How long will my battery last?
A 10 kWh battery runs a typical South African household's essential loads for about 20h 09m. Essentials here means lights, wifi, a fridge and a TV, totalling roughly 420 watts. Only about 85% of a battery's nameplate capacity reaches your appliances, because lithium batteries are not discharged below about 10% and the inverter loses a few percent converting DC to AC. The formula is usable kWh divided by load in kW. Heating appliances change the picture completely: adding a 2 000 W kettle to the same load drops runtime to roughly 3h 30m, which is why geysers, kettles and stoves are normally left off the backed-up circuit.
Nameplate capacity, as quoted. Usable capacity is lower, see below.
Runtime
Usable capacity is 90% of nameplate (depth of discharge) times 94% inverter efficiency. Real runtime is shorter if loads cycle on and off, as a fridge does.
Worked example
A 10 kWh battery running household essentials.
| Nameplate capacity | 10 kWh |
| Depth of discharge | 90% |
| Inverter efficiency | 94% |
| Usable capacity | 8.5 kWh |
| Essential load | 420 W |
| Runtime | 20h 09m |
| Runtime with a kettle on | 3h 30m |
How this is calculated
Usable capacity = nameplate kWh × 0.9 × 0.94. The first factor is depth of discharge, the second is DC-to-AC inverter loss.
Runtime = usable kWh ÷ (load in watts ÷ 1000).
Appliance figures are typical continuous draw, not peak. A fridge is shown averaged, because its compressor cycles rather than running constantly. Real runtime with a fridge on the circuit is usually a little better than this calculation suggests.
Not modelled: inverter standby draw, battery efficiency changes at low temperature, or motor start-up surge, which matters for pool pumps and borehole pumps. Treat the result as a planning figure.
Battery runtime: common questions
How long will a 10 kWh battery last during load shedding?
About 20h 09m running essentials: lights, wifi, fridge and TV, roughly 420 W in total. That comfortably covers a four-hour load shedding block with capacity to spare. Add heating appliances and it falls sharply: the same battery with a kettle running lasts about 3h 30m.
Why can't I use the battery's full capacity?
Two losses. Lithium batteries are not discharged below roughly 10% of capacity, to protect cycle life, and the inverter loses about 6% converting DC to AC. A 10 kWh nameplate battery therefore delivers around 8.5 kWh to your plugs.
What should I leave off the backup circuit?
Anything that makes heat. A geyser element draws about 3 000 W, a kettle 2 000 W and a stove plate similar. Any one of them will empty a household battery in under two hours. Standard practice is to keep the backed-up circuit to lights, plugs, wifi, fridge and entertainment, and let heating loads run only when the grid is up.
How big a battery do I actually need?
Work backwards from the longest outage you want to cover. Multiply your essential load in kW by the hours you want, then divide by 0.85 to allow for depth of discharge and inverter losses. A 420 W essential load over 6 hours needs about 3 kWh usable, so a 5 kWh battery is comfortable and 10 kWh gives real headroom.