Battery backup time calculator and tables
How long an inverter battery lasts at your load, for every common Ah size, tubular and lithium. Worked out with the same engine as our inverter calculator.
- Calibrated to manufacturers’ published backup tables
- 12 V and 24 V banks
- Lithium (LiFePO4) too
Backup time by battery size
One 12 V tubular battery on a home inverter. 100 W is a fan and four lights; 400 W is three fans, six lights, a TV and Wi-Fi.
| Battery (12 V tubular) | 100 W | 200 W | 300 W | 400 W | 500 W |
|---|---|---|---|---|---|
| 100 Ah | 9 h 30 min | 4 h 05 min | 2 h 20 min | 1 h 25 min | 1 h 05 min |
| 120 Ah | 11 h 30 min | 5 h 10 min | 3 h | 2 h | 1 h 20 min |
| 135 Ah | 13 h | 6 h | 3 h 35 min | 2 h 25 min | 1 h 35 min |
| 150 Ah | 14 h 25 min | 6 h 50 min | 4 h 05 min | 2 h 50 min | 2 h |
| 160 Ah | 15 h 25 min | 7 h 20 min | 4 h 30 min | 3 h | 2 h 10 min |
| 165 Ah | 15 h 50 min | 7 h 35 min | 4 h 40 min | 3 h 10 min | 2 h 20 min |
| 180 Ah | 17 h 20 min | 8 h 25 min | 5 h 10 min | 3 h 35 min | 2 h 35 min |
| 200 Ah | 19 h 10 min | 9 h 30 min | 5 h 55 min | 4 h 05 min | 3 h |
| 220 Ah | 21 h 05 min | 10 h 35 min | 6 h 35 min | 4 h 40 min | 3 h 30 min |
| 230 Ah | 22 h 05 min | 11 h | 7 h | 4 h 55 min | 3 h 40 min |
| 250 Ah | 24 h | 12 h | 7 h 40 min | 5 h 30 min | 4 h 05 min |
| 260 Ah | 25 h | 12 h 30 min | 8 h 05 min | 5 h 40 min | 4 h 20 min |
Lithium (LiFePO4) batteries
Using 80% of the stored energy, the depth of discharge in Loom Solar’s LiFePO4 datasheet, and 80% inverter efficiency.
| Lithium battery | 100 W | 200 W | 300 W | 400 W | 500 W |
|---|---|---|---|---|---|
| 12.8 V 100 Ah lithium | 8 h 10 min | 4 h 05 min | 2 h 40 min | 2 h | 1 h 35 min |
| 25.6 V 100 Ah lithium | 16 h 25 min | 8 h 10 min | 5 h 30 min | 4 h 05 min | 3 h 20 min |
| 51.2 V 100 Ah lithium | 32 h 50 min | 16 h 25 min | 10 h 55 min | 8 h 10 min | 6 h 35 min |
| 51.2 V 200 Ah lithium | 65 h 30 min | 32 h 50 min | 21 h 50 min | 16 h 25 min | 13 h 05 min |
Battery backup questions
How do I work out battery backup time?
Multiply the battery voltage by its Ah to get watt-hours, take off inverter losses, and divide by the load in watts. Lead-acid gives less than its label at higher loads, so the tables here use a curve fitted to manufacturers’ published backup tables instead of the label alone.
Why is my backup shorter than the label suggests?
Inverter batteries are rated at C20, a 20-hour discharge. A typical home load drains them much faster than that, and lead-acid delivers less of its capacity at higher current. Ageing, heat and an inverter that cuts off early shorten it further.
Does a 24 V inverter give more backup?
Two 12 V batteries in series make 24 V and roughly double the stored energy, so at the same load the backup roughly doubles. Each battery also discharges at a lower current, which helps lead-acid a little more.
How accurate are these figures?
For 150 to 250 Ah tubular batteries they match Exide’s published Inva Zen backup tables within about 10% at 200 to 400 W. At very light loads (100 W) our figures are lower than Exide’s, so treat them as a safe minimum.