· SH Energy editorial
Backup sizing is a measurement exercise, not a catalogue lookup. Two houses with identical floor plans can need very different systems once you account for the actual loads that stay on, the surge behaviour of the appliances behind them, and how long the outage lasts on the stage your area experiences.
1. Start with the load list, not the battery
Write down what must stay on during an outage, then separate that from what you would like to keep on. The two lists have very different costs. A common failure is to size for the comfortable list but budget for the essential one, which produces a system that trips or dies early and gets blamed on the battery.
- Essential: fridge or freezer, lighting circuits, internet router, the alarm, a few plug points for phones and a laptop.
- Comfort: microwave, kettle, geyser, air conditioning, pool pump, tumble dryer.
A geyser or a kettle can draw more than everything on the essential list combined. If they are on the backup circuit, the inverter rating stops being about the battery and starts being about the largest single load.
2. Measure the load instead of trusting the label
Appliance labels state a maximum, not an average. A fridge cycles, so its continuous draw is a fraction of its rated current, but its compressor start can pull several times its running current for a moment. That start is what makes a small inverter shut down under an otherwise easy load.
Practical approach: use a plug-in energy meter on the essentials for a few days and record both the steady draw and the peak. If you cannot measure, use conservative figures and say so in the design, rather than presenting an estimate as a measurement. Whichever route you take, record the numbers in the design file so the next person can check them.
3. Convert the load into energy, then into a battery
Energy needed is the average power of the essential list multiplied by the hours you want to cover. Then correct for two losses that are easy to forget: usable depth of discharge and round-trip efficiency. A battery quoted at a nameplate capacity cannot deliver all of it every cycle, and not all of the energy you put in comes back out.
Do the arithmetic twice: once for a typical outage on your stage, once for the longest outage you consider plausible. Sizing only for the typical case is what makes systems fail on the day the stage goes higher.
4. Size the inverter for the surge, not only the running load
Continuous rating is the easy number. What decides whether the system works is the short surge when a compressor, pump or tool starts. Look at the manufacturer’s surge rating and its duration, and check whether the unit limits motor starts by design. If the answer is only “continuous power”, that is not a complete specification.
Also confirm the unit’s behaviour when the battery is nearly empty and the load is heavy. Voltage collapse at low state of charge turns an undersized design into a system that appears to have failed for no reason.
5. Plan the recharge window
During a loadshedding cycle the grid returns between outages, and that window is when the battery has to recover. If the recharge current is too low for the window, the battery starts each outage partially charged and usable runtime falls day by day. Ask what the recharge power is, and check it against the interval your area normally gets — not against a best case.
6. Verify with one real overnight test
Before accepting an installation, run the essential list through a real outage and log the result: start voltage or state of charge, time to the low-battery threshold, and every appliance that was on. One honest test tells you more than a specification sheet, and it gives you a baseline to compare against in a year when the battery has aged.
Copy-ready load list to send to a supplier
Can one battery size cover both a house and a small shop?
Only if the essential load lists are similar. A shop normally adds refrigeration, a till, lighting and sometimes a three-phase machine, so the inverter rating and the surge requirement usually decide the design before battery capacity does. Size each site from its own measured list.
Should the geyser be on the backup circuit?
Usually not. A geyser is a large resistive load that can empty a battery quickly and rarely needs to run during an outage. Keeping it off the backup circuit often reduces both the inverter and the battery requirement.
How long should backup last?
It is a choice, not a standard. Decide the number of hours you want to cover, and also decide what happens beyond that: whether the system shuts down gracefully, sheds loads, or keeps a smaller essential set alive.
Does a bigger battery always mean a better system?
No. If the inverter cannot carry the surge, or the recharge power cannot recover the battery between outages, extra capacity does not fix either problem. Match capacity, inverter rating and recharge power to the same load list.
Discuss a sourcing or sizing enquiry
Send your proposed configuration, the load list you measured and the documents you already hold to jimmy@sunhongenergy.com, or contact Jimmy on WhatsApp. Related reading: the battery storage RFQ checklist, the battery and inverter compatibility checklist and our quality and documents page.
Sources and scope
Reviewed 19 October 2026. This article describes a sizing method taken from general electrical design practice and from the operating behaviour of battery-based backup systems; it asserts no specific capacity figure, product performance number or outage duration for any site. Local supply arrangements and the rules for connecting a grid-tied system are set by the municipality or distributor that supplies the site; the national reference point for small-scale embedded generation in South Africa is the municipal SSEG resource portal at sseg.org.za. Where a figure is given by a supplier, ask for the document that states it and treat an unverifiable claim as unconfirmed. This is a preparation method for installers and buyers, not electrical, legal or municipal advice, and SH Energy does not claim that any model is approved, listed or certified for a particular utility.