A battery's label gives amp-hours, but what you want to know is how long it will run your fridge, lights or inverter. That takes two steps: converting amp-hours to watt-hours, then working out how much of that energy you can actually use.
Watt-hours = amp-hours × volts, so a 100 Ah 12V battery stores about 1,200 Wh. With the calculator's assumptions, a lithium iron phosphate battery delivers about 933 Wh of that, running a 100-watt load for about 9.3 hours; a lead-acid battery about 502 Wh, or 5 hours.
12V Battery Watts Calculator
Runtime estimate for 12V DC systems, planning tool, not a design specification.
Watts, watt-hours and amp-hours
- Watts measure power: how fast a device uses energy right now. Watts = volts × amps.
- Watt-hours measure energy: a 100-watt load for 1 hour uses 100 Wh.
- Amp-hours measure charge: 100 Ah can supply 1 amp for 100 hours, or 10 amps for 10 hours, in theory.
- Milliamp-hours (mAh) are amp-hours ÷ 1,000, so a 20,000 mAh pack is 20 Ah; kilowatt-hours (kWh) are watt-hours ÷ 1,000.
The calculator uses 12 volts. A lithium iron phosphate battery's nominal voltage is closer to 12.8, so its true energy is a few percent higher.
How much of the energy you can use
The calculator multiplies the stored energy by three factors, all assumptions you can change:
- Usable depth of discharge: about 90% for lithium iron phosphate and 50% for lead-acid, since draining lead-acid deeper shortens its life. Check your battery's specs.
- System efficiency: 90%, for inverter and wiring losses.
- Temperature: capacity drops in the cold; at 68°F the calculator applies 96% for lithium and 93% for lead-acid.
12V battery chart
At 68°F, with the calculator's default assumptions:
| Battery | Stored energy | Usable, lithium iron phosphate | Usable, lead-acid |
|---|---|---|---|
| 35 Ah | 420 Wh | 327 Wh | 176 Wh |
| 55 Ah | 660 Wh | 513 Wh | 276 Wh |
| 75 Ah | 900 Wh | 700 Wh | 377 Wh |
| 100 Ah | 1,200 Wh | 933 Wh | 502 Wh |
| 150 Ah | 1,800 Wh | 1,400 Wh | 753 Wh |
| 200 Ah | 2,400 Wh | 1,866 Wh | 1,004 Wh |
How long a 100 Ah battery runs common loads
Runtime = usable watt-hours ÷ the load's watts:
| Load | Lithium iron phosphate | Lead-acid |
|---|---|---|
| LED strip, 24 W | 38.9 hours | 20.9 hours |
| 12V fridge, 45 W | 20.7 hours | 11.2 hours |
| Water pump, 60 W | 15.6 hours | 8.4 hours |
| 300 W through an inverter | 3.1 hours | 1.7 hours |
A fridge cycles on and off, so its average draw is usually below its rated watts, and it runs longer than the table shows.
Recharging
Charge at the rate your battery's maker specifies. The calculator's charge-time estimate assumes 10% of capacity, 10 amps for a 100 Ah battery, plus about 15% for charging losses: about 11.5 hours from empty to full. Faster charging is possible where the battery and charger are rated for it.
Wiring
At 12 volts, power means high current: 300 watts draws 25 amps. Size wires and fuses for that current under the NEC for buildings or ABYC E-11 for boats; see our wire gauge chart.
Watt-hours for any battery
The same formula, watt-hours = volts × amp-hours, works for any battery once you use its nominal voltage. The calculator below converts volts and amp-hours, or milliamp-hours, to watt-hours for any chemistry, from an AA cell to a home battery bank.
Battery Watt-Hours Calculator
Runtime, days-per-charge, cycle life, planning estimate for battery banks.
Nominal voltage by chemistry
Use the battery's nominal voltage, its average during discharge, not the higher voltage when fully charged:
| Chemistry | Nominal voltage per cell | Common packs |
|---|---|---|
| Alkaline | 1.5 V | AA, AAA, C, D |
| Nickel-metal hydride | 1.2 V | Rechargeable AA, AAA |
| Lithium-ion | 3.6 to 3.85 V | Phones, laptops, power banks |
| Lithium iron phosphate | 3.2 V | 4 cells = 12.8 V |
| Lead-acid | 2.0 V | 6 cells = 12 V |
Examples
| Battery | Voltage × capacity | Watt-hours |
|---|---|---|
| AA alkaline | 1.5 V × 2.5 Ah | 3.75 Wh |
| Phone | 3.85 V × 4.5 Ah | 17.3 Wh |
| Power bank, 20,000 mAh | 3.7 V × 20 Ah | 74 Wh |
| Lead-acid, 12 V, 100 Ah | 12 V × 100 Ah | 1,200 Wh |
| Lithium iron phosphate, 100 Ah | 12.8 V × 100 Ah | 1,280 Wh |
| Home battery bank, 200 Ah | 51.2 V × 200 Ah | 10,240 Wh (10.2 kWh) |
A power bank's mAh rating is at its internal cells' voltage, about 3.7 volts, not the 5 volts it delivers over USB; converting it to watt-hours avoids that confusion.
Airline watt-hour limits
The FAA allows spare lithium-ion batteries of up to 100 Wh in carry-on bags, and 101 to 160 Wh with airline approval; spares aren't allowed in checked bags. A 74 Wh power bank is under the limit, so check the Wh on the label, or work it out from mAh and voltage.
Frequently asked questions
How many watts is a 100Ah 12V battery?
It stores about 1,200 watt-hours. You can use about 933 Wh from a lithium iron phosphate battery and about 502 Wh from lead-acid under the calculator's assumptions.
How long will a 12V battery run a 100-watt load?
A 100 Ah lithium iron phosphate battery runs it about 9.3 hours, and a lead-acid battery about 5 hours, at 68°F.
How many amps does a 12V device draw?
Divide its watts by 12. A 60-watt pump draws 5 amps; a 300-watt inverter load about 25 amps.
Why can I only use half of a lead-acid battery?
Draining lead-acid deeper shortens its life sharply, which is why the calculator assumes a shallower working limit for it than for lithium. Lithium iron phosphate tolerates much deeper discharge.
How many watt-hours can I take on a plane?
Spare lithium-ion batteries up to 100 Wh in carry-on bags, and 101 to 160 Wh with the airline's approval, according to the FAA. Spares aren't allowed in checked bags.