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Home power backup systems

Electrical engineers answer your questions

Eskom has made it clear that ‘load-shedding’ isn’t going to end any time soon.  This reality, and President Cyril Ramaphosa’s announcement during his annual State of the Nation speech on 9 February 2023 that tax incentives for solar power use are imminent (confirmed in the Budget on 22 February 2023), means that many people are considering alternative electricity supply systems for their homes.

However, deciding on the best system isn’t a simple matter.  There’s a bewildering array of jargon to sift through and many elements to consider, from the right kind of inverter to the size of your solar panels.

The authors are electrical engineers who are working on a standalone charger for small electric vehicles with the South African National Energy Development Institute as part of the Long-Term Joint European Union—African Union Research and Innovation Partnership on Renewable Energy.  The way the charger is designed resembles the sort of system needed for domestic power cut solutions.  Our full and detailed instructions for designing a load-shedding system will be available online soon**.

Therefore, we’re able to answer a few questions for those who feel overwhelmed by the options.

What is an inverter?

This is a key component of any alternative power system.  It’s an electronic device that changes direct current (like energy stored in a battery) into alternating current (power for your home).

There are a few kinds of inverters.  Some are grid-tied (synchronous) with Eskom’s power grid.  They are typically used with solar systems that augment the Eskom supply, but they are not suitable for load-shedding solutions.  You also get off-grid (grid-forming) inverters, which form their own mini-grid and can operate during power cuts.

We recommend a hybrid inverter, which can be grid-tied to augment supply and seamlessly continue operation as an off-grid solution during load-shedding.  You want one that can connect to the grid, battery backup, and to solar panels.

To extract the maximum power from the solar panels, be sure to get one that has maximum power point tracking (MPPT).

What size should the system be?

This purchase should be a long-term investment.  The inverter must be able to carry the sum of all the loads that are drawing power at any instant in time, and the battery must be able to supply the energy required.

To reduce both the upfront capital cost and operational expenditure, you need to decide what is essential.  Lights?  Your washing machine?  The stove and electric kettle?  Then you need to make sure they are as energy efficient as possible before you size the backup system.  For example, old incandescent lights use ten times more energy than LED lights do.

Why do I need batteries for my system?

It is theoretically possible for an inverter to generate electricity for household use directly from solar panels.  However, the supply from panels is intermittent and often not powerful enough to reliably supply power to the varying loads in the house.  To overcome this problem, energy is stored in the batteries.  This provides a more stable source of power which responds to demand during loadshedding.

Battery capacity is specified as kWh (kilowatt hours) or Ah (ampere-hours).  This determines the amount of energy it can supply.  A battery with a capacity of 5kWh can theoretically supply 5kW for an hour.

However, if a lithium battery is discharged beyond 20% of its capacity, it loses capacity and ages faster.  A 5kWh battery, therefore, has an effective capacity of only 4kWh.  It can supply 4kW for one hour, or 1kW for four hours.

As a practical example, if you want to power only 20 10W LED lights and a medium-sized LED TV, drawing a total of 0.5kW, a 3.5kWh battery will suffice for four hours.

Are solar panels crucial for a backup system?

No.  The batteries store energy to provide a stable supply to the inverter when needed.  Technically, you can use the mains power to charge the batteries, rather than rely on solar panels.  Solar panels are merely there to augment the supply of electricity and could give you a bit more range during loadshedding if the sun is shining.

However, if everyone installs backup systems without solar panels, we are just using batteries to carry us through power cuts.  That increases the load on Eskom outside load-shedding periods, as the batteries must be replenished.  This will neuter Eskom’s ability to use load-shedding as a grid management tool.  It could destabilise the grid and lead to a complete blackout.

Also, given the tax breaks for solar generation expenses announced by Finance Minister Enoch Godongwana in his Budget speech, solar panels will have to be part of your setup if you want to benefit from these incentives.  This is therefore a good time to make the switch to a solar-powered backup system, for your peace of mind and future savings.

How much does it all cost?

Inverter prices are falling as the market grows and vary across suppliers.  Currently, they cost about R3 000 per kW for bottom-of-the-range and closer to R7 000 per kW for top-of-the-range.  Most households will get by with a 3kW to 5kW inverter if its loads are managed well, costing between R9 000 and R35 000.

Lithium (LiFePO4) batteries also vary in cost, but normally retail for about R5 000 to R7 000 per kWh.  Most households will get by with a 5kWh to 10kWh battery if the loads are optimised and managed well, so you’re looking at a cost of between R25 000 and R70 000 for the batteries.

Solar panels tend to range from R8 000 to R10 000 per kWp (a measure of how high the panels’ power output is).  Again, they’re not crucial but are necessary if you want the system to pay for itself over time.

Can I install this system myself?

No, unless you’re a certified electrician.  The inverter needs to be installed into the distribution board—and the cost will depend on how many of your circuit breakers need to be moved to the backup, as well as how easy the solution is to install.  The inverter must also be approved by the municipality if you want to feed back into the grid.

Installation costs for the inverter typically range from R10 000 to R20 000.  Installing solar panels is separate, and costs vary widely.

WRITTEN BY Thinus Booysen and Arnold Rix

MJ (Thinus) Booysen is a professor in engineering and is chair of the Internet of Things at Stellenbosch University.

Arnold Rix is a senior lecturer in electronic and electrical engineering at Stellenbosch University.

This article is republished from The Conversation under a Creative Commons Attribution-NoDerivatives (version 4.0 International) license.

While every reasonable effort is taken to ensure the accuracy and soundness of the contents of this publication, neither writers of articles nor the publisher will bear any responsibility for the consequences of any actions based on information or recommendations contained herein.  Our material is for informational purposes and should not be construed as financial advice.

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