IT technician checking a desktop UPS and power meter to estimate backup runtime

Power backup planning

UPS Runtime Calculator Estimate Backup Time & Battery Ah

Calculate a realistic runtime range, check both VA and watt headroom, and reverse-size the battery capacity required for your target backup time.

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Advanced UPS Runtime Calculator

Enter measured load and the real battery-bank layout. The model accounts for chemistry, discharge rate, reserve, temperature, health and inverter efficiency.

Advanced battery model

Estimate UPS runtime and required battery capacity

Model the battery bank, connected load, efficiency, reserve, temperature and battery health. Results are a planning range, not a guarantee.

Calculated locally in your browser
Quick presets
Connected load and UPS
Battery bank
Advanced assumptions and reverse sizing

Planning estimate

UPS runtime result

Enter your system details
Estimated runtime — —

The calculator will show a conservative range after accounting for discharge rate and battery condition.

Battery bank—
Nominal energy—
Usable energy—
Estimated load—
UPS load—
Ah for desired runtime—

Runtime across common load levels

LoadWattsEstimated runtime
What this result includes

The estimate models bank voltage and Ah, inverter loss, reserve, temperature, battery health and Peukert discharge behavior. Manufacturer runtime curves remain the best final reference.

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Advanced UPS Runtime Calculator is a free, browser-based online utility that lets you estimate UPS battery backup time with load watts, VA, power factor, battery chemistry, series and parallel banks, Peukert effect, health and temperature.

  • Cost: Free, no signup
  • Install: None — runs in the browser
  • Privacy: Runs locally, no uploads
  • Platforms: Windows, macOS, Linux, Android, iOS
  • Time: Under a minute

Reliable planning workflow

How to estimate UPS battery backup time

Runtime is not simply watt-hours divided by watts. High discharge current, battery age, temperature and the UPS inverter all reduce usable energy.

01 Measure the real load Use a wall power meter and enter watts plus the load power factor instead of adding nameplate maximums.
02 Describe the battery bank Enter voltage and Ah per battery, batteries in series, parallel strings, chemistry and estimated health.
03 Review both limits A UPS must have enough watt capacity and enough VA capacity. The stricter percentage sets the headroom verdict.
04 Validate before relying on it Compare the range with the manufacturer curve, then run a controlled self-test with safe shutdown configured.
UPS runtime calculator battery backup time UPS load calculator battery Ah sizing Peukert effect series and parallel batteries
Planning estimate only. Battery manufacturing variation, UPS firmware, waveform, cut-off voltage and sudden load changes can move actual runtime outside the range.

UPS calculator FAQ

UPS runtime calculation questions

How is UPS runtime calculated?

The calculator derives bank voltage and Ah from the series and parallel layout, estimates DC current, then applies inverter efficiency, reserve, health, temperature, chemistry depth of discharge and Peukert discharge behavior.

Why is VA different from watts?

Watts measure real power while VA measures apparent power. A load can remain below the UPS watt rating but exceed its VA rating when power factor is low, so both limits matter.

What is the Peukert effect?

Lead-acid batteries deliver less usable capacity when discharged faster than their rated test rate. A higher Peukert exponent produces a larger high-current penalty. Lithium batteries usually have a smaller effect.

How do series and parallel batteries change capacity?

Series batteries add voltage while Ah stays the same. Parallel strings add Ah while voltage stays the same. The calculator models both dimensions separately.

Can this calculator replace a manufacturer runtime chart?

No. It provides a transparent planning range and overload check. The exact UPS model runtime chart and a controlled battery test are the final references.

Windows app

KeyboardTester.click is also on Microsoft Store

Install the official Windows shortcut or keep using the same free browser tools.

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