Battery Life Calculator

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ups battery runtime calculator

UPS runtime: calculator by battery and load

In a UPS, runtime comes from the energy in the batteries divided by the load power. Build the pack in the Cell pack tab or add the batteries by hand, enter the load in watts and put the inverter efficiency under Your assumptions.

UPS batteries in series and in parallel

UPS units use one or more sealed 12 V batteries. The wiring sets the bus voltage: in series the voltages add and the Ah capacity does not change; in parallel the voltage stays and the Ah add. In energy the result is the same, the sum of the Wh of each battery. Two 12 V, 7 Ah batteries give 168 Wh, whether in series (24 V, 7 Ah) or in parallel (12 V, 14 Ah). What changes is the voltage the UPS inverter expects.
The load is the real power in watts that the equipment draws, not the VA rating of the UPS. Add up the power of each connected device. The inverter efficiency, which you enter, makes the battery deliver more than the load receives: at 85%, a 60 W load draws about 70.6 W from the batteries. The accepted discharge limits the fraction of the energy that enters the math, and both fields are your assumptions, with no ready-made number.
This is an energy estimate. Battery voltage drops during discharge, old batteries hold less, and manufacturers publish runtime tables by load that reflect the actual UPS curve. For a server or medical equipment, rely on the manufacturer's table and use the calculator to compare scenarios.

UPS scenarios

Two 12 V, 7 Ah batteries, 100 W load

Input: 168 Wh, 100 W
Expected output: 1 h 41 min

1.68 h in the ideal case.

One 12 V, 7 Ah battery, 60 W load, 85% efficiency

Input: 84 Wh x 85% / 60 W
Expected output: 1 h 11 min

1.19 h.

12 V and 100 Ah, 300 W, 80% discharge

Input: 1.200 Wh x 80% / 300 W
Expected output: 3 h 12 min

3.2 h.

Two 12 V, 100 Ah in series, 600 W, 90% and 50%

Input: 2.400 Wh x 50% x 90% / 600 W
Expected output: 1 h 48 min

1.8 h with a 24 V bus.

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

What is the difference between VA and watts on a UPS?

VA is the apparent power of the UPS and watts is the real power the load consumes. The runtime math uses watts. If the equipment label only gives VA or amps, multiply by the voltage and the power factor, or measure the consumption.

How do I know my inverter efficiency?

Check the UPS datasheet or manual. The tool does not ship a default number: leave it at 100% for the ideal limit or enter the maker's value and compare the two results.

Can I add batteries of different capacities?

In energy terms the sum works, but in practice mixing different batteries wears out the weaker ones and is not recommended. Use identical batteries of the same age and model.

Next steps

UPS batteries in series and in parallel

UPS units use one or more sealed 12 V batteries. The wiring sets the bus voltage: in series the voltages add and the Ah capacity does not change; in parallel the voltage stays and the Ah add. In energy the result is the same, the sum of the Wh of each battery. Two 12 V, 7 Ah batteries give 168 Wh, whether in series (24 V, 7 Ah) or in parallel (12 V, 14 Ah). What changes is the voltage the UPS inverter expects.

The load is the real power in watts that the equipment draws, not the VA rating of the UPS. Add up the power of each connected device. The inverter efficiency, which you enter, makes the battery deliver more than the load receives: at 85%, a 60 W load draws about 70.6 W from the batteries. The accepted discharge limits the fraction of the energy that enters the math, and both fields are your assumptions, with no ready-made number.

This is an energy estimate. Battery voltage drops during discharge, old batteries hold less, and manufacturers publish runtime tables by load that reflect the actual UPS curve. For a server or medical equipment, rely on the manufacturer's table and use the calculator to compare scenarios.

UPS scenarios

Two 12 V, 7 Ah batteries, 100 W load

Input: 168 Wh, 100 W
Expected output: 1 h 41 min

1.68 h in the ideal case.

One 12 V, 7 Ah battery, 60 W load, 85% efficiency

Input: 84 Wh x 85% / 60 W
Expected output: 1 h 11 min

1.19 h.

12 V and 100 Ah, 300 W, 80% discharge

Input: 1.200 Wh x 80% / 300 W
Expected output: 3 h 12 min

3.2 h.

Two 12 V, 100 Ah in series, 600 W, 90% and 50%

Input: 2.400 Wh x 50% x 90% / 600 W
Expected output: 1 h 48 min

1.8 h with a 24 V bus.

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

What is the difference between VA and watts on a UPS?

How do I know my inverter efficiency?

Check the UPS datasheet or manual. The tool does not ship a default number: leave it at 100% for the ideal limit or enter the maker's value and compare the two results.

Can I add batteries of different capacities?

In energy terms the sum works, but in practice mixing different batteries wears out the weaker ones and is not recommended. Use identical batteries of the same age and model.