Inverter Battery Backup Calculator
Enter your load (watts), battery capacity (Ah) and number of batteries to see how many hours of backup you get in a power cut and what VA inverter you need.
Quick answer
500 W load, 150 Ah battery: about 2.9 hours of backup.
How it is calculated
- Battery energy = Ah × volts × batteries = 150 × 12 × 1 = 1,800 Wh.
- Backup hours = energy × efficiency ÷ load = 1,800 × 0.8 ÷ 500 = 2.88.
- Inverter VA = load ÷ 0.8 (power factor).
Worked example
A 500 W load (fans, lights, TV) on one 12 V, 150 Ah battery at 80% efficiency gives about 2.9 hours of backup; choose at least a 625 VA inverter.
| Load | 500 W |
| Battery capacity | 150 Ah |
| Battery voltage | 12 V |
| Batteries | 1 |
| Efficiency | 80 % |
| Backup time | 2.9 hours |
| Inverter size | 625 VA |
Formula
Backup time = Ah×Bv×Nb×Ef/100/Ld Inverter size = Ld/0.8
Good to know
- Fully draining the battery shortens its life; plan for a little less backup.
- Fridges and motors draw extra current when they start.
Questions people ask
How much backup from a 150 Ah battery?
About 2.9 hours at 500 W, about 4.8 hours at 300 W.
What VA inverter for a home?
Total watts ÷ 0.8. For a 600 W load, at least 750 VA.
What do two batteries do?
Roughly double the backup (in a 24 V system).
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Updated 13 Nov 2026 · Desktop N Calculator v1.2.0