How is UPS runtime calculated?

In Ah mode: Runtime (hours) = (Ah × Volts × Efficiency) / Load (W). In VA mode, this quick estimate uses Wh ≈ VA × 0.084 for a typical small lead-acid UPS, then Runtime (minutes) = (Wh × Efficiency / Load) × 60. Actual battery configurations vary, so use Ah mode or manufacturer runtime data when available.

Why does VA mode use a 0.084 factor?

A typical small 1000 VA UPS with a 12 V 7 Ah battery stores about 84 Wh, or 0.084 Wh per rated VA. This is only an empirical estimate: larger and extended-runtime systems can have very different battery energy, so use Ah mode when you know the battery specifications.

What efficiency should I use?

Lead-acid batteries (standard in most UPS units) deliver about 90% of rated capacity at moderate loads. Lithium-ion UPS batteries reach 95%. Both values decrease at high loads or with aged batteries.

Why does actual runtime differ from the manufacturer spec?

Manufacturer runtime figures use a specific reference load (often 50% of rated VA) and a new battery at room temperature. Higher loads, aged batteries, or low ambient temperature all reduce runtime. This calculator gives a working estimate; always add a safety margin of 20–30%.

How many batteries does a UPS have?

Small desktop UPS units (600–1500 VA) typically use one 12 V battery. Larger rack-mount units (2000–3000 VA) often use two or four batteries in series/parallel. Check the UPS specification sheet or the physical unit to confirm.

Can I use this for lithium-ion UPS units?

Yes. Select "Lithium-ion" to apply the 95% efficiency factor. Use Ah mode with the battery's rated capacity and voltage from the spec sheet for the most accurate result.

UPS Runtime Calculator

Enter your UPS capacity and load to instantly see how long the battery will last — in minutes and hours.

UPS Battery Details

Input Mode

Uses rated UPS VA — quick estimate for standard lead-acid units.

UPS Capacity (VA)

Connected Load (W)

If you know load in VA, multiply by 0.8 to get watts.

Battery Type

Target Runtime (minutes) — optional

Enter a target to see the recommended UPS size.

Fill in the details and click Calculate

Estimated Runtime

— min

Total Energy

—

Watt-hours available

Usable Energy

—

After efficiency loss

Load

—

Watts connected

Efficiency

—

Battery type factor

Runtime by Charge Level

Charge	Minutes	Hours

Estimates only. Actual runtime varies with battery age, temperature, and load profile. Add 20–30% safety margin for critical systems.

Summary

UPS runtime depends on battery capacity, connected load, and battery chemistry efficiency. This calculator offers two modes: VA mode (enter rated UPS VA and load watts, using the typical Wh/VA factor for lead-acid batteries) and Ah mode (enter battery amp-hours, voltage, number of batteries, and load watts for a precise result). Lead-acid batteries deliver roughly 90% usable energy; lithium-ion UPS batteries reach about 95%. The tool also estimates the UPS VA size needed to sustain a target runtime.

How it works

Choose an input mode: VA (quick, uses rated UPS size) or Ah + Voltage (precise, for known battery specs).
In VA mode, select a common UPS size or enter a custom VA value, then enter the load in watts.
In Ah mode, enter battery amp-hours, voltage, and number of batteries, then enter the load in watts.
Select battery type (Lead-acid or Lithium-ion) to apply the correct efficiency factor.
Click Calculate to get runtime in minutes and hours, plus a reverse estimate of the UPS size needed for a custom target runtime.

Use cases

Verify a 1500 VA UPS will keep a server online for the minimum time needed for graceful shutdown.
Size a UPS for a home office setup — PC, monitor, and router — based on measured load.
Estimate runtime for a 1000 VA UPS powering network equipment during a power outage.
Compare lead-acid versus lithium-ion UPS runtime for the same load.
Plan battery replacements by comparing calculated runtime against rated runtime from the manufacturer.
Determine the VA rating needed for a 30-minute backup target given a known load.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu