Why Sizing Matters
An undersized solar system leaves you without power during peak demand or at night. An oversized system means you paid for capacity you never use. South African solar installers vary significantly in how rigorously they size systems — some conduct thorough load assessments, others quote a standard package and hope it fits. Understanding the basic maths yourself means you can evaluate quotes intelligently and avoid both extremes.
Step 1 — Understand Your Electricity Consumption
Your monthly electricity bill shows your total kWh consumption. Most South African households use between 400 and 1,200 kWh per month. Divide your monthly figure by 30 to get your daily average consumption in kWh.
A 600 kWh/month household uses 20 kWh/day on average.
But averages hide peaks. Check your bills over 12 months and identify your highest month — this is your design month for battery sizing. Also identify what drives consumption: geysers typically account for 40–50% of a household's electricity use; pool pumps, air conditioners, and electric stoves are other major consumers.
Step 2 — Decide What You Want to Power
Solar systems are sized for specific loads. You have three broad options:
- Essential loads only — lights, plugs, router, TV, and perhaps a fridge. Excludes geysers, stoves, and pool pumps. A 3–5kWp system with 5–10kWh of storage suits this.
- Whole-home backup — everything except the stove and geyser (which are best addressed separately). A 5–8kWp system with 10–20kWh of storage.
- Full energy independence — including geyser (via heat pump or diverter), electric vehicle charging, and all loads. 8–15kWp+ with 20–40kWh of storage.
Step 3 — Calculate Panel Requirements
South Africa averages 4.5–6 peak sun hours per day depending on the region (higher in the Northern Cape, lower in the coastal Cape). A 1kWp (kilowatt-peak) solar array produces approximately 4–5 kWh per day in Gauteng and 5–6 kWh per day in the Northern Cape.
To produce 20 kWh/day in Gauteng: 20 ÷ 4.5 ≈ 4.4 kWp of panels required. In practice, systems are sized at 20–30% above the calculated minimum to account for losses (shading, temperature, inverter efficiency, wiring).
A typical residential panel is 400–550Wp. A 5kWp array requires approximately 10–12 panels. Check your roof has adequate unshaded south-north facing space — each panel is approximately 1.7m × 1.1m.
Step 4 — Calculate Battery Requirements
Batteries are sized to cover your consumption during outages or at night. The standard approach: multiply the loads you want to run through an outage by the expected outage duration, then add 20% for inefficiency.
If you want to run 2 kW of essential loads through a 6-hour Stage 6 outage: 2 × 6 = 12 kWh of load. With LFP lithium batteries at 90% efficiency and 80% usable depth: 12 ÷ 0.9 ÷ 0.8 = 16.7 kWh of nominal battery capacity required.
For whole-home backup through a full night (8 hours at 1.5 kW average): 12 kWh of load → approximately 17 kWh nominal battery capacity for LFP.
Step 5 — Inverter Sizing
The inverter must handle your peak simultaneous load. If you run a 2kW air conditioner, a 1kW kettle, 300W of lights, and a 200W fridge simultaneously, your peak load is 3.5kW. The inverter should be sized at least 20% above this peak: 4.2kW minimum, so a 5kW inverter is appropriate.
Common South African inverter sizes: 3kVA, 5kVA, 8kVA, 10kVA. For most three-bedroom homes, a 5kVA single-phase inverter is the entry point for whole-home backup.
Using This to Evaluate Installer Quotes
When an installer quotes you, ask them to walk through their load assessment. How did they calculate the recommended system size? What assumptions did they make about your daily consumption? A good installer will share their load assessment spreadsheet or calculation. One who says "this is the standard package for a house your size" without site-specific calculation deserves a more careful look.