How Long Will My Battery Last During Load Shedding?
The most common question about backup power is: how long will my battery last? The answer depends on your battery capacity, the load you are running, and the battery type. Here is how to calculate it.
Key Takeaways
- Battery runtime = battery capacity (Wh) / load (W)
- A 100Ah 12V battery = 1200Wh of stored energy
- Lithium batteries can be discharged to 80-90%, lead-acid only to 50%
- Running fewer appliances dramatically extends battery life
- Use the NavigateZA calculator for a precise estimate
The basic formula
Battery runtime in hours = Battery capacity in Watt-hours (Wh) divided by your load in Watts. A 100Ah battery at 12V = 1200Wh. Running a 300W load = 4 hours of runtime (before accounting for depth of discharge).
- Runtime (hours) = Battery Wh / Load W
- 100Ah x 12V = 1200Wh
- 1200Wh / 300W load = 4 hours
- Always account for depth of discharge limits
Depth of discharge matters
You cannot fully drain most batteries. Lead-acid batteries should only be discharged to 50% to preserve their lifespan. Lithium batteries can go to 80-90%. This significantly affects usable capacity.
- Lead-acid: use only 50% of rated capacity
- Lithium (LiFePO4): use 80-90% of rated capacity
- A 100Ah lead-acid = 50Ah usable = 600Wh usable
- A 100Ah lithium = 80-90Ah usable = 960-1080Wh usable
Practical examples
A typical South African home running essentials (fridge, lights, TV, router) uses about 300-500W. A 200Ah lithium battery bank would give you 3-5 hours of runtime for a Stage 4 load shedding session.
- Essentials load: 300-500W
- 200Ah lithium = 1920-2160Wh usable
- Runtime: 4-7 hours at 300W load
- Add more batteries to extend runtime
Ready to see your own numbers?
Use the Inverter & Battery Sizing Calculator