UPS Battery Backup Time Calculator
Find out how long your UPS will run on batteries. Enter your load, battery bank details and inverter efficiency, and get backup hours plus a battery bank planner.
Battery Bank Details
Formula used
backup_hours = (voltage × Ah × batteries × depth_of_discharge × inverter_efficiency) ÷ load_watts
Battery Bank Planner (reverse)
Already know how many backup hours you want? Enter the target and the tool tells you the battery bank size you need.
How to use
- Enter your total load in watts — add up every device the UPS will power during an outage.
- Enter your battery bank: voltage, Ah of one battery, number of batteries, inverter efficiency and usable depth of discharge.
- Click Calculate Backup Time to see estimated hours and minutes plus stored energy.
- Use the Battery Bank Planner to work backwards from the backup hours you actually want.
Tips
- UPS ratings in VA are not watts — check the actual watt load of your devices for an accurate result.
- Doubling the batteries doubles the backup time, but halving the load doubles it too.
- Lead-acid batteries last longest when you keep depth of discharge around 50–80%.
- Everything runs locally in your browser — nothing is uploaded.
- Convert between Ah, mAh, Wh and kWh with the Battery Capacity Converter.
- Check whether your appliances are safe on an inverter and get a recommended inverter size with the Inverter Load Planner.
Frequently Asked Questions (FAQ)
What formula does the UPS backup calculator use?
Backup hours = (battery voltage x battery Ah x number of batteries x depth of discharge x inverter efficiency) divided by the load in watts. The tool uses 0.80 depth of discharge and 0.85 inverter efficiency by default, and you can change both.
Which inputs do I need for a backup time estimate?
Enter your total load in watts (add up everything the UPS will run), each battery voltage and Ah rating, how many batteries are connected, plus the inverter efficiency and usable depth of discharge percentages.
Can it tell me how many batteries I need for a target backup time?
Yes. Enter your desired backup hours in the Battery Bank Planner section and the tool works the formula backwards to tell you the total battery Ah you need and how many batteries that means with your chosen voltage and Ah rating.
Why is my real backup time shorter than the calculated time?
Common reasons are old or weak batteries that no longer hold their rated Ah, a higher actual load than entered, poor ventilation heating the inverter, and loose or corroded battery terminals. Treat the result as an estimate for healthy new batteries.
Does the calculator work for lithium batteries?
Yes. The defaults of 80% depth of discharge and 85% inverter efficiency suit lead-acid batteries. For lithium batteries, raise depth of discharge to 90-100%, since they can safely use nearly their full rated capacity.