UPS Battery Runtime Calculator

Calculate UPS Battery Runtime

Connected Load

Power Factor

UPS Rating

Battery Voltage Per Unit

Batteries In Series

Parallel Strings

Battery Capacity Per Unit

Inverter Efficiency

Allowed Depth Of Discharge

Battery Health Factor

Temperature Capacity Factor

Reserve Capacity

Peukert Exponent

Rated Discharge Time

Example Data Table

Use Case	Load	Battery Bank	Assumed Losses	Estimated Runtime
Home router backup	80 W	12 V, 100 Ah	90% efficiency, 80% discharge	About 10.8 hours
Office desktop backup	250 W	24 V, 100 Ah	90% efficiency, 80% discharge	About 6.9 hours
Small server backup	900 W	48 V, 200 Ah	90% efficiency, 80% discharge	About 7.7 hours

Formula Used

DC bus voltage = battery voltage × batteries in series

Total amp hours = battery Ah × parallel strings

Nominal energy = DC bus voltage × total amp hours

Battery current = load watts ÷ (DC bus voltage × inverter efficiency)

Peukert factor = (rated current ÷ battery current)^{Peukert exponent − 1}

Usable energy = nominal energy × efficiency × discharge limit × health factor × temperature factor × reserve factor × Peukert factor

Runtime hours = usable energy ÷ connected load watts

How To Use This Calculator

Enter the real connected load in watts.
Enter the power factor to estimate apparent load.
Add the UPS rating if you want a load percentage check.
Enter one battery voltage and the number of batteries in series.
Enter battery amp hours and parallel strings.
Adjust efficiency, discharge limit, age, temperature, and reserve values.
Use a Peukert exponent near 1.05 to 1.25 for many lead acid batteries.
Press calculate and review runtime, energy, current, and load status.

Why UPS Runtime Matters

A UPS protects equipment when utility power fails. Runtime shows how long the battery can support the connected load. A good estimate prevents surprise shutdowns. It also helps with battery sizing, maintenance planning, and safer electrical decisions.

What This Calculator Checks

This tool uses load watts, battery voltage, amp hour rating, series batteries, and parallel strings. It also adjusts for inverter efficiency, allowed depth of discharge, battery age, temperature loss, reserve time, and Peukert behavior. These options make the result more practical than a simple watt hour division.

Important Input Ideas

Load watts should include every device connected to the UPS. Servers, routers, monitors, switches, and chargers all matter. Battery voltage is the voltage of one battery. Series count increases voltage. Parallel strings increase amp hour capacity. Efficiency covers conversion loss between battery power and output power.

Runtime Planning Tips

Use realistic values before buying batteries. New batteries may perform well. Older batteries can lose capacity quickly. Heat also reduces usable energy. Keep a reserve percentage when graceful shutdown time is needed. A lower reserve gives longer displayed runtime, but it leaves less safety margin.

Practical Electrical Notes

Power factor affects apparent load in volt amps. That value helps compare the load against the UPS rating. A UPS can be overloaded even when the battery has enough energy. Check both runtime and load percentage. Replace weak batteries before capacity drops too far.

Battery Sizing Guidance

Start with the measured running load, not the label rating alone. Many devices draw less than their nameplate value. Some devices draw more during startup. Add a margin for future equipment. Choose a battery bank that meets the target minutes after losses.

Reading the Results

The main runtime value is shown in minutes and hours. Nominal energy shows the battery bank before losses. Usable energy shows the adjusted energy after efficiency, discharge limit, battery condition, temperature, reserve, and Peukert correction. Battery current helps judge cable size, fuse ratings, and cell stress.

Maintenance Reminder

Test runtime during downtime. Record each result. Compare it with the estimate. Falling runtime often means aging batteries, hot storage, loose connections, or excess load. Keep terminals clean. Confirm charger settings. Follow the UPS manual for service work.

FAQs

What is UPS battery runtime?

It is the estimated time a UPS can power connected equipment during an outage. It depends on load, battery capacity, efficiency, battery condition, temperature, and reserve settings.

Why is actual runtime lower than calculated runtime?

Actual runtime may drop because of battery age, high temperature, weak cells, heavy discharge current, poor efficiency, charger issues, or extra devices connected after calculation.

What battery voltage should I enter?

Enter the voltage of one battery. For example, enter 12 if each battery is 12 volts. Then enter the number of batteries connected in series.

What does parallel strings mean?

Parallel strings are battery groups connected side by side. They increase amp hour capacity while keeping the same system voltage. More strings usually increase runtime.

What is Peukert exponent?

Peukert exponent estimates capacity loss at higher discharge current. A value near 1 means little loss. Larger values reduce runtime under heavy load.

Should I enter watts or volt amps?

Enter the real connected load in watts. The calculator uses power factor to estimate volt amps, which helps compare the load with the UPS rating.

How much reserve capacity should I keep?

A reserve of 10% to 25% is useful for graceful shutdown, battery protection, and unexpected load changes. Critical systems may need more reserve.

Can I use this result for critical systems?

Use it as a planning estimate only. Critical systems need measured load testing, manufacturer data, correct protection devices, safe wiring, and professional review.