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 — Plan for future expansion
The case for an oversized inverter: a 5 kW inverter running 8 panels today can accept 4 more panels later if you buy a hybrid inverter with a higher PV input rating from day one.
Most 5 kW hybrid inverters (Deye, SunSynk) accept 6.5–7.5 kW of PV input — enough for 12 × 550W panels. Starting with 8 panels and adding 4 later is a cost-effective upgrade path.
Similarly, most hybrid inverters support parallel battery expansion. Starting with one 5.12 kWh battery and adding a second later is straightforward with Pylontech, Freedom Won, or Dyness.
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.
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