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Battery backup planning tool

UPS Runtime Calculator

Estimate how long a UPS battery bank can support computers, networking equipment, security systems, servers, or other essential loads during a power outage.

Battery voltage, Ah, and quantity
Inverter losses and safe discharge limit
Runtime in hours and minutes

Calculate UPS battery backup time

Use battery-label values and the real connected load for the best estimate.

Live calculation

Battery bank details

Enter values for identical batteries used in the UPS.

Use the voltage printed on one battery.
Ah
Enter the total number of identical batteries.
W
W
Internal electronics and cooling losses.
%
%
%
Use 100% for a new, fully charged bank.
hr
Used to estimate how many matching batteries are needed.
Simple process

How the UPS runtime calculator works

The calculator starts with nominal battery energy, then reduces it to a practical usable amount before dividing that energy by the total watts the UPS must support.

1

Enter battery data

Add the voltage, amp-hour capacity, and total number of identical batteries in the UPS bank.

2

Add the real load

Enter the combined wattage of all equipment that must continue operating during an outage.

3

Apply real-world losses

Adjust inverter efficiency, safe discharge, battery health, and UPS self-consumption.

4

Review backup time

Read the estimated hours and minutes, usable energy, and batteries required for your target.

UPS runtime formula

Nominal energy is calculated by multiplying battery voltage, amp-hours, and battery count. The result is then adjusted for the energy that can safely and realistically reach the load.

Runtime (hours) = V × Ah × batteries × DoD × health × efficiency ÷ total load watts

Total load watts include both connected equipment and the UPS unit's own operating demand. The optional target-runtime calculation reverses the same formula and rounds up to a whole battery.

Improve the estimate

  • Measure the equipment load with a plug-in watt meter when possible.
  • Use the battery manufacturer's recommended discharge limit.
  • Reduce battery health for old, hot, or heavily cycled batteries.
  • Confirm the UPS supports the battery chemistry and bank voltage.
  • Keep reserve time for shutdown, battery ageing, and unexpected load changes.
Helpful answers

UPS runtime calculator FAQs

Understand the assumptions that make battery-backup calculations more realistic and useful.

How is UPS runtime calculated?

The calculator converts battery voltage and amp-hours into watt-hours, multiplies that energy by battery count, then applies inverter efficiency, allowed depth of discharge, and battery health. Usable watt-hours are divided by the connected load plus UPS self-consumption.

Why is actual UPS runtime lower than the basic Ah formula?

A basic voltage × amp-hours calculation assumes every stored watt-hour reaches the load. Real UPS systems lose energy through inverter conversion, internal electronics, cabling, battery ageing, temperature, and high discharge rates.

What depth of discharge should I use?

For many lead-acid, AGM, and gel batteries, 50 percent is a conservative planning value that supports battery life. Many LiFePO4 batteries can use 80 to 90 percent, but the battery manufacturer and UPS charging system should be checked.

Can I enter several batteries?

Yes. Enter the total number of identical batteries in the UPS bank. Multiplying voltage, amp-hours, and battery count gives the same total nominal watt-hours whether identical batteries are arranged in series, parallel, or matched series-parallel strings.

What should I enter as the connected load?

Use the real power in watts for every device that will operate during an outage. A plug-in power meter or equipment specification is more reliable than using the UPS VA rating as though it were watts.

Does a larger UPS VA rating always provide longer runtime?

No. VA rating mainly describes the power the UPS can supply at one time. Runtime depends primarily on battery energy, load watts, inverter efficiency, discharge limits, and battery condition.

Why does battery age matter?

Battery capacity decreases with age, heat, repeated cycling, storage conditions, and maintenance. The battery health setting reduces the calculation so an older battery bank does not appear to provide new-battery runtime.

Important: This calculator provides a planning estimate, not a guaranteed runtime. Critical medical, safety, server, telecom, and security loads should be verified with manufacturer runtime charts, measured load data, battery testing, and a qualified UPS or electrical professional.