Battery Runtime Calculator South Africa
Calculate how long your backup battery will last during loadshedding
Actual battery runtime depends on battery health, temperature, and inverter efficiency. These are estimates.
Frequently Asked Questions
What is Depth of Discharge (DoD)?
DoD is how much of the battery you can safely use. Lithium batteries allow 80-90% DoD. Lead-acid should not exceed 50% or you damage them permanently.
How long does a battery last during loadshedding?
A 48V/100Ah lithium battery (4.8kWh) with 80% DoD running a 500W load will last approximately 7.7 hours — enough for Stage 4 loadshedding.
Lithium vs lead-acid for backup?
Lithium lasts 3,000-5,000 cycles vs 300-500 for lead-acid. Lithium allows deeper discharge (80-90% vs 50%). Lithium costs more upfront but is cheaper per cycle.
Actual battery runtime depends on battery health, temperature, and inverter efficiency. These are estimates.
How the battery runtime calculator works
Runtime = usable battery energy ÷ load. Usable energy is the nominal capacity × depth of discharge × inverter efficiency — a lithium battery is not emptied to zero, and the inverter loses something in the conversion.
A 5.1 kWh battery at 90% depth of discharge and 90% efficiency gives 5.1 × 1000 × 0.90 × 0.90 = 4131 Wh usable. At a 400 W load that is 10.3 hours.
Double the load to 800 W and the runtime halves to 5.2 hours. This is why the fridge, the kettle and the microwave decide the answer, not the number of lights.
What changes the answer in South Africa
Lithium iron phosphate batteries are usually rated for 6 000 cycles at 80–90% depth of discharge; lead-acid batteries should not go below 50% and last far fewer cycles, so the same nominal capacity gives roughly half the usable energy. Inverter efficiency is typically 90–95%, and idle consumption of 30–50 W is real over a long outage.
Common questions about the battery runtime calculator
Why can I not use the full battery capacity?
Because depth of discharge is limited to protect the cells — 90% here — and the inverter loses about 10% in conversion.
How long will a 5 kWh battery run a fridge?
A modern fridge drawing about 400 W on average gives roughly 10.3 hours. An old fridge can draw double that, halving the runtime.
Does a bigger battery always mean longer runtime?
Only at the same load. Runtime is energy divided by load, so adding loads cancels the extra capacity.