How to size a battery for load-shedding

Batteries are usually the most expensive part of a backup system, so it pays to size them for what you actually need rather than for everything in the house. The method is simple arithmetic, and you can do a first pass yourself before anyone visits.

1. List what must stay on

Walk through the house and write down what you would want running during a cut: the fridge, some lights, the Wi-Fi router, a TV, a laptop or desktop for work, perhaps a gate motor. Leave out anything that heats: geysers, kettles, ovens, heaters and hair dryers draw so much power that they would need a much bigger system on their own.

2. Add up the watts

Each appliance has a power rating on its label or in its manual. Add them up for everything that would run at the same time. A fridge cycles on and off, so its average draw is lower than its label, but its motor needs a short burst of extra power to start. That burst matters for the inverter (see step 4), not for the battery.

3. Multiply by the longest cut

Battery capacity is measured in kilowatt-hours (kWh): power multiplied by time. Take your total load and multiply it by the longest cut you want to ride through. Then allow for the fact that you should not drain a battery completely. Check the datasheet, but planning on using about 80% of the rated capacity is a sensible starting point.

A worked example: a fridge, six LED lights, the Wi-Fi and a TV come to roughly 280 W. A 4-hour cut needs 280 W × 4 h = 1.12 kWh. A 4.8 kWh battery at 80% gives about 3.84 kWh, so it covers that cut with plenty to spare, even two cuts back to back.

4. Check the inverter and the recharge

The battery stores the energy; the inverter decides how much can flow at once. Its continuous rating has to cover everything running together, and it needs headroom for motors starting up. An undersized inverter will trip even with a full battery.

The battery also has to refill between outages. If they come close together, a battery that only recharges from the grid in the gaps may never get back to full. Solar panels sized to recharge it during the day solve that.

Try it on the calculator

The battery calculator on our home page does this sum for you. Pick a battery size, tick the appliances you would keep on, and it shows how long the battery would run them if it took over at 6 pm. Treat it as a first estimate. A site visit gives you real figures, including the recharge side.