Solar
How to Calculate Battery Backup Time for Your Solar System or Inverter
Short answer
Backup time in hours ≈ usable battery energy (kWh) × inverter efficiency ÷ load (kW). For example, a 5 kWh battery with 90% usable capacity and a 0.9 inverter efficiency, powering a 500 W load, lasts about 5 × 0.9 × 0.9 ÷ 0.5 ≈ 8 hours. Lead-acid batteries have less usable capacity than lithium.
The formula
Hours = (battery kWh × usable fraction × inverter efficiency) ÷ load in kW.
Battery kWh = volts × amp-hours ÷ 1000. A 48 V 100 Ah battery is 4.8 kWh. A 12 V 200 Ah battery is 2.4 kWh.
Usable fraction (depth of discharge)
| Battery type | Typical usable fraction |
|---|---|
| Lithium iron phosphate (LiFePO4) | About 80 to 100% |
| Gel or AGM lead-acid | About 50% |
| Flooded tubular lead-acid | About 50% |
Check the manufacturer datasheet; going deeper shortens lead-acid life.
Worked examples
- 48 V 100 Ah lithium (4.8 kWh, 90% usable), 1 kW load: 4.8 × 0.9 × 0.9 ÷ 1 ≈ 3.9 hours.
- 2 × 12 V 200 Ah lead-acid in series (24 V, 4.8 kWh, 50% usable), 300 W load: 4.8 × 0.5 × 0.9 ÷ 0.3 ≈ 7.2 hours.
Real-world factors
- Loads vary: fridges cycle on and off, so use average, not peak, power.
- Batteries lose capacity with age and heat.
- High loads reduce lead-acid capacity further.
- Solar charging during the day extends backup when the sun is out.
Next
Read lithium vs lead-acid and system sizing.
Common questions
How many batteries do I need for one night?
Estimate your night-time energy use in kWh, then divide by usable energy per battery.
Why does my battery drain faster than calculated?
Higher loads, battery age, cold or heat and inverter standby use all reduce runtime.
Is more Ah always better?
More Ah at the same voltage means more energy. Make sure the inverter supports the battery bank voltage.