“How big a battery do I need?” is the question we hear most from homeowners. The honest answer is that it depends on what you want to keep running and for how long. This guide shows you how to work it out, so you can have a better conversation with any installer, including us.
kWh and kW, explained simply
Two numbers matter, and they are often confused:
- kW (kilowatts) is power: how much the system can supply at one moment. It is mostly set by the inverter. If your kettle, microwave and borehole pump all start together, the inverter needs enough kW to carry them.
- kWh (kilowatt-hours) is energy: how much the battery stores. A 10 kWh battery could, in theory, supply 1 kW for 10 hours or 2 kW for 5 hours.
Think of kW as the width of a pipe and kWh as the size of the tank. You need both to be right: a big battery behind a small inverter can’t run heavy loads, and a big inverter with a small battery runs out quickly.
Step 1: list your essential circuits
Start with what you want on during an outage. For most Gauteng homes that means the gate motor, alarm and outside lights, Wi-Fi and the fibre router, fridges and freezers, indoor lights, TVs and laptops, and a borehole or pressure pump if you have one. Our guide to grid outages in Gauteng explains why these come first.
Leave out, at least at first, the big heating loads: geysers, stoves, ovens, kettles, heaters, pool pumps and air-conditioners. They can be backed up, but each one adds significantly to the inverter and battery size.
Step 2: a worked example
The figures below are illustrative only. They are round numbers to show the method, not measurements from any home and not a recommendation. Your real figures come from your own appliances and usage.
| Load (example) | Average draw | Hours | Energy |
|---|---|---|---|
| Fridge and freezer | 0.15 kW | 4 | 0.6 kWh |
| Lights | 0.2 kW | 4 | 0.8 kWh |
| Wi-Fi, router, alarm, cameras | 0.1 kW | 4 | 0.4 kWh |
| TV and laptops | 0.3 kW | 4 | 1.2 kWh |
| Gate motor and borehole pump (short runs) | – | – | 0.5 kWh |
| Total for a 4-hour outage | 3.5 kWh |
In this example, a four-hour evening outage needs about 3.5 kWh of usable energy. Add a margin for conversion losses and battery reserve, and for nights when the battery is not full when the outage starts. The inverter must also handle the peak: the moment the pump, fridge compressor and gate motor start together.
Now change one thing. If the same home wants backup through a whole evening and night, or wants to run the house from the battery after sunset every day to save on the bill, the energy needed can be several times larger. That is why two homes with the same floor area can need very different systems.
Why we size from your real usage
Appliance labels and guesses are a starting point, but they are rarely accurate. Fridges cycle, pumps start under load, and families use power differently. Orilla Solar uses data logging to record how your home actually uses electricity before we design the system. The logged data shows your real evening peak, your overnight base load and the short surges the inverter has to carry, so the battery and inverter are sized for your home rather than an average one.
If you want a first estimate before that, our savings calculator starts from your monthly bill.
Backup hours versus everyday savings
A battery can do two jobs. For backup, it holds energy in reserve for outages. For savings, it stores solar energy during the day and powers the home in the evening, when you would otherwise buy from the grid. Most modern systems do both, and you can set how much of the battery to keep in reserve.
The more you want to save, the more solar and battery capacity you need. The more backup hours you want, the more battery you keep in reserve. A good design balances the two for how you live, and that balance usually becomes clear once the usage data is in.
Adding batteries later
You don’t have to buy all the capacity on day one, provided the inverter and battery system allow expansion. Both systems Orilla Solar installs for homes are built for this:
- Sigenergy: SigenStor and SigenStor Neo are both premium systems with one to six battery modules per controller. Sigenergy says each SigenStor battery pack has its own DC-DC optimiser, so packs can be connected in parallel and added later [1].
- Sungrow: the SBH high-voltage battery comes in 5 kWh modules, from 10 to 40 kWh per unit, and works with compatible Sungrow hybrid inverters [2].
Check with your installer that any future modules will be compatible, and that adding them is covered by the manufacturer’s warranty terms. See home solar and battery backup for the full options, and finance options if you want to spread the cost.
Send us a recent electricity bill and the Orilla Solar team will come back to you.
Sources
- Sigenergy, SigenStor product page
- Sungrow, SBH100–400 high-voltage LFP battery (product page)
- Sungrow, SBH100–400 datasheet V4
This article is general information, checked against the sources above in November 2026. It is not legal, tax or engineering advice for your specific situation.