A backup system for load shedding has two main parts: an inverter, which turns battery power into normal mains power, and batteries, which store the energy. Solar panels can recharge the batteries during the day, but the first job is to size the inverter and batteries for what you want to run. This guide shows how.
Step 1: list what must keep running
Write down everything you want on during an outage, with its power in watts from its label: lights, the router, a TV, a fridge, a laptop, perhaps a gate motor. Add up everything that could run at the same time. That total is your load. Leave out the geyser, kettle, oven and heaters: their elements use so much power that they would need a far bigger system.
Step 2: size the inverter
An inverter has two ratings. The continuous rating is the power it can supply indefinitely, and the peak or surge rating is what it can supply briefly, when a motor starts. SA Home Loans’ guide puts it simply: choose an inverter whose continuous rating at least matches the total load, and make sure it can handle the surge of start-up loads. Their example: an appliance drawing 5 A continuously and 15 A at start-up, on 230 V, needs about 1,150 W continuous and 3,450 W surge, so an inverter of about 1,500 W continuous and 3,500 W peak.
Surge ratings are not a fixed multiple. Energy Bee’s comparison shows several popular 5 to 10 kW inverters rated at 1.5 times their continuous power, but one 12 kW model only 1.1 times for 60 seconds. Check the datasheet of the model you choose against your worst motor start. For example, with 1,000 W of load and a fridge whose compressor briefly draws an extra 1,500 W when it starts, the inverter needs at least 1,000 W continuous and 2,500 W surge.
Step 3: work out the energy
Formula: energy (Wh) = load (W) × hours of backup
Running 1,000 W for 4 hours needs 4,000 Wh, or 4 kWh.
Step 4: size the batteries
A battery can’t be fully emptied without shortening its life, so only part of its rated capacity is usable. That part is the depth of discharge. Pylontech’s US3000C, a common lithium module, holds 3.55 kWh nominally and may be discharged to 95%, so about 3.37 kWh is usable.
Formula: modules = energy ÷ (module kWh × depth of discharge), rounded up.
For the 4 kWh example: 4 ÷ 3.37 = 1.19, so two modules, giving about 6.7 kWh usable and roughly 6.7 hours at 1,000 W. Inverter and battery losses make real backup time a little shorter. The inverter and battery calculator works this out with any module size and depth of discharge from the datasheet.
Step 5: think about recharging
Between outages the batteries recharge from the grid, from solar panels, or both. If load shedding is frequent, the batteries must recharge fully in the time between outages. Solar panels can do much of the recharging. To size them, divide the energy you use in an outage by what one kilowatt-peak (kWp) of panels produces in a day in your area’s worst month. For panels facing north at 26°, the European Commission’s PVGIS database gives from about 2.9 kWh per kWp a day in Cape Town in June to about 4.6 in Mahikeng. For the 4 kWh example in Johannesburg, whose worst month gives 4.30, that is 4 ÷ 4.30 = about 0.93 kWp, or roughly 930 W of panels. The inverter and battery calculator does this for 14 cities. Shade, the roof’s direction and its pitch all change the output, so have your installer confirm the layout for your roof.
Safety and compliance
- A fixed inverter installation is part of the electrical installation. Have a registered electrician wire it and issue a Certificate of Compliance; see electrical CoCs and voltage drop.
- Put the backup on its own distribution board circuits so only the essentials run from the inverter.
- Mount batteries where they are ventilated, dry and protected, as the maker specifies.
- Check what your municipality requires before connecting a system that can feed power back into the grid.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.