How is UPS runtime calculated?

In Ah mode: Runtime (hours) = (Ah × Volts × Efficiency) / Load (W). In VA mode: Wh = VA × 0.6 (the typical watt-hour capacity of a lead-acid UPS at rated VA), then Runtime (minutes) = (Wh × Efficiency / Load) × 60. These are estimates; actual runtime varies with battery age, temperature, and load curve.

Why does VA mode use a 0.6 factor?

A 1000 VA UPS with a standard 12 V 7 Ah lead-acid battery holds about 84 Wh (≈ 0.084 × 1000). The 0.6 factor is the typical ratio of watt-hours to VA for common small UPS units. Larger or multi-battery UPS systems have higher ratios — use Ah mode for those.

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

Name: UPS Runtime Calculator
Availability: InStock
Author: Nham Vu

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

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

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: 2026-07-01 · Reviewed by Nham Vu