How do I convert watt hours to amp hours?
Divide by the voltage. 1,000 Wh at 12 V is about 83 Ah; at 51.2 V it is about 19.5 Ah.
How long will a 100 Ah battery run a 100 W load?
About 5 hours from a 12 V AGM (600 usable Wh, less inverter losses) and about 10–11 hours from a 12.8 V LiFePO4.
Is mAh the same as Ah?
1,000 mAh = 1 Ah. Multiply Ah by the voltage to get Wh: 20,000 mAh at 3.7 V is 74 Wh.
Guides that use this calculator
- How Long Will a 1,000 Wh Power Station Actually Last?
- Can a Portable Power Station Run a CPAP? Runtime by Size