The most common backup-power question we get in Sandton and Midrand is also the most expensive to get wrong: what size inverter and battery do you actually need? Oversize it and you have paid for capacity you never use; undersize it and the system trips the moment the fridge and microwave run together. Here is how to size it properly.
Step 1 — Work out your essential load, not your whole house
Backup power is about keeping the essentials running, not powering everything as if nothing happened. List what you genuinely need during load-shedding: lights, Wi-Fi router, TV, phone chargers, the fridge or freezer, and maybe an alarm and gate motor. Add up the running watts. For most homes the essential load lands between 800W and 2,000W.
Step 2 — Match the inverter to peak demand
- 3kVA — lights, electronics, Wi-Fi, TV and a fridge. The right choice for most households that just want to ride out the slot.
- 5kVA — the above plus a microwave, pool pump or a few extra circuits running together.
- 8kVA+ — near-whole-home backup, usually paired with solar. Needed if you want to include heavy loads.
The inverter must handle the peak moment when several appliances switch on at once, not just the average.
Step 3 — Size the battery to how long you need
Battery capacity (kWh) decides how long you last. A 5kWh lithium battery carries a typical essential load through a 2 to 4 hour slot. Step up to 10kWh for back-to-back slots or larger loads. Always choose lithium (LiFePO4) over lead-acid — you can safely use most of its capacity and it lasts three to four times longer.
Keep the geyser off the battery
A geyser draws around 3kW and will drain a home battery in well under an hour. Far smarter to fit a GeyserWise controller or move to a solar geyser, and keep the inverter for lights, electronics and the fridge.
What it costs and why the COC matters
A starter 3kVA + 5kWh system starts around R55,000 installed; a 5kVA + 10kWh system from about R95,000; add solar panels to recharge during the day and you are in hybrid territory. Every install is wired through your DB board and signed off with an Electrical Certificate of Compliance — see our inverter installations and solar power pages.
The bottom line
Size to your essential load, choose lithium, keep the geyser separate, and insist on a COC. Not sure where to land? Send us your appliance list and we will size it for you, free.