How do I calculate battery backup time?

Convert the battery bank to nominal watt-hours, multiply by the usable battery fraction, system efficiency, battery-condition factor and any reserve factor, then divide the usable watt-hours by the connected load in watts.

How do series and parallel batteries affect backup time?

Series batteries increase bank voltage while keeping amp-hour capacity per string the same. Parallel strings increase amp-hour capacity. Total stored watt-hours increase with the total number of identical batteries, but the inverter or UPS must match the configured bank voltage.

Why can actual battery runtime be lower than the estimate?

Actual runtime can be reduced by battery age, temperature, discharge rate, chemistry, cutoff voltage, inverter standby use, changing loads and manufacturer-specific discharge behavior.

BATTERY RUNTIME · UPS / INVERTER BACKUP · REQUIRED AH

Battery Backup Calculator

Q: How do I calculate the battery Ah needed for a target runtime?

Multiply load watts by required hours, then divide by system voltage and the usable-capacity, efficiency, condition and reserve factors. The calculator shows both the exact figure and a rounded planning figure.

Estimate how long a battery bank can run a load, or calculate the battery capacity needed for a target backup time.

Battery backup workspace

Choose the direction of the calculation, enter your battery/load values, and see the result with the full working beside the inputs.

🔒 Runs locally in your browser

Use this mode when you already know your battery bank and want to estimate how long it can power the connected load.

Capacity per battery

Voltage per battery (V)

Batteries in seriesSeries increases bank voltage.

Parallel stringsParallel increases bank Ah.

Connected load

Watts is preferred. Amps are treated at the calculated battery-bank voltage.

Quick examplesChoose an example to fill the fields and calculate instantly.

Advanced loss & reserve settings

Usable battery capacity (%)Equivalent to the usable depth-of-discharge assumption.

Inverter / system efficiency (%)Use 100% only when you intentionally want no conversion-loss allowance.

Battery condition (%)Reduce this only when you have a reasonable capacity/condition assumption.

Reserve margin (%)Leaves part of the otherwise usable energy unused.

Battery behavior is not perfectly linear. For critical systems, compare this energy estimate with the battery and UPS/inverter manufacturer's runtime or discharge data.

Your result will appear hereCalculate to see runtime or required battery capacity, key figures and the complete working beside the inputs.

Calculated result

Planning and Safety Notes

This calculator estimates energy capacity and runtime. It does not size inverter VA/kVA, surge capacity, battery charging current, DC cables, fuses, breakers, ventilation or installation clearances. Battery systems can involve high current and hazardous energy. For installation or safety decisions, follow the equipment manufacturer's documentation and applicable electrical requirements.

All calculations run in your browser. You can copy the complete result, save a clean result image with the working steps, or share the result without sending your entered values to an OfficeCalculator.Net calculation server.

Frequently Asked Questions

How long will a 100 Ah 12 V battery run a 300 W load?

Using 80% usable battery capacity and 90% system efficiency, the simple energy estimate is about 2.88 hours. Real runtime can be lower or higher depending on battery condition, discharge rate, temperature, cutoff behavior and equipment losses.

Can I calculate the battery size needed for a specific backup time?

Yes. Select Find required battery Ah, enter the load, battery-bank voltage and required time, then set usable capacity, efficiency, condition and reserve. The calculator returns the exact capacity estimate and a rounded planning figure.

Should I enter watts or amps?

Watts is usually the clearest input for an AC load powered through a UPS or inverter. If you select amps, the calculator treats the value as an equivalent current at the stated battery-bank voltage. Set the efficiency field appropriately for the system you are modeling.

Does a bigger inverter give longer battery backup?

Not by itself. Runtime is primarily an energy question: battery-bank energy and losses compared with the connected load. Inverter power rating determines what load it can support, while battery capacity largely determines how long that load can run.

Why Actual Battery Runtime Can Be Different

Battery runtime is affected by more than nominal Ah. Battery age and condition, temperature, chemistry, discharge rate, cutoff voltage, inverter standby consumption and a changing connected load can all change the real result. The usable-capacity, efficiency, condition and reserve fields let you make the arithmetic more conservative, but they cannot reproduce every manufacturer's discharge curve.

For direct DC loads, you may choose a system efficiency close to 100% if that matches the setup you are intentionally modeling. For inverter or UPS loads, use an efficiency assumption supported by the equipment documentation when available.

Series vs Parallel Batteries

With identical batteries, connecting batteries in series increases the battery-bank voltage while the amp-hour capacity of one string stays the same. Adding parallel strings increases amp-hour capacity while keeping the string voltage the same. Either arrangement adds total stored energy when more batteries are added.

For example, two 12 V 100 Ah batteries in series form a 24 V 100 Ah bank with about 2,400 Wh nominal energy. Two 12 V 100 Ah batteries in parallel form a 12 V 200 Ah bank, also about 2,400 Wh. The correct wiring arrangement depends on the equipment and is outside this calculator; use the bank configuration required by the UPS, inverter or battery system manufacturer.

Worked Battery Backup Examples

Runtime example: a 12 V, 100 Ah battery stores about 1,200 Wh nominally. At 80% usable capacity and 90% system efficiency, it provides about 864 Wh to the load. At 300 W, the energy-based estimate is about 2.88 hours, or roughly 2 hours 53 minutes, before any additional reserve or battery-condition reduction.

Capacity example: a 300 W load for 4 hours needs 1,200 Wh at the load. At 12 V, 80% usable capacity, 90% efficiency and a 10% reserve, the planning requirement is about 154.3 Ah. The calculator rounds this upward only as a convenient planning figure; available commercial sizes vary.

Battery Backup Formula

For a battery bank rated in amp-hours, nominal energy is approximately Ah × volts. For multiple identical batteries, the calculator derives bank voltage and total stored energy from the number in series and the number of parallel strings.

Estimated backup hours = nominal battery Wh × usable fraction × efficiency × battery condition × (1 − reserve) ÷ load watts.

To size a battery for a target runtime, the calculation is reversed: required Ah = load watts × hours ÷ [system volts × usable fraction × efficiency × battery condition × (1 − reserve)].

Battery Backup Time and Required Capacity

This calculator works in both directions. Estimate backup runtime when you know the battery bank and connected load, or switch to Find required battery Ah when you know the load and the backup time you want. The result panel shows nominal energy, usable output energy, bank voltage, current estimate and the formula steps used.

The calculation uses watt-hours as the common energy unit. That makes it possible to compare batteries rated in amp-hours with loads measured in watts while still accounting for usable battery capacity, inverter or system efficiency, battery condition and an optional reserve.