"My battery goes off at 2am. Why?" This simple formula answers it.
The formula
Backup hours = Usable battery (kWh) ÷ Load (kW)
Usable battery: lithium gives about 85 to 90% of the label.
Load: what is switched on, in kilowatts. 500W = 0.5kW.
Examples
| Battery | Usable | 300W load | 500W load | 1,200W (with AC) |
|---|---|---|---|---|
| 3.2kWh | ~2.9kWh | ~9.5 hrs | ~5.8 hrs | ~2.4 hrs |
| 5kWh | ~4.5kWh | ~15 hrs | ~9 hrs | ~3.7 hrs |
| 8kWh | ~7kWh | ~23 hrs | ~14 hrs | ~5.8 hrs |
| 10kWh | ~8.5kWh | ~28 hrs | ~17 hrs | ~7 hrs |
Why your battery may finish early
- It did not fill up in the day. Too few panels, or a very cloudy day.
- Your load is bigger than you think. An old fridge or freezer uses more.
- The battery is ageing. Lead-acid loses power fast.
- Things run that you forget, like a pump or iron at night.
How to find your load
- Read the load on your inverter screen at night.
- Or add up appliance labels. See how to calculate your load.
How to make it last longer
- Switch to LED bulbs.
- Use an inverter AC at 24–25°C.
- Run heavy items in the day.
- Add more battery if you need more hours.
