Battery Life Calculator

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how long does a 12v 100ah battery last

How long a 12 V, 100 Ah battery lasts

Pick the 12 V 100 Ah UPS example in the How long it lasts tab: it brings 100 Ah, 12 V and a 300 W load. Swap the load for watts or amps, and adjust efficiency and discharge for a more conservative estimate.

The math in three steps

First, the energy: 12 V x 100 Ah = 1,200 Wh. Second, the load power in watts. If the load comes in amps, multiply by the voltage at which it is measured: 5 A at 12 V is 60 W. Third, divide: 1,200 Wh / 100 W = 12 h in the ideal case, 1,200 / 300 = 4 h, 1,200 / 60 = 20 h. The relation is inverse, so doubling the load halves the runtime.
The ideal case almost never holds. An inverter between the battery and the load loses some energy, so the battery delivers more power than the load receives: at 85% efficiency, a 100 W load pulls 117.6 W from the battery. And lead-acid batteries are usually discharged only so far, because deep and frequent discharges shorten their life. With 50% accepted discharge, the 1,200 Wh become 600 Wh usable. Both fields are your own assumptions and start at 100%, with no losses.
The math is about energy, not a discharge curve. High currents reduce the usable capacity of lead-acid batteries (the Peukert effect), cold does too, and an old battery delivers less than its label. If the project is critical, use the maker's discharge table and treat the calculator number as a starting point.

Runtimes for 12 V and 100 Ah

100 W load, ideal

Input: 1.200 Wh, 100 W
Expected output: 12 h

No losses and full discharge.

300 W load, ideal

Input: 1.200 Wh, 300 W
Expected output: 4 h

Three times the load, one third of the time.

5 A load at 12 V

Input: 5 A x 12 V = 60 W
Expected output: 20 h

1,200 Wh / 60 W.

100 W with 50% accepted discharge

Input: 1.200 Wh x 50% / 100 W
Expected output: 6 h

Half the usable energy, half the time.

100 W, 85% efficiency and 50% discharge

Input: 1.200 Wh x 50% x 85% / 100 W
Expected output: 5,1 h

Equals 5 h 6 min.

Full tool FAQ

How do I convert mAh to Wh?

Multiply the mAh by the voltage in volts and divide by 1,000. A 20,000 mAh power bank at 3.7 V holds 20,000 x 3.7 / 1,000 = 74 Wh. To go back, Wh x 1,000 / V.

How long does a 12 V, 100 Ah battery last?

What are efficiency and depth of discharge here?

Why does a power bank deliver fewer mAh than advertised?

How do I estimate the runtime of a UPS?

What is the Wh limit for carrying batteries on a plane?

How do I calculate a series and parallel cell pack?

Does the calculator consider temperature, ageing and the Peukert effect?

How do I size a battery for a load and a runtime?

Does my data leave the browser?

Frequently asked questions

Can I discharge the battery to zero?

The tool computes whatever you ask, but fully discharging usually shortens the life of lead-acid batteries. That is why the discharge field is yours: adjust it per your battery maker's recommendation.

Do two identical batteries double the time?

In parallel, yes: the voltage stays at 12 V and the capacity adds to 200 Ah, or 2,400 Wh. In series, the voltage rises to 24 V and the capacity stays at 100 Ah, also 2,400 Wh; the load must accept 24 V.

Why does a 100 W device draw more from the battery?

Because of converter efficiency. The inverter turns part of the energy into heat, so the battery must deliver the load power divided by the efficiency. Enter yours under Your assumptions.

Next steps

The math in three steps

First, the energy: 12 V x 100 Ah = 1,200 Wh. Second, the load power in watts. If the load comes in amps, multiply by the voltage at which it is measured: 5 A at 12 V is 60 W. Third, divide: 1,200 Wh / 100 W = 12 h in the ideal case, 1,200 / 300 = 4 h, 1,200 / 60 = 20 h. The relation is inverse, so doubling the load halves the runtime.

The ideal case almost never holds. An inverter between the battery and the load loses some energy, so the battery delivers more power than the load receives: at 85% efficiency, a 100 W load pulls 117.6 W from the battery. And lead-acid batteries are usually discharged only so far, because deep and frequent discharges shorten their life. With 50% accepted discharge, the 1,200 Wh become 600 Wh usable. Both fields are your own assumptions and start at 100%, with no losses.

The math is about energy, not a discharge curve. High currents reduce the usable capacity of lead-acid batteries (the Peukert effect), cold does too, and an old battery delivers less than its label. If the project is critical, use the maker's discharge table and treat the calculator number as a starting point.

Runtimes for 12 V and 100 Ah

100 W load, ideal

Input: 1.200 Wh, 100 W
Expected output: 12 h

No losses and full discharge.

300 W load, ideal

Input: 1.200 Wh, 300 W
Expected output: 4 h

Three times the load, one third of the time.

5 A load at 12 V

Input: 5 A x 12 V = 60 W
Expected output: 20 h

1,200 Wh / 60 W.

100 W with 50% accepted discharge

Input: 1.200 Wh x 50% / 100 W
Expected output: 6 h

Half the usable energy, half the time.

100 W, 85% efficiency and 50% discharge

Input: 1.200 Wh x 50% x 85% / 100 W
Expected output: 5,1 h

Equals 5 h 6 min.

Full tool FAQ

How do I convert mAh to Wh?

Multiply the mAh by the voltage in volts and divide by 1,000. A 20,000 mAh power bank at 3.7 V holds 20,000 x 3.7 / 1,000 = 74 Wh. To go back, Wh x 1,000 / V.

How long does a 12 V, 100 Ah battery last?

What are efficiency and depth of discharge here?

Why does a power bank deliver fewer mAh than advertised?

How do I estimate the runtime of a UPS?

What is the Wh limit for carrying batteries on a plane?

How do I calculate a series and parallel cell pack?

Does the calculator consider temperature, ageing and the Peukert effect?

How do I size a battery for a load and a runtime?

Does my data leave the browser?

Frequently asked questions

Can I discharge the battery to zero?

Do two identical batteries double the time?

Why does a 100 W device draw more from the battery?

Because of converter efficiency. The inverter turns part of the energy into heat, so the battery must deliver the load power divided by the efficiency. Enter yours under Your assumptions.