UPS Runtime Calculator

Estimate ideal UPS runtime from battery energy, load and efficiency

Your result will appear here.

About the UPS Runtime Calculator

UPS Runtime Calculator estimates how long an uninterruptible power supply may support a connected load during a power outage. It is useful for computers, networking gear and other equipment that needs enough backup time for shutdown or short outages.

Battery nameplate energy does not translate directly into load runtime. Inverter losses, battery age, discharge rate, low-voltage cutoff and the UPS manufacturer’s internal reserve all reduce usable energy.

How to Use the UPS Runtime Calculator

  1. Enter UPS battery voltage and amp-hour capacity or the supported battery energy value.
  2. Enter the connected load in watts.
  3. Enter efficiency or usable-capacity assumptions if available.
  4. Calculate runtime and compare the result with the manufacturer’s published runtime curve for the specific UPS.

How the Calculation Works

Nominal battery energy ≈ voltage×amp-hours in watt-hours for a simple DC battery estimate.

Usable AC energy = nominal energy × usable fraction × inverter efficiency. Estimated runtime in hours = usable watt-hours ÷ load watts. Real UPS units can deviate because battery capacity falls at high discharge rates.

Worked example

UPS Runtime Calculator Worked Example

24 V, 9 Ah battery bank, 120 W load, 80% usable and 85% efficient

Nominal energy = 24×9 = 216 Wh. Usable AC energy ≈216×0.80×0.85 = 146.9 Wh. Runtime ≈146.9÷120 = 1.22 hours.

Result guide

Understanding Your UPS Runtime Calculator Result

The result is a planning estimate for the entered battery and load. A higher load normally shortens runtime more than a simple proportional model can predict on some battery chemistries.

Before you rely on the answer

Assumptions and Limitations

  • Battery configuration and capacity are known correctly.
  • Load watts reflect equipment draw during backup operation.
  • Efficiency and usable fraction are realistic for the UPS and battery condition.

Practical tip

Using This Result in Practice

For a production system, test runtime under a controlled outage or use the vendor’s load-versus-runtime chart. Battery aging can cut runtime significantly even when the UPS still reports a normal status.

Related calculations

Useful Calculations to Continue With

If you want a general battery-and-load runtime estimate outside a UPS, use the Battery Runtime Calculator.

If you need to understand how aggressively the battery is being discharged relative to its capacity, use the Battery C-Rate Calculator.

Common questions

UPS Runtime Calculator FAQ

Why is UPS runtime shorter than battery Wh divided by load W?

Inverter losses, reserve capacity, battery discharge behavior and cutoff voltage reduce usable energy.

Does doubling the load always halve runtime?

Not exactly for every battery because high discharge rates can reduce usable capacity.

Should I trust an old UPS battery’s nameplate capacity?

No. Aging can materially reduce actual capacity and runtime.