Start with your bill, not your appliances
Most online guides tell you to add up your appliances and pick a system. That's not how SA solar is done professionally. The correct starting point is your monthly kWh consumption — the number on page 2 of your Eskom or municipal bill.
Why? Because appliance lists are guesswork. You don't know exactly how many hours your fridge compressor runs, how often the geyser element fires, or what your standby load really is. Your bill already captures all of this — accurately, over a full billing period.
Step 1 — Find your kWh consumption
Your electricity bill shows kilowatt-hours (kWh) consumed. Here's where to find it:
- Eskom bill: Look for "Total kWh" or "Energy consumption" on the statement.
- City Power (Johannesburg): The kWh figure is on the detailed consumption page.
- Municipal bills: If your bill only shows a Rand amount, divide by your tariff rate. For City Power 2026 high-tier: approximately R3.48/kWh. For Eskom Homepower: approximately R2.75/kWh.
Typical SA household consumption: 600–1 200 kWh/month (R1 800–R4 000 bill).
Step 2 — Apply peak sun hours by province
Peak sun hours (PSH) represent how many hours of equivalent full-strength sunshine your location receives per day, averaged across the year. This is the key variable in system sizing. Note that these calculations assume your panels are installed at the optimal tilt and direction (read our solar panel angle guide for details).
| Province | City | Avg PSH | Summer PSH | Winter PSH |
|---|---|---|---|---|
| Gauteng | Johannesburg | 4.9 | 5.8 | 3.9 |
| Western Cape | Cape Town | 5.5 | 7.2 | 3.8 |
| KwaZulu-Natal | Durban | 4.7 | 5.9 | 3.6 |
| Eastern Cape | Gqeberha | 5.2 | 6.7 | 3.8 |
| Northern Cape | Upington | 6.3 | 7.7 | 5.2 |
| Limpopo | Polokwane | 5.4 | 6.2 | 4.5 |
| Mpumalanga | Mbombela | 4.8 | 5.6 | 3.8 |
| Free State | Bloemfontein | 5.6 | 6.8 | 4.2 |
| North West | Mahikeng | 5.5 | 6.5 | 4.4 |
Formula: System size (kW) = Daily kWh needed ÷ Peak sun hours × 1.25
The 1.25 factor accounts for system losses (inverter efficiency, cable losses, temperature derating, dust).
- Monthly bill: R2 500 → approximately 720 kWh/month → 24 kWh/day
- PSH: 4.9 hours
- System size: 24 ÷ 4.9 × 1.25 = 6.1 kW → round to 5 kW system (common size)
- Monthly bill: R2 500 → approximately 720 kWh/month → 24 kWh/day
- PSH: 5.5 hours
- System size: 24 ÷ 5.5 × 1.25 = 5.5 kW → round to 5 kW system
Step 3 — Match to inverter and panel count
Standard SA system combinations:
| System Size | Inverter | Panels (550W) | Battery | Monthly Output (GP) | Typical Bill Range |
|---|---|---|---|---|---|
| 3.6 kW | 3.6 kW hybrid | 4–6 | 3.6 kWh | 450–550 kWh | R600–R1 800 |
| 5 kW | 5 kW hybrid | 8–10 | 5.12 kWh | 650–800 kWh | R1 800–R4 000 |
| 8 kW | 8 kW hybrid | 12–14 | 10.24 kWh | 1 000–1 250 kWh | R4 000–R7 500 |
| 12 kW | 12 kW hybrid | 16–20 | 15.36 kWh | 1 500–1 850 kWh | R7 500+ |
Step 4 — Size your battery separately
Battery sizing is about backup duration, not solar production. You size panels for daytime generation; you size batteries for how long you need power when the grid is off.
Formula: Battery kWh needed = Hours of backup × Average load (kW) × 1.2
The 1.2 factor accounts for depth of discharge (DoD) and inverter conversion losses.
| Load Shedding Stage | Outage Duration | Essentials Only (0.8 kW) | Comfort (1.5 kW) |
|---|---|---|---|
| Stage 2 | 2–4 hours | 2.4–3.8 kWh | 3.6–7.2 kWh |
| Stage 4 | 4–6 hours | 3.8–5.8 kWh | 7.2–10.8 kWh |
| Stage 6 | 6–8 hours | 5.8–7.7 kWh | 10.8–14.4 kWh |
Step 5 — How many panels can your inverter accept?
There is no safe universal panel count for a “5 kW inverter”. The exact inverter model and panel model determine the answer. Panel wattage is only one part of the calculation.
Ask the designer to provide a string calculation that checks all of the following against the inverter and panel datasheets:
- Maximum DC input power for the exact inverter model.
- MPPT operating range and absolute maximum input voltage, including the panel string's open-circuit voltage (Voc) in the coldest expected conditions.
- Maximum input current and short-circuit current for each MPPT.
- Number of MPPT trackers and permitted strings per tracker, especially where roof sections face different directions or receive different shading.
For a future upgrade, request a written design showing spare MPPT capacity and a compatible expansion layout before buying equipment. See the add panels or battery guide for the wider upgrade decision.
If your home has a three-phase supply
Do not assume that every inverter proposal will use all three phases, or that every circuit will be backed up. Ask the designer to state the proposed single- or three-phase arrangement, the exact inverter model or models, which household circuits sit on each phase, and which circuits will remain powered during an outage.
Before accepting the quote, request a single-line diagram showing the generation connection point, isolation, protection and backup circuits. Also confirm that the installer will use your electricity supplier's current application and sign-off process. Eskom's December 2025 SSEG application form requires the exact planned inverter output and supply information, while its 2026 EGI compliance report explicitly covers both single- and three-phase low-voltage installations. Municipal requirements can differ, so get the applicable approval and paperwork responsibilities in writing.
Common sizing mistakes
- Sizing for peak summer only: winter shortfall, especially in Cape Town where winter PSH drops to 3.8.
- Not accounting for geyser or stove: unrealistic expectations about going "off-grid" without understanding the base load.
- Undersizing battery for Stage 6: the battery lasts 3 hours but outages are 6 hours.
- Choosing a non-expandable inverter: stuck with the initial panel count and battery capacity forever.
Once you have sized your system components correctly, ensure your installer sets up every part according to standard requirements on the day by following our solar installation day checklist.
Is your system producing what it should?
Once the system is running, the most common question is whether the numbers in the monitoring app are normal. Two different readings get confused:
- kW (power right now) — the instantaneous reading. A clear midday figure that looks below the panel nameplate is usually normal, not a fault.
- kWh (energy over time) — the daily and monthly total. This is the number your savings come from, and the only one worth comparing season to season.
Panels are rated at standard test conditions: 25 °C cell temperature and 1 000 W/m² of sunlight. A panel's own datasheet states how much its peak power falls for every degree above 25 °C — commonly quoted in the region of −0.30% to −0.45% per °C. On a hot roof the cells run well above 25 °C, so a 3.36 kWp array (6 × 560 W panels) showing roughly 3 kW at midday on a clear day is expected, not a defect.
To sanity-check the daily total: Expected daily kWh ≈ array kWp × peak sun hours × 0.75–0.85. Use the peak sun hours for your province from the provincial solar potential guide, and use the winter figure when you want a worst-case number. The 0.75–0.85 factor is a planning assumption for temperature derating, dust, cable and inverter losses, and any orientation losses — your own monitoring data is the real answer.
| 3.36 kWp example (6 × 560 W, north-facing, optimal tilt) | Peak sun hours | Expected daily output |
|---|---|---|
| Johannesburg — annual average day | 4.9 | ~12–14 kWh |
| Johannesburg — winter day | 3.9 | ~10–11 kWh |
| Cape Town — summer day | 7.2 | ~18–20 kWh |
| Cape Town — winter day | 3.8 | ~9.5–11 kWh |
Ranges are estimates built on the peak sun hours in the table above and a 0.75–0.85 derating factor. They are not a guarantee of output.
Low readings: check these before calling the installer
- Shading — one shaded panel can pull down every panel on the same string. Compare figures from before and after the obstruction appeared; see the shading guide.
- Soiling — dust build-up after a dry spell lowers output gradually. Rain alone does not always clear it.
- Direction and tilt — the estimates above assume north-facing and optimal tilt. East/west splits spread production across the day and lower the midday peak without reducing the daily total as much as owners expect.
- Battery full and low house load — where there is no export agreement or an export limit is set, many hybrid inverters throttle solar once the battery is charged and the house is not using the power. Midday curtailment is normal in that setup.
- Clipping — if the DC array is larger than the inverter's rated AC output, the inverter caps production at its limit during the strongest hours.
- Cloud and haze — a single cloudy day is not a pattern. Compare full sunny days only.
- What the app is actually measuring — metering position and inverter settings decide whether a figure represents PV production, house load or export. Ask the installer to label each number in the app before you interpret it.
How to check it properly
- Record daily kWh — not peak kW — for five to seven consecutive clear days.
- Note the dates, the weather, and whether the battery reached full charge.
- Compare each day against the expected seasonal range for your province.
- Screenshot the string or MPPT input figures so both strings can be compared. A large and consistent gap between two identical strings points to shading, a wiring issue or an MPPT problem that settings alone will not explain.
- Send the installer the dates and screenshots and ask them to explain the variance — a remote settings check is not a diagnosis.
If the numbers fall consistently below the seasonal range for genuinely clear days, get the explanation and the remedy in writing before you accept the job as closed. If the shortfall is simply that you need more generation, work through the add panels or battery guide. Where an installer will not engage with measured evidence, the installer red-flags checklist sets out how to escalate.
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