A 5 kWh battery is not five hours of backup power, because the hours are not stored in the battery. They are stored in the load list. A battery holds energy; the circuits you moved to the backup side of the distribution board decide how fast that energy leaves. Take the usable energy, which is the rated capacity multiplied by the depth of discharge the manufacturer allows, divide it by the average draw of the backup subset rather than by the whole house, and what falls out is the hours. Two identical 5 kWh batteries on the same street, on the same load shedding schedule, deliver completely different evenings, and the difference is not the box. It is which breakers were left on the essential side.
I worked that out the hard way, standing in a passage at half past eight at night with a torch in my teeth, watching the inverter display drop from 64 per cent to 61 while the kettle boiled, and then listening to the thing complain anyway when the fridge compressor chose that exact moment to start. Three minutes of kettle is almost nothing in energy terms. It was still enough to trip an overload.
So I did the walk I should have done before anyone quoted me anything. Main switch off. Notebook. Every room, every plug, every breaker, writing down what was still alive and what it was drawing.
The number on the box is a ceiling, not a promise
The first subtraction is depth of discharge. A 5 kWh battery does not hand you 5 kWh, because the manufacturer will not let it run to zero, and if it did, you would be replacing it sooner than you planned. Some lithium chemistries are comfortable being taken down to 80 or 90 per cent of rated capacity. Lead-acid categories are commonly worked to around half. That single spec, buried in a datasheet, removes a quarter to a half of the figure the salesperson put at the top of the page.
Then there are the losses. Inverter conversion, battery round-trip, and the standing draw of the inverter itself. A hybrid inverter sitting there with nothing running still eats somewhere in the region of 30 to 60 watts, and over a twelve-hour outage that is real energy gone from the same bucket.
Then there is the reserve. If you spend every last watt-hour on the first slot of the day, the second slot starts in the dark. Most people I know hold something back, usually somewhere between 20 and 30 per cent, and then argue with themselves about whether the last 10 per cent is worth saving when the children are doing homework by cellphone light. That argument is a preference, not a formula.
The list comes before the number
Here is the arithmetic, with every assumption labelled, because the method is the part that transfers and the numbers are not.
Say a 5 kWh battery, 85 per cent depth of discharge, so 4.25 kWh of usable energy. Knock off roughly a tenth for conversion and round-trip losses: about 3.8 kWh at the socket. Hold back 20 per cent as reserve: about 3 kWh you are actually willing to spend on this outage.
Now the backup side. A fridge that cycles, averaged over a day, might sit near 80 to 100 watts, more when the door is being opened by people who miss load shedding. A router, 10 to 15. A television, 60 to 80. Four LED lights at 5 watts each, 20. Chargers, 15. Add the inverter's own 40 and you are in the region of 240 to 260 watts, not the 2 to 3 kilowatts the whole house pulls when the geyser and the oven are both on.
Divide 3 kWh by 250 watts and you land near twelve hours. That is not a claim about your house. It is a demonstration that the same battery can carry a small subset through several slots, or one geyser element for under an hour.
Which is the whole point.
The kettle and the geyser are not the same problem
Almost every explanation of this flattens high-draw appliances into one category, and that is where people make expensive mistakes.
A geyser element of 3 to 4 kilowatts running for an hour is 3 to 4 kilowatt-hours. That is the entire battery, spent on hot water, and it is a problem of energy.
A 2 kilowatt kettle boiling for three minutes is roughly 0.1 kilowatt-hours. In energy terms it is a rounding error. The reason it still wrecks a backup system is that it is a problem of power: the inverter has to supply 2 kilowatts on top of whatever else is running, and the DC side sags for a moment when it does. Leave the kettle off the essential side because the inverter is not built for it, not because the battery cannot afford another hundred watt-hours. Get the two reasons the right way around and the decisions get easier, including the decision about what actually needs upgrading.
This is also why the load list is not only a battery document. It is an inverter document. It tells you the continuous load, the start-up surge of things like a fridge compressor, and the worst-case combination that will ever be on at once. A bigger battery behind an inverter that trips on a kettle buys you nothing but a longer wait in the dark.
Size for the day, not for the slot
Load shedding does not arrive in convenient two-hour parcels. At the lower stages a household might see one slot in a day. As stages climb, the same household can see several, sometimes with a gap too short to refill the battery before the next one starts. Each stage adds roughly two hours of outage to a day in a given area, though the blocks and the timing differ by municipality, and anyone who has watched a schedule shift by an hour knows how much that matters.
Size for the longest daily total you reasonably expect, then decide how much of that day you want to come from battery rather than from the grid. Sizing for one slot is how a system ends up flat by seven in the evening during a higher stage, and it is the most common form of disappointment I see after a new install.
What actually moves the hours
If you want to know why one house gets through the evening and another does not, these are the levers, roughly in the order they matter:
- Which circuits were moved to the essential side. Nothing else comes close.
- The depth of discharge the manufacturer allows for the chemistry in question.
- Conversion and round-trip losses, which are never zero.
- The inverter's standing draw, running all night whether you notice it or not.
- The reserve you choose to keep.
- The number of slots the schedule delivers that day.
- The length of the recharge gap between them.
Notice that the rated capacity of the battery is not on that list as a separate item. It is already inside the first line of the arithmetic. It is just not the thing that decides the answer.
What the municipal bill is for
A municipal bill tells you what the whole house consumed over a month. That is the right input for sizing panels and for the paperwork around an SSEG application. It is close to useless for sizing the backup.
For the backup side you want something closer to the plug. A plug-in energy monitor left on the fridge for a week will teach you more than a month of statements, because it shows the cycling, the defrost, the spike when the compressor starts. Everything else you can estimate well enough from the labels, then check by turning the main switch off for two hours on a quiet Sunday and reading what the inverter reports. That one exercise, main switch off and notebook open, is the closest thing to a real answer you will get before you spend money.
And write the split down. The electrician doing the changeover and the CoC will want the essential and non-essential sides named anyway, and having that list in your own handwriting makes the conversation shorter.
The 5 kWh was never the answer to how many hours. It is the size of the bucket. The hours are how far you open the tap, and you decide that at the distribution board, not at the till.
