Like countless other people in South Africa, I have invested in an Inverter with a Lithium battery. The battery is a significant part of this investment, so it is essential to get as much runtime out of a single charge as possible.

My inverter is connected to my DB board, meaning all the sockets and appliances in my house are powered by the inverter during a power outage, which, in South Africa, happens frequently and for hours at a time.

Having “always on” power in all the sockets is great and super convenient, but not always smart. As winter encroaches, we start plugging in appliances that draw substantial amounts of power, like electric heaters and blankets. Some electric heaters can draw up to 2 kilowatts of power per hour. Given that the typical inverter system is paired to a 5-kilowatt battery, which means a single electric heater can almost drain the entire capacity in just two hours.

To solve this problem, I built a “smart extension cord” that turns off when the power fails and turns back on once the power returns.

Sure, you can manually turn off the heater when the power fails, but this is not so easy for appliances with hard-to-reach plugs, like power-hungry dishwashers & washing machines.

Another obvious solution is to split your supply in your house into essential and non-essential, and only run the essential circuits through the inverter. Easier said than done, as most houses do not have separate wiring for essential and non-essential plugs, other than geysers and stoves.

So how can we make this easy, so we don’t drain the inverter battery with high-demand appliances that may be left on inadvertently during periods of power unavailability?

There are costly solutions available that require an electrician to install, like the Sonoff Loadshedder. Alternatively, you can configure a scene using the Ewelink or Google Home apps, but all these solutions require an active internet connection and keep your data on a third-party server in the cloud.

The solution I built at home does not use any cloud servers and does not require an active Internet connection. I already have a Raspberry Pi running in my house, connected to my SunSynk inverter, so I am using this to trigger on and off signals to my electric heater when the power fails and returns.
To make this work, I used a Shelly 1 smart relay. These are tiny, and relatively inexpensive. You can get hold of one online for around R270.

The advantage of using a Shelly relay is that besides being controllable via a mobile app, it also runs a local webserver on the device itself, meaning it has a locally available API. Once configured, you can make a simple API call to enable or disable the relay inside the unit.

Wiring it up was easy as well, as clear wiring diagrams for various scenarios are available online. I put the Shelly unit inside a plug box and attached a short lead. This “smart extension cord” is then plugged in between the electric heater and the wall socket.

The final step was to modify the existing code on my Raspberry Pi to call the API whenever it detects a power failure or the resumption of power. So, every time the power fails, and my electric heater happens to be on, it will automatically switch off as if the wall socket was turned off manually. Once the power returns, the smart socket turns back on.

I plan on building more of these units and will be installing one behind the dishwasher, the washing machine, etc. In all those hard-to-reach places.

If you know how to program, you can go much further. For example, I recently bought a “smart” panel heater from Builders Warehouse. This panel heater only consumes 400W, so I am happy to let it run all day whilst I am working at my desk. It comes with a Sonoff controller built in, so it is straight forward to program it to turn on at 8 am and off again at 5 pm, weekdays only. I took this a step further, and automated this heater to also turn off when the power fails. I programmed it to only turn back on if the power resumes and the outside temperature is below 18 degrees. This prevents it from needlessly turning on when it is not required.

Please note, whenever working with 220v electricity, always ensure all safety precautions are adhered to, and get an electrician to check your work. If the installation is permanent, then a CoC must be obtained.