Back-up solar power in South Africa: How to size a home system properly

Key Takeaways

A well-sized system starts with your own energy use, not a package label. Work through the loads you want to run, then check that the battery, inverter and panels fit those needs.

• Write down which appliances matter when grid power is unavailable.
• Estimate energy use in kWh by combining each load’s power and run time.
• Size the battery first, then the inverter, then the solar array.
• Compare system types and quote line items rather than relying on a headline price.
• Use your own electricity bill and load list to judge the financial case.

What "back-up solar power" actually means

Most solar quotes can feel like car prices with the stickers scraped off: a bundle of parts, but little explanation of why it suits your home. You can make the numbers easier to assess by understanding what each part does and what it needs to support. That matters whether you want more control over your energy supply or a way to use solar electricity at home.

The three parts of every system

A home solar system usually brings together panels, an inverter and a battery, although not every design includes all three. Panels make electricity from sunlight, the inverter converts it for household use, and a battery stores electricity for use later. The system design determines how those parts work together and which household circuits they can supply.

Solar water heating is a separate technology: it captures heat rather than generating electricity for household circuits. The Wikipedia: Solar water heating article reports that global solar hot-water thermal capacity reached 560 GWth at the end of 2023, up 3% from 2022, according to Wikipedia. That figure describes solar thermal systems, not rooftop PV, so it should not be used to size an electrical system.

How the system behaves when the grid is up vs when it is down

When grid power is available, a solar system can supply the home while the grid remains an additional source, depending on the design and settings. When grid power is unavailable, only equipment designed to operate in that condition can supply selected circuits. Ask which circuits are connected and how the system transfers between sources; do not assume every solar installation provides backup.

A solar thermal collector also differs from a PV panel in what it produces. The Wikipedia: Solar thermal collector page notes that absorptivity in modern collector absorber plates can exceed 93%, according to Wikipedia. That is a measure related to heat collection, not a rating for the electrical output of your PV panels.

Decide what you want to keep on during an outage

The useful starting point is not the size of your roof or a package name. It is the list of appliances you want available when grid power is out, and how long you expect to use them. That list sets the energy the battery must provide and the power the inverter must deliver at one time.

Walk through a typical home's essential loads

Think through the routines you want to maintain, from keeping food cold to staying connected. A typical essential-load list might include refrigeration, selected lights, a router and a few small devices. Heating appliances, pumps and cooking equipment can add much heavier demand, so include them only if you genuinely want them supported.

It helps to sort the list by how you use each item, rather than treating the whole home as one load. These groups make that conversation clearer:

• Appliances that run for long periods, such as a fridge.
• Short-use devices, such as a kettle or microwave.
• Loads that need a motor to start, such as a pump.
• Optional equipment that can be switched off to preserve stored energy.

This is not a final design. It is a practical way to spot which items use energy continuously, which draw a high load briefly, and which you can leave out.

How to add up the watts and the hours

For each appliance, note its power in watts and the hours you expect it to run. Convert watts to kilowatts, then multiply by the run time in hours to estimate energy use in kWh. Add the estimates for the loads you want supplied together. The result is a working daily energy estimate, not a guarantee of actual use.

Starting with this list makes it easier to check system sizing against real household needs rather than a vague idea of a “whole-home” system. Record appliances that may run at the same time as well, because their combined power affects the inverter choice. Your list can be refined after a site assessment and a check of the actual equipment.

Sizing the battery, inverter and panels in that order

Sizing works best as a sequence. First estimate how much energy you want stored, then check the inverter can supply the loads, and finally estimate how much solar generation is needed to serve the home and replenish the battery. Each step answers a different question, so one large component cannot compensate for a mismatch elsewhere.

Battery size in kWh for a typical household

Battery capacity is measured in kWh, the unit that describes stored energy. Use your load list to estimate the energy you need across the period when you expect to rely on stored power. A household that wants only a few essential appliances running has a different requirement from one that includes cooking or other heavy loads.

Also check how much of the stated capacity is usable, and whether the design allows room for the battery to recharge from solar. A battery that looks large on a quote may still not match your intended run time if your daily energy estimate is higher than expected. Your usage pattern matters more than a generic label such as “medium home”.

