Solar Calculator SA: How Much Solar Do You Need?

One of the most common questions South Africans ask when going solar is: "How many panels and batteries do I actually need?" The answer depends on your daily electricity consumption, your location, and what you want to power. This step-by-step guide walks you through the exact calculation — with three worked examples for different home sizes. No engineering degree required.

Step 1: Find Your Daily kWh Consumption

Look at your last 3–6 electricity bills (or prepaid token history). Find the monthly kWh consumed and divide by 30 for your daily average.

• Eskom bill: Usually shows kWh used in the billing period
• Municipal bill: Look for "units" or "kWh" line item
• Prepaid: Divide your monthly spend by the rate per kWh to get monthly kWh

Use summer AND winter bills and average them — solar produces less in winter (shorter days, lower sun angle), but you also typically use more electricity in winter (heating, longer lighting hours).

Step 2: Peak Sun Hours by SA Region

Peak sun hours (PSH) is the daily average number of hours when solar irradiance is at its peak (1,000 W/m²). South Africa is exceptionally sunny — here are realistic annual averages:

Note: Use the winter PSH for sizing if you want year-round self-sufficiency. If grid backup is acceptable in winter, use the annual average.

Step 3: Calculate Required Panel Capacity

Formula: Panel kW = Daily kWh ÷ Peak Sun Hours ÷ System Efficiency

System efficiency accounts for inverter losses (~5%), wiring losses (~3%), temperature derating (~5%), and panel degradation (~3%). Use 0.82 (82%) as a realistic total efficiency factor.

Example: 30kWh/day in Johannesburg (5.5 PSH): 30 ÷ 5.5 ÷ 0.82 = 6.65kW of panels needed

Step 4: Battery Bank Sizing

Batteries cover your night-time and overcast usage. For a hybrid system (grid as backup), size for 1–1.5 days of essential loads. For off-grid, size for 2–3 days.

Formula: Battery kWh = Essential Daily Load (kWh) × Days of Autonomy ÷ DoD

DoD = Depth of Discharge: 0.8 for lithium LiFePO4, 0.5 for lead-acid.

Example: 10kWh essential load per day, 1.5 days, lithium: 10 × 1.5 ÷ 0.8 = 18.75kWh battery bank (≈ 8 × Pylontech US2000)

Step 5: Inverter Sizing

Your inverter must handle your peak simultaneous load — the maximum watts you'll draw at any one moment. Add up your largest concurrent loads:

• Rule of thumb: inverter kVA = peak load kW ÷ 0.8 (power factor)
• Always upsize by 20–30% for startup surge capacity
• For solar, the inverter's PV input capacity must exceed your panel array kW

Worked Examples for Three SA Home Sizes

Example A: Small Flat or Apartment (15kWh/day, Johannesburg)

Example B: Average 3-Bedroom House (30kWh/day, Cape Town)

Example C: Large Home with Pool (50kWh/day, Durban)