How to Size an Inverter and Battery for Your Home

The two numbers that decide whether your backup system works are the inverter's power rating (in kW) and the battery's storage capacity (in kWh). Get them right and the lights stay on. Get them wrong and you either overpay or trip the system every time the kettle goes on.

Step 1: list what you actually need to run

Backup power is about essentials, not running the whole house. Walk through your home and note the appliances you genuinely need during an outage, with their rough draw:

Step 2: size the inverter (kW)

Add up the power draw of everything you might run at the same time. That peak figure — plus roughly 20–30% headroom for appliance startup surges — is the minimum inverter size you need.

• Lights, Wi-Fi, TV and charging together rarely exceed 1 kW — a small system copes easily.
• Add a fridge and the occasional microwave and you're looking at 3–5 kW.
• Want to run a geyser or several heavy appliances together and you'll need 8 kW+.

A common mistake is buying a small inverter then trying to boil a kettle on it — the surge trips it instantly. Either size up, or simply avoid heating appliances during load-shedding.

Step 3: size the battery (kWh)

The battery decides how long you can run, not how much you can run at once. Multiply your average running load (in kW) by the number of backup hours you want:

• Running about 500 W (lights, Wi-Fi, TV) for 4 hours needs roughly 2 kWh.
• Running about 1 kW (add a fridge) for 4 hours needs roughly 4 kWh.
• A typical home wanting comfortable evening backup lands on a 5 kWh battery, the most common single-battery size sold in South Africa.

Add a margin: lithium (LiFePO4) batteries are happiest not fully discharged, and you lose a little to conversion losses. Sizing for about 20% more than your calculation is sensible.

Step 4: think about recharging

During stage 4–6 load-shedding you may only get a few hours of grid power between slots. A battery that can't recharge fully in that window will slowly run down over a day. Two fixes: a bigger battery bank, or solar panels to recharge during daylight regardless of the grid.

A worked example

A family wants lights, Wi-Fi, TV, phone charging and the fridge to survive a 4-hour evening slot.

• Peak simultaneous draw: about 1.2 kW → a 3–5 kW inverter is comfortable.
• Average running load: about 0.8 kW over 4 hours = 3.2 kWh → a 5 kWh battery gives headroom.

That's the classic "5 kW inverter + 5 kWh battery" starter system many South African homes buy. If you later want to run more, or recharge off-grid, add solar panels.

Don't over-buy

Installers earn more on bigger systems, so it pays to know your own numbers before the quote. Size for the essentials you listed in Step 1, not for every appliance in the house — and always insist on a written quote after a site assessment.