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How to Size a Solar System for Your Home

One of the biggest mistakes people make is buying a solar system that's too small or too big. Here's how to work out exactly what size you need.

Key Takeaways

  • Check your electricity bill to see how many kWh you use per month
  • A typical SA home uses 600-900 kWh/month
  • You need about 1kW of panels for every 4-5 kWh of daily usage
  • Battery size depends on what you want to run during load shedding
  • Start smaller — you can always add more panels later

Step 1: Check your electricity usage

Look at your electricity bill or prepaid purchases for the last 3 months. Find the kWh number. This tells you how much electricity you use.

  • Small home / flat: 300-500 kWh/month
  • Medium home: 600-900 kWh/month
  • Large home (pool, geyser, aircon): 1,000-1,500 kWh/month
  • If you use prepaid, add up your monthly purchases and divide by the tariff rate

Step 2: Decide what you want to achieve

There are three levels of solar:

  • Level 1 — Reduce bill only (grid-tied, no battery): Cheapest. Saves money during the day. No backup during load shedding.
  • Level 2 — Reduce bill + load shedding backup: Medium cost. Saves money AND keeps essentials running during outages.
  • Level 3 — Go fully off-grid: Most expensive. No Eskom bill at all. Needs large battery bank.

Step 3: Size your panels

In South Africa, we get about 4.5 'peak sun hours' per day on average. A 1kW panel system produces about 4.5 kWh per day. So:

  • 3kW system → 13.5 kWh/day → good for small homes
  • 5kW system → 22.5 kWh/day → good for medium homes
  • 8kW system → 36 kWh/day → good for large homes
  • 10kW+ system → 45+ kWh/day → large homes or small businesses

Step 4: Size your battery (if needed)

Battery size depends on what you want to run during load shedding and for how long:

  • 5kWh battery: Lights, WiFi, TV, phone charging for 4-6 hours
  • 10kWh battery: Above + fridge, microwave for 4-6 hours
  • 15kWh+ battery: Most of the house for 6-8 hours
  • Remember: batteries lose capacity over time (expect 80% after 10 years)

Common mistakes to avoid

Don't make these errors when sizing your system:

  • Don't size based on your TOTAL usage if you only want daytime savings
  • Don't forget that panels produce less in winter (about 30% less)
  • Don't buy the cheapest panels — quality matters for the 25-year lifespan
  • Don't skip the inverter size — it must handle your peak load
  • Do get at least 3 quotes and compare like-for-like

Ready to see your own numbers?

Use the Solar ROI Calculator