How do I calculate inverter battery backup time?
Short answer: Backup time (hours) = battery Ah × battery voltage × number of batteries × inverter efficiency × usable depth of discharge ÷ load in watts. Example: one 150Ah 12 V battery stores 1,800 Wh; with about 85% inverter efficiency and 80% usable depth, about 1,224 Wh is usable, so a 300 W load runs for about 4 hours.
| Battery | 200 W load | 300 W load | 500 W load |
|---|---|---|---|
| 100 Ah | 4.1 h | 2.7 h | 1.6 h |
| 150 Ah | 6.1 h | 4.1 h | 2.4 h |
| 200 Ah | 8.2 h | 5.4 h | 3.3 h |
Tip: Older batteries hold less charge: a 4–5 year old lead-acid battery may give only 60–70% of its original backup.
Using this calculator outside India? Outside India? Pick your currency at the top of the page and enter the per-kWh rate printed on your bill. Typical residential averages: USA 18.3¢/kWh (EIA, July 2026; California 33.6¢, Texas 15.9¢), UK about £0.25/kWh, Germany about €0.38/kWh, UAE about AED 0.23–0.38/kWh, Canada about C$0.13–0.20/kWh and Australia about A$0.30–0.35/kWh. The kWh result is the same in every country; only the cost changes.