Inverter size in kW and why it must match the battery

The inverter’s kW rating describes the rate at which it can supply power. Compare that rating with the total load you expect to run at once, and pay special attention to appliances with motors or heating elements. A system may have plenty of stored energy but still be unable to run several high-power appliances together if the inverter is undersized.

The battery also needs to be able to provide power at a rate the inverter can use. Ask for the battery’s continuous output rating and any relevant peak rating, then compare them with the inverter and your expected loads. This is where the design must be checked as a complete system, not as three separate catalogue choices.

How many panels you actually need, in plain words

Panel quantity depends on how much electricity you use, how much you want solar to provide, roof orientation and available roof space. In South Africa, roofs generally receive their best sun when they face north, though the final design also depends on shading and the roof itself. Panels also need to make enough energy to serve daytime loads and, where the design allows, recharge the battery.

Ask the designer to show the expected solar production and the assumptions behind it. The SARS page describes a personal income-tax rebate of 25% of the cost of new and unused PV panels, up to R15 000, as stated by SARS. Treat that as information about the published tax rule, not as a panel-sizing input; eligibility depends on the applicable tax rules.

Grid-tied, hybrid or full off-grid

These labels describe how a system relates to the grid, battery storage and household loads. The right choice depends on how much independence you want, what the property can support and how you intend to use electricity. Ask the installer to explain the system’s operating modes in ordinary language, not only through a product name.

What a hybrid system does that the others do not

A hybrid system combines solar generation with battery storage and a grid connection. That gives it more flexibility than a grid-tied system without storage, while still keeping the grid as part of the supply arrangement. The exact behaviour depends on the selected equipment and the system design, so confirm which circuits remain supplied when grid power is unavailable.

ZenSolar sells Fox ESS systems. When comparing any hybrid proposal, focus on the documented configuration, the battery and inverter ratings, and the circuits included. A brand name on its own does not establish that the system suits your load list.

When full off-grid makes sense for a home

Full off-grid means the home is designed to operate without relying on the grid. It can suit a property where a grid connection is not available or where the owner has a clear reason to manage the full energy supply independently. It also requires careful planning for periods of low solar production and for the largest loads in the home.

Before choosing it, compare the cost and practical demands of supplying every load with the value of keeping some grid connection. The design needs to account for the household’s seasonal use, roof space, battery capacity and any backup generation plan. For many homes, the decision is less about a label and more about what level of independence is worth paying for.

Solar backup vs a generator, side by side

A battery system and a generator can both provide electricity when grid supply is unavailable, but they operate differently. A battery stores energy, while a generator makes electricity using fuel. Compare them against the loads you want to supply and the role you expect them to play in your home.

Noise, fuel and automatic switchover

A battery has no engine running beside the house and needs no fuel tank. A generator makes noise while operating and needs fuel, servicing and a suitable place to run safely. Whether either option switches over automatically depends on the equipment and how it is installed, so get the transfer behaviour confirmed in writing.

A generator may be considered where long periods without grid supply are part of the property’s operating plan. A battery can support selected circuits and may recharge from solar when conditions allow. Neither option should be assumed to run every appliance unless the design and ratings support that load.

Running cost over a year

Compare more than the purchase price. For a generator, include fuel, routine servicing and the cost of storing fuel safely. For a battery system, consider how it is charged, how often you use stored energy and any maintenance or replacement costs stated in the quote.

The best comparison uses your own pattern of use. If you rarely need a generator, its standing costs may matter more than the cost of fuel during operation. If you use a battery regularly, the value depends on the electricity it supplies and the tariff you would otherwise pay.

What this costs you in rand, without the sales spin

A quote should explain what is included, not just offer a single total. The final cost depends on the size and type of system, the selected equipment, installation requirements and any electrical work needed at the property. Without your load list and site details, a generic rand figure would not tell you whether the system is correctly sized.

The line items that make up a quote

Look for separate descriptions of the panels, inverter, battery, mounting equipment, protection devices, cabling, labour and any changes to the existing electrical installation. Check whether monitoring, commissioning and required paperwork are included, and ask what falls outside the quoted scope. Clear line items make it easier to compare two proposals on equal terms.

