Solar Irradiance Calculation Guide

“Photovoltaic” is a blend of two words: “photo” (Greek for light) and “voltaic” (related to voltage). A photovoltaic (PV) system works by converting light into electricity using semiconductor junctions—and it all starts with sunlight.

The performance of a PV system is directly tied to how much sunlight it receives. This is measured by solar irradiance—the amount of solar power received per unit area.

When designing a solar plant, irradiance is typically the first parameter evaluated. Platforms like ARKA 360’s Design Studio automate this process, generating accurate irradiance maps remotely—saving time and reducing errors.

In this article, we’ll explore the different types of solar irradiance and how to calculate them for your site.

What Is Solar Irradiance?

The sun emits energy in the form of heat and light. Solar photovoltaics focus on the light component, which includes a wide range of electromagnetic radiation: visible light, ultraviolet (UV), infrared (IR), radio waves, X-rays, and more.

Together, these make up solar radiation. The intensity of this radiation at a specific location is known as solar irradiance, measured in watts per square meter (W/m²).

💡 Earth receives an average of 1,400 W/m² (1.4 kW/m²) at the outer atmosphere. However, actual irradiance at ground level varies depending on location, time of day, season, terrain, and weather.

Types of Solar Irradiance

Understanding the three main components of irradiance is key to accurate solar design:

☀️ Total Solar Irradiance (TSI)

The total solar power received per unit area at the Earth’s outer atmosphere, measured across all wavelengths.

🔆 Direct Normal Irradiance (DNI)

The portion of sunlight received in a straight line from the sun. It excludes light scattered by the atmosphere and is measured on a surface perpendicular to the sun’s rays.

🌥️ Diffuse Horizontal Irradiance (DHI)

Sunlight that has been scattered by molecules and particles in the atmosphere and reaches the surface from all directions.

📈 Global Horizontal Irradiance (GHI)

The total solar radiation received on a horizontal surface:
GHI = DHI + DNI × cos(θ)
Where θ = solar zenith angle

Calculating Irradiance at Your Site

Let’s take an example:

🗺️ Location: San Francisco, CA

• Coordinates: 37.7749° N, 122.4194° W
• Time Zone: UTC -8

🕒 Solar Time vs Clock Time

Solar time is based on the sun’s position in the sky—not the time on your clock. On June 15 at 1:00 PM local time, solar noon occurs at 1:09 PM. Therefore, the solar time is about 11:50 AM.

📐 Solar Angles on June 15

• Solar declination: 23.31°
• Solar altitude: 75.38°
• Solar azimuth: 9.06° East
• Hour angle: 2.48°
• Air mass ratio: 1.033

Total Insolation

Solar panels must be positioned correctly to maximize insolation (the solar energy received per square meter). This involves optimizing:

• Azimuth angle (orientation relative to true south)
• Tilt angle (panel inclination from the horizontal)

❗ A misaligned azimuth can reduce system output by up to 35%.

🔍 For San Francisco (June 15):

• Optimal azimuth: 13.21°
• Fixed tilt angle: 38°
• Beam insolation: 807.35 W/m²
• Diffuse radiation: 103.28 W/m²
• Reflected radiation: 20.50 W/m²
• Total insolation: 931.13 W/m²

Sun Path and Hours of Sunlight

On June 15, San Francisco receives approximately 14.77 hours of sunlight. By plotting the solar azimuth, altitude, and irradiance over time, you can map the sun’s path for the day—crucial for panel positioning and shading analysis.

The Impact of Shading on Irradiance

Shade is the enemy of solar power. Even minor shading can drastically reduce energy output.

⚠️ As per Prof. Gil Masters, shading a single cell in a 36-cell panel can reduce output by over 75%.

ARKA 360’s Design Studio uses GPU acceleration to simulate over 5,000 sun positions for a single site—generating detailed irradiance heatmaps in seconds. This enables precise design decisions before even stepping on-site.

🎁 Bonus Benefit with ARKA 360

By simulating irradiance and shading remotely, ARKA 360 helps reduce customer acquisition costs by up to 55%, eliminating the need for manual site visits during pre-sales.

Final Thoughts

Solar irradiance is the foundation of every good solar design. Understanding it—not just as a number, but as a function of time, geography, and environmental conditions—allows you to create systems that are accurate, efficient, and optimized for performance.

Whether you're designing rooftop systems or large-scale solar farms, tools like ARKA 360 simplify irradiance analysis and help you make smarter, faster decisions.

📞 Ready to see it in action?

Book a FREE Demo with ARKA 360 →

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