UPS Battery Runtime Calculator
Find out how long your UPS backup will last in a power cut. Enter your battery details, the load you run on it, and inverter efficiency to get the estimated backup time.
Battery and load details
Usable capacity: use 50 for lead-acid batteries, 80 to 90 for lithium batteries. Efficiency is usually 80 to 90 for home inverters.
Estimated backup time
| Total stored energy | — |
|---|---|
| Usable energy (after depth of discharge) | — |
| Energy reaching your load (after efficiency) | — |
| Runtime in minutes | — |
| Runtime in hours (decimal) | — |
This is a planning estimate. Battery age, temperature and wiring losses change the real runtime.
How to use
- Enter how many batteries you have, the Ah rating written on each battery, and the voltage (usually 12 V).
- Add the total watts of everything you run on the UPS, for example fans, lights and a TV.
- Set inverter efficiency and usable capacity, then press Calculate Runtime.
- Read the backup time and the energy breakdown in the results card.
Note: To measure your real load, add up the wattage printed on each device label. A typical ceiling fan uses 70 to 80 watts and an LED bulb uses 9 to 12 watts.
Tips
- Lead-acid batteries live longest when you keep usable capacity at 50 percent.
- Running a smaller load doubles the runtime, so switch off heavy devices during long cuts.
- Old batteries lose capacity over time, so reduce the usable capacity setting for batteries older than 2 years.
- High temperature shortens backup time, so keep the UPS and batteries in a cool, ventilated place.
Frequently Asked Questions (FAQ)
How is UPS runtime calculated?
The tool multiplies battery capacity by voltage to get stored energy, applies your usable depth of discharge and inverter efficiency, then divides by your load in watts.
Why does depth of discharge matter?
Lead-acid batteries last much longer if you only use about 50 percent of their capacity. Using more of it on every cycle shortens battery life.
Is this estimate exact?
No. Real runtime depends on battery age, temperature, wiring quality and load changes. Treat the result as a planning estimate.
What inputs does the runtime calculator need?
The number of batteries, capacity per battery in Ah, battery voltage, your load in watts, inverter efficiency percent and usable capacity percent.
What should I enter for usable capacity?
Use 50 for lead-acid batteries and 80 to 90 for lithium batteries. Lead-acid batteries last longer when only about half their capacity is used on each cycle.