How long will the inverter last in a power cut? Work out the backup time for your load.
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An inverter battery stores energy in amp-hours (Ah) at 12 V. A 150 Ah battery holds 1,800 Wh, but lead-acid batteries should not be drained fully and the inverter loses some energy, so a 300 W load runs for about 3.5 hours.
Enter your load, battery size and type. The calculator gives the backup time, the battery capacity needed for the backup you want, and the minimum inverter VA rating for your load.
| Total load | 300 W |
| Battery capacity | 150 Ah |
| Battery voltage | 12 V |
| Number of batteries | 1 |
| Battery type | Tall tubular lead-acid (use 70%) |
| Inverter efficiency | 85% |
| Backup you want (optional) | 4 hours |
| Backup time | 3 h 34 min |
| Battery bank | 1,800 Wh (1 × 150 Ah × 12 V) |
| Usable energy | 1,071 Wh (70% depth of discharge, 85% inverter efficiency) |
| Inverter size for this load | at least 500 VA |
| For 4 hours of backup | you need 170 Ah per battery (1 × 12 V) |
3 h 34 min.
| Total load | 600 W |
| Battery capacity | 200 Ah |
| Battery voltage | 12 V |
| Number of batteries | 2 |
| Battery type | Tall tubular lead-acid (use 70%) |
| Inverter efficiency | 85% |
| Backup you want (optional) | 5 hours |
| Backup time | 4 h 46 min |
| Battery bank | 4,800 Wh (2 × 200 Ah × 12 V) |
| Usable energy | 2,856 Wh (70% depth of discharge, 85% inverter efficiency) |
| Inverter size for this load | at least 950 VA |
| For 5 hours of backup | you need 220 Ah per battery (2 × 12 V) |
4 h 46 min.
About 3–3.5 hours at 300 W with a tubular battery, or about 6–7 hours at 150 W.
About 270 W of load, so a 500–600 VA inverter with a 150 Ah battery is typical.
Only with a large inverter (2–3 kVA) and big battery bank or solar; a 1.5-ton AC would drain a normal 150 Ah battery in under an hour.