How We Size a Solar Energy System. The Exact Formula We Use

Why Proper Sizing Matters More Than Anything Else

In the solar industry, the most common reason customers are disappointed with their systems is that they were incorrectly sized. Either too small and the client still has high electricity bills or occasionally oversized, resulting in excess capacity they paid for unnecessarily.

At RC Solar & Electrical, we spend significant time on system sizing before we ever touch a screwdriver. It's what separates us from cheap installers who just guess, or use generic sizing without understanding your actual usage.

Step 1: Understanding Your Energy Consumption

The foundation of every solar system design is your actual energy consumption, measured in kilowatt-hours (kWh). Your Eskom or municipal electricity bill will show either your consumption directly, or the cost from which we can calculate it.

The Cape Town Tariff Calculation

This is why we always ask for your electricity bill amount, it's the fastest way to understand your baseline consumption. The tariff rate may change annually, so we always use the current approved City of Cape Town tariff when making calculations.

Step 2: How Many Panels Do You Need?

Once we know your monthly kWh requirement, we calculate how many solar panels are needed to produce that amount. The formula uses panel wattage (we typically work with 545W monocrystalline panels) and Cape Town's peak sun hours.

What Are Peak Sun Hours?

A "peak sun hour" is one hour of sunlight at 1,000W/m² intensity the standard intensity at which panel ratings are measured. Cape Town receives approximately 5–6 peak sun hours per day in summer, dropping to 3–4 in winter. For summer production estimates, we conservatively use 5 hours.

The Production Formula (Summer)

This means 6 × 545W panels can produce approximately 490 kWh per month in summer nearly a perfect match for a client spending R1,500/month on electricity in Cape Town.

Step 3: Accounting for Winter & System Losses

Summer calculations give us the ceiling of production. In winter, Cape Town's shorter days and increased cloud cover reduce output significantly. We plan for:

• Winter sun hours of 3–4 per day (vs 5 in summer)
• System losses of approximately 15–20% (inverter efficiency, cable losses, temperature derating)
• Panel degradation of ~0.5% per year over the system lifespan

This is why in Cape Town we often recommend slightly oversizing the panel array to ensure adequate winter production. A system that perfectly covers summer needs will typically cover 65–75% of needs in mid-winter.

Step 4: Battery Sizing for Backup Power

If load shedding protection is the goal, we also need to determine battery capacity. This is based on:

• Your average consumption during load shedding hours (typically evening)
• How many hours of backup you need (2hrs = Stage 2, 4hrs = Stage 4, 8hrs = Stage 6+)
• The battery's usable depth of discharge (DoD) typically 80–90% for LiFePO4 lithium

Why Cape Town Is Excellent for Solar

The Western Cape has some of the highest solar irradiance in South Africa. With 300+ sunny days per year and relatively mild temperatures (high heat actually reduces panel efficiency slightly), Cape Town provides near-ideal conditions for solar energy production.

The combination of high tariffs (R3.08/kWh), excellent solar resources, and the ongoing load shedding crisis makes the ROI calculation for solar in Cape Town extremely compelling.

How We Apply This to Your Property

Every property is different. Roof orientation, shading from trees or neighbouring buildings, available roof space, and structural load capacity all affect the final system design. The formula above gives us a starting point but we always conduct a physical site assessment before finalising any design.

Our process ensures that the system we install actually matches your energy needs not a generic template, not an oversell, not an undersell.