---
title: "UPS Runtime Calculator: Battery Backup Time for Any Load"
description: "Estimate how long a UPS keeps your computer, router or server running in a power cut, from its battery Ah and voltage and the load in watts."
canonical: https://batteryruntimecalculator.com/ups-runtime-calculator
language: en
updated: 2026-11-10
source_revision: 59f9d32
maintainer: Battery Runtime Calculator (hello@batteryruntimecalculator.com)
---

# UPS Runtime Calculator

A UPS calculator for battery backup time: estimate how long your uninterruptible power supply keeps a computer, router or NAS running when the power goes out. 

Updated November 10, 2026 

Quick answer A 1,000 VA UPS with one 12 V 9 Ah battery runs a 100 W load for about **28 min** and a 20 W router and modem for about **2 h 57 min**. Runtime falls faster than the load rises, because small lead-acid batteries lose capacity at high currents.

Estimated runtime

28 min

≈ 0.46 hours on 108 Wh at 100 W

Usable energy

86 Wh

Battery current

9.8 A

Power from battery

118 W

Discharge rate

1.09C

-   This load pulls 1.1C from the battery. Lead-acid batteries deliver noticeably less than their rating at this rate, and the voltage may sag below the inverter cut-off early.
-   Draining lead-acid past 50% shortens its life. 80% is fine for occasional outages but not for daily cycling.

How this was worked out

1.  Battery energy: 108 Wh rated
2.  Power drawn from the battery: 100 W ÷ 85% efficiency = 118 W
3.  Rate effect (Peukert 1.15): the battery delivers 63% of its rating at this load
4.  Runtime: 108 Wh × 63% × 80% usable ÷ 118 W = 0.46 h

Runtime at other loads

| Load | Runtime |
| --- | --- |
| 25 W | 2 h 17 min |
| 50 W | 1 h 2 min |
| 100 W | 28 min |
| 150 W | 17 min |
| 200 W | 13 min |

## UPS runtime chart

Estimated runtime for common UPS battery setups, using sealed lead-acid (AGM) batteries in good condition, 85% inverter efficiency, and the UPS shutting down at about 80% of the battery’s capacity.

| UPS battery | 50 W | 100 W | 200 W | 400 W |
| --- | --- | --- | --- | --- |
| 1 × 12 V 7 Ah (small 600–850 VA UPS) | 46 min | 21 min | 9 min | 4 min |
| 1 × 12 V 9 Ah (1000 VA) | 1 h 2 min | 28 min | 13 min | 6 min |
| 2 × 12 V 9 Ah (1500 VA) | 2 h 17 min | 1 h 2 min | 28 min | 13 min |
| 2 × 12 V 18 Ah (2000–3000 VA) | 5 h 3 min | 2 h 17 min | 1 h 2 min | 28 min |

Doubling the load more than halves the runtime. A 7 Ah battery at 400 W is discharging at several times its rated current, and lead-acid delivers far less of its rated capacity that fast.

## How to find your UPS load in watts

-   **Read the UPS display or software.** Most units show the load in watts or as a percentage of capacity. Multiply the percentage by the UPS watt rating (not the VA).
-   **Use a plug-in power meter** on each device for a few minutes of normal use.
-   **Estimate from typical values:** a cable modem and Wi-Fi router draw 10–25 W together, a laptop 30–90 W, an office desktop with monitor 100–250 W, and a gaming PC 300–600 W under load.

## How UPS runtime is calculated

The calculator turns the battery into watt-hours (Ah × V × number of batteries), takes off the inverter loss inside the UPS, and applies Peukert’s law for sealed lead-acid batteries. That last step matters more for a UPS than for almost any other battery: UPS batteries are small and the loads are large compared to them, so they routinely discharge in 10 to 30 minutes, where a lead-acid battery gives well under its 20-hour rating. The full method is on [how battery runtime is calculated](https://batteryruntimecalculator.com/how-battery-runtime-is-calculated).

Planning a bigger backup for a home office or network rack? Use the [battery size calculator](https://batteryruntimecalculator.com/battery-size-calculator) to work out the capacity for the hours you need, or compare with a [portable power station](https://batteryruntimecalculator.com/power-station-runtime-calculator).

## Frequently asked questions

### How long will a UPS last during a power outage?

It depends on the battery and the load. A typical 1,000 VA UPS with one 12 V 9 Ah battery runs a 100 W load (a desktop PC at idle, or a router plus a laptop) for about 28 min, and a 20 W router alone for 2 h 57 min. Desktop UPS units are built to give you time to save work and shut down, not to ride out long outages.

### How do I find my UPS battery capacity?

The replacement battery label shows voltage and amp-hours, for example 12 V 9 Ah. Check the UPS manual or the battery compartment, and count the batteries: units of 1,500 VA and up usually have two 12 V batteries in series (24 V).

### Is the VA rating the same as runtime?

No. VA and watts are the maximum load the UPS can power, not how long it runs. Two UPS units with the same VA rating can have very different batteries and runtimes. Runtime comes from the battery capacity and your actual load.

### Why does my UPS run for less time than the calculator shows?

UPS batteries are small sealed lead-acid batteries that wear out in 3 to 5 years, and capacity drops long before they fail completely. Set battery health under Advanced settings to 70–80% for a battery that is a few years old. High loads also cut lead-acid capacity sharply, which the calculator models with Peukert’s law.

### How can I get more runtime from a UPS?

Plug only the essentials into the battery-backed outlets (the router, modem and the computer itself, not printers or speakers), replace old batteries, or choose a UPS that accepts an external battery pack. For hours of runtime, a LiFePO4 power station or a home inverter suits better than a desktop UPS.

## More battery calculators

-   [Battery runtimeRuntime for any battery and load, in Ah or Wh, AC or DC.](https://batteryruntimecalculator.com/)
-   [Battery life (mAh)mAh and mA to hours, with sleep current and duty cycle.](https://batteryruntimecalculator.com/battery-life-calculator)
-   [Battery sizeHours you need in, battery Ah, Wh and count out.](https://batteryruntimecalculator.com/battery-size-calculator)
-   [Solar battery bankDaily kWh and days of autonomy to a battery bank size.](https://batteryruntimecalculator.com/solar-battery-calculator)
-   [Inverter backupHome inverter backup hours for 150 Ah to 220 Ah batteries.](https://batteryruntimecalculator.com/inverter-battery-backup-calculator)
-   [Power station runtimeWh rating to runtime for portable power stations.](https://batteryruntimecalculator.com/power-station-runtime-calculator)
-   [12V battery runtime12V deep-cycle runtime for RVs, vans and boats.](https://batteryruntimecalculator.com/12v-battery-runtime-calculator)
-   [Ah to Wh converterAmp hours, watt hours and kWh at any voltage.](https://batteryruntimecalculator.com/ah-to-wh-calculator)
-   [mAh to Wh convertermAh to Wh for power banks, with the 100 Wh flight limit.](https://batteryruntimecalculator.com/mah-to-wh-calculator)
-   [Charge timeHours to charge from any state of charge, by charger amps.](https://batteryruntimecalculator.com/battery-charge-time-calculator)
-   [How runtime is calculatedThe formulas behind every calculator, with worked examples.](https://batteryruntimecalculator.com/how-battery-runtime-is-calculated)