ZenSolar sells Sigenergy systems by quote and site assessment only. A proposal involving Sigenergy should therefore set out the assessment and the equipment being considered, rather than presenting an off-the-shelf price. Fox ESS is also sold by ZenSolar, so compare the exact components and scope in each proposal rather than assuming two brand names represent equivalent systems.

Why the Eskom tariff climb has changed the math

The value of solar depends in part on the electricity you would otherwise buy from the grid. The CSIR reports that the national average electricity tariff increased by an average of 10% per year from 2014 to 2024, compared with average inflation of 5.2% over that period, according to CSIR. Those figures describe the past period, not a forecast for your future bill.

Use your own bill and consumption pattern to test the economics of a system. Solar production, self-consumption, battery use and the tariff on your account all affect the result. ZenSolar’s solar ROI assessment is one way to organise your own figures before comparing a proposal.

Common questions South African buyers ask

Many questions have straightforward answers, but the details depend on the system design and the rules that apply to the property. Ask for answers that refer to the equipment in your proposal, rather than relying on a general promise. Your electricity account and household use are part of the picture too.

Do I still pay Eskom if I have solar?

Usually, a home connected to the grid keeps an electricity account, even if solar reduces how much electricity it buys. The amount depends on your tariff, your usage and any fixed charges on your account. Check the terms that apply to your meter and municipality.

Does rain or clouds stop the system from working?

No. Solar panels can still generate electricity in cloudy weather, though production is generally lower than under clear, sunny conditions. A battery can store energy for later use, but its available energy depends on how much it has charged and how much you consume.

Can I start with batteries and add panels later?

Sometimes, but do not assume every system can be expanded easily. Check whether the inverter supports the planned arrangement, whether the battery can be expanded and whether the later panel design will fit the roof and electrical setup. If you want to add panels later, make that part of the original design discussion.

How to read any backup solar quote with confidence

A useful quote should connect the equipment to your actual household needs. It should also show the system’s limits, the work included and the assumptions behind the expected production. If those details are missing, ask for them before comparing totals or signing.

The five questions to ask before you sign

Ask the installer to walk through the design against your load list. These questions help expose gaps without requiring you to be an electrical engineer:

1. Which circuits will the system supply, and which will not?
2. What battery capacity is usable, and what run time does the design assume?
3. Can the inverter handle the loads that may run at the same time?
4. What solar production does the design expect, and what assumptions shape that estimate?
5. Which equipment, installation work and exclusions are included in the quoted total?

Then compare the answers with your original list and your electricity bill. If the design meets your priorities and the quote explains its scope, you have a sound basis for comparing proposals. You can also book a discovery call to discuss your own numbers before making a decision.

Conclusion

Sizing back-up solar power is a practical exercise in matching your household’s loads to the battery, inverter and panels. A clear load list gives you a way to test whether a quote makes sense, while your bill helps you judge the financial case. When you are ready to discuss your own usage, book a free solar consultation with ZenSolar.

Frequently Asked Questions

What does back-up solar power mean?

It is a solar system designed to supply selected household loads when grid electricity is unavailable, usually using a battery and an inverter.

How do I work out the battery size I need?

List the appliances you want supplied, estimate their energy use over the period you need, and compare that total with the usable battery capacity in the proposal.

What is the difference between kW and kWh?

kW describes the rate of power being used or supplied. kWh describes an amount of energy used or stored over time.

Does a solar system always include a battery?

No. Some systems are designed without battery storage. Confirm whether the system you are considering can supply selected circuits when grid power is unavailable.

Can solar panels produce electricity on cloudy days?

Yes, panels can still produce electricity in cloudy conditions, but output is generally lower than in clear, sunny weather.

Can I run the whole house from a battery?

That depends on the battery capacity, inverter rating and combined power demand of the appliances you want to run. A design should state which circuits it can supply.

What should I compare between solar quotes?

Compare the equipment, usable battery capacity, inverter output, expected solar production, circuits included, installation scope and exclusions, not only the total price.