What Size Solar System Does a South African Home Need?

Most homeowners ask 'what size solar system do I need?' and expect a number in kilowatts. But a solar system has two separate ratings: instantaneous power (kW) and energy over time (kWh). Your home uses both. A kettle might draw 2 kW while it boils, but only for a few minutes, so it uses a small amount of energy. A pool pump might draw 0.75 kW for six hours, using 4.5 kWh. The size of your solar system must cover both the peak power you draw at any moment and the total energy you use across a day.

The only reliable way to size a system is to look at your actual consumption. Your municipal or Eskom bill shows monthly kWh, but that single number hides when you use the energy. Two homes can use 900 kWh per month: one mostly in the evening, the other mostly during the day. The daytime-heavy home will get far more value from solar panels without a large battery. The evening-heavy home needs either a battery or a change in habits.

So before you accept any quote, gather at least a few months of bills and, if possible, half-hourly or hourly data from your meter. If you do not have interval data, a simple energy monitor that clamps around your main breaker can show your daily pattern. This step costs little and prevents an expensive mistake.

A solar panel is rated in watts (W) or kilowatts (kW) under standard test conditions. A 450 W panel produces 450 W at noon on a cool, clear day. In real South African conditions, with heat, dust and imperfect angles, you will often see less. The inverter is also rated in kW, and it converts the direct current from the panels into alternating current for your home. If your inverter is 5 kW, it can supply up to about 5 kW of instantaneous power, regardless of how many panels you have.

Energy is measured in kilowatt-hours (kWh). One kWh is 1 kW of power used for one hour. Your bill is in kWh. A battery is also rated in kWh, which tells you how much energy it can store. A 5 kWh battery can, in theory, run a 1 kW load for five hours, though real-world usable capacity is often less to protect battery life.

The practical rule: size the inverter and panels for your peak kW and daily kWh, and size the battery for the energy you need after sunset or during an outage. Do not confuse the two. A system with 6 kW of panels and a 5 kW inverter might produce 25–30 kWh on a good day, but it will not power a 7 kW instantaneous draw unless the grid or battery assists.

A load profile is a record of when you use electricity and how much. You can build one on paper. List your main appliances, their power rating in watts, and the hours per day they run. Multiply watts by hours and divide by 1000 to get kWh. Add them up for a daily total. Then mark which ones run during the day and which at night.

For example, a household might have: fridge 150 W for 24 hours (3.6 kWh), lights 100 W for 5 hours (0.5 kWh), TV 100 W for 4 hours (0.4 kWh), washing machine 2 kW for 1 hour (2 kWh), dishwasher 1.5 kW for 1 hour (1.5 kWh), pool pump 0.75 kW for 6 hours (4.5 kWh), and geyser 3 kW for 3 hours (9 kWh). That totals about 21.5 kWh per day. The geyser alone is over 40% of the energy. If you heat it during the day with solar, you can avoid a large battery.

Your peak demand might be 6 kW if the geyser, kettle and microwave run together. That tells you the inverter and wiring must handle at least that, or you must manage loads so they do not overlap. A load profile turns a vague question into specific numbers.

Your roof may limit the system size more than your budget or energy use. A typical 450 W panel is about 1.7 m by 1.1 m, so it needs roughly 2 square metres. A 5 kW array of 12 panels needs about 24 square metres of unshaded roof. If your roof has dormers, trees, or multiple slopes, you may fit fewer panels or need optimisers, which add cost.

In South Africa, north-facing roofs get the most sun over the year. East and west-facing roofs produce less total energy but can spread production across the day, which may suit a household that uses more power in the morning and late afternoon. Shading is the biggest killer of solar production. Even a small shadow from a chimney or tree branch can disproportionately reduce output from a string of panels.

Also consider the structural condition of your roof and the age of your waterproofing. Installing solar on an old roof may mean removing and reinstalling panels later. If you plan to expand, leave physical space and ensure the inverter and wiring can handle additional panels. A reputable installer will assess your roof and be honest about what fits.

Suppose a family of four uses 900 kWh per month, or about 30 kWh per day. Their load profile shows 12 kWh during daylight hours and 18 kWh in the evening and night. They want to reduce their bill and have some backup during outages. Their roof has space for 12 panels facing north with no shade.

Option A: They install 5 kW of solar panels and a 5 kW inverter, no battery. On a sunny day, the system might generate 20–25 kWh. Because only 12 kWh is used during the day, the rest is exported to the grid if they have a feed-in tariff, or wasted if they do not. Their evening use still comes from the grid. This option reduces the bill but provides no backup.

Option B: They add a 10 kWh battery. Now they can store excess daytime solar and use it in the evening. If the battery is 10 kWh and they use 18 kWh at night, they still draw about 8 kWh from the grid. To cover all night use, they would need a larger battery or to shift more loads to the day. This shows why the battery size depends on evening energy, not on the solar panel size alone.

Option C: They first change habits: run the geyser, pool pump and washing machine during the day. Their daytime use rises to 18 kWh and evening use falls to 12 kWh. Now a 5 kW solar system with a 5 kWh battery covers most of their needs. The cheapest kilowatt-hour is often the one you do not need to store.

Solar is modular, but not infinitely so. Inverters have a maximum input, and batteries have limits on how many can be added. If you think you may buy an electric vehicle or add a heat pump later, tell your installer now. It may be cheaper to buy a larger inverter or a battery system that can be expanded than to replace equipment in two years.

However, do not oversize on speculation. A larger inverter than your panels can support may be inefficient, and batteries degrade over time. The best approach is to design a system that meets your needs for the next three to five years, with a clear path to add panels or batteries if your consumption grows. Ask your installer for a single-line diagram and a written explanation of the expansion limits.

Also consider the non-technical side: municipal approval processes, home insurance, and warranty terms vary. Check with your municipality and insurer before installing. VumaTech does not provide legal or compliance advice, and you should confirm requirements with the relevant authorities.

Use this checklist to move from guesswork to a informed decision. It will help you ask better questions and compare quotes fairly.

Remember that the cheapest system is not always the best value. A well-sized system that matches your load profile will outperform a larger, poorly designed one. Take your time, get multiple quotes, and ask for references.