UPS Battery Backup Time Calculator
Enter the load in watts and your battery bank's voltage, capacity and chemistry. The calculator estimates the runtime after inverter losses and safe depth of discharge. It also works out the battery capacity you need for a target backup time.
Estimate
- Backup time with this battery
- 7 h 47 min
- Usable energy
- 3.89 kWh
- Capacity needed for 4 h at 24 V
- 103 Ah
- Suggested minimum UPS rating
- 700 VA
Estimates assume a steady load. Battery age, heat and high discharge rates reduce real backup time, so size with a margin.
How the calculation works
How do you calculate UPS backup time?
Backup time (hours) = battery voltage × capacity (Ah) × usable depth of discharge × inverter efficiency ÷ load (W). For example, a 24 V 200 Ah lithium bank at 90% usable and 90% efficiency runs a 500 W load for about 7.8 hours.
Why is lithium usable capacity higher than lead-acid?
Lead-acid batteries last much longer if you use only about half their capacity, while lithium (LiFePO4) batteries can safely use about 80–90% on every cycle.
Should I size for the whole office or just IT?
For the longest backup at the lowest cost, put only critical IT on the UPS: servers, network, CCTV and POS. Air-conditioning and heavy loads are better served by a generator or a separate solar system.
Get a free quote
Tell us a few lines about your business and what you need. We reply the same working day with questions or a fixed written quote.
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