About the Battery Runtime Calculator
What is the Battery Runtime Calculator?
How long will the inverter last during a power cut? This battery backup time calculator divides the battery's usable energy by your load.
Usable energy depends on how deeply the battery can be discharged – about 50% for flat-plate lead-acid, 70–80% for tubular and 80–90% for lithium – and on inverter losses.
How to use the Battery Runtime Calculator
- Battery Capacity (Ah) (sample: 150)
- Battery Bank Voltage (V) (sample: 12)
- Load (W) (sample: 300)
- Usable Depth of Discharge (%) (sample: 50)
- Inverter Efficiency (%) (sample: 85)
- Press Calculate Battery Backup (results also refresh as you type). The result card shows the Backup Time, along with Backup in hours, Usable energy, Stored energy and Average battery current.
Formula used
Backup (hours) = Ah × Voltage × DoD × Inverter efficiency ÷ Load (W)
- DoD = usable depth of discharge
At high loads, lead-acid batteries deliver less than their rated Ah (Peukert effect), so real backup is shorter.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Battery Capacity (Ah) | 150 |
| Battery Bank Voltage (V) | 12 |
| Load (W) | 300 |
| Usable Depth of Discharge (%) | 50 |
| Inverter Efficiency (%) | 85 |
| Result | Value |
|---|---|
| Backup Time | 2 h 33 min |
| Backup in hours | 2.55 h |
| Usable energy | 765 Wh |
| Stored energy | 1,800 Wh |
| Average battery current | 29.4 A |
Tips for using the Battery Runtime Calculator
- Turn off heavy loads like irons and geysers during outages to stretch backup.
- Top up distilled water in flooded batteries regularly.