GHI (Global Horizontal Irradiance)

Global Horizontal Irradiance (GHI) is the total amount of solar radiation received per unit area on a horizontal surface. It is one of the most fundamental solar resource metrics used in PV modeling, system design, shading analysis, and annual production forecasting.

GHI represents the combined effect of direct sunlight, diffuse sky radiation, and ground-reflected light, making it crucial for accurately estimating a project’s energy yield—whether residential, commercial, or utility-scale.

Solar designers rely on GHI data when creating energy models inside software platforms such as Solar Designing and shading tools like Shadow Analysis. It forms the basis for performance simulations like POA irradiance, specific yield, and loss modeling.

Key Takeaways

GHI (Global Horizontal Irradiance) is the total solar radiation a horizontal surface receives.
It is foundational for solar energy modeling and performance forecasting.
GHI includes direct, diffuse, and ground-reflected radiation.
Accurate GHI data leads to better POA modeling, shading loss calculations, and energy yield predictions.
Essential for all solar project types — residential, commercial, and utility-scale.

What Is GHI?

GHI is the measurement of overall solar power hitting a flat, horizontal surface at ground level.

It includes three components:

Direct Normal Irradiance (DNI) — sunlight coming straight from the sun
Diffuse Horizontal Irradiance (DHI) — scattered light from the sky dome
Ground-Reflected Irradiance — light bouncing off the ground

The formula is:

GHI = DNI × cos(θ) + DHI + Ground Reflected Irradiance

Where θ is the solar zenith angle (the angle between the sun and vertical).

Because it reflects real-world weather conditions, cloudiness, pollution, and atmospheric scattering, GHI is a primary determinant of energy production potential.

Related terms include POA Irradiance, Insolation, and Shading Analysis.

How GHI Works

1. The sun emits radiation toward the earth.

Some of it reaches the ground directly (DNI), while some is scattered by clouds, dust, and air molecules (DHI).

2. A horizontal surface receives both direct and diffuse components.

Because the surface is flat, the angle of the sun affects how much direct radiation reaches it.

3. Atmospheric conditions modify GHI.

Factors such as humidity, aerosols, pollution, clouds, and altitude significantly alter GHI values.

4. Solar modeling tools incorporate GHI data.

Platforms use GHI to compute POA irradiance, shading loss, energy yield, PR, and kWh-output forecasts.

To analyze angles, designers often use the Sun Angle Calculator.

Types / Variants Related to GHI

1. DNI (Direct Normal Irradiance)

Measured on a surface pointed directly at the sun.

2. DHI (Diffuse Horizontal Irradiance)

Scattered sky radiation.

3. POA Irradiance (Plane of Array)

Irradiance measured on the tilted surface of the PV modules.

4. Global Tilted Irradiance (GTI)

GHI adjusted for module tilt and orientation.

5. Albedo-Adjusted Irradiance

Includes ground-reflected radiation (important for bifacial systems).

How GHI Is Measured

GHI is typically measured using:

Pyranometers

Precision instruments that detect total solar irradiance.

Weather Station Data

Meteorological stations record GHI continuously.

Satellite Models

Large-scale datasets (e.g., for planning and long-term forecasts).

Typical Meteorological Year (TMY) Files

Used for simulation inputs in PV design tools.

GHI is expressed in:

W/m² (instantaneous)

kWh/m²/day (daily energy)

kWh/m²/year (annual resource potential)

Typical Values / Ranges

Daily values commonly range from 2–7 kWh/m²/day depending on season and cloud cover.

Practical Guidance for Solar Designers & Installers

1. Always use site-specific GHI data

Accurate resource data directly impacts yield predictions.

2. Combine GHI with shading analysis

Use Shadow Analysis to identify real-world loss factors.

3. Convert GHI into POA irradiance for your tilt

Use the Sun Angle Calculator to optimize tilt and azimuth.

4. Use reliable datasets (TMY, satellite, weather station)

Consistency ensures accurate long-term predictions.

5. Adjust for bifacial gain when applicable

GHI + ground albedo conditions affect rear-side production.

6. Use GHI early in project planning

Sizing tools like the Solar Panel Sizer depend on accurate irradiance inputs.

Real-World Examples

1. Residential Rooftop in India

A designer uses local GHI values of 5.2 kWh/m²/day, resulting in strong annual performance for a 6 kW PV system on a south-facing roof.

2. Commercial Building in the U.S. Southwest

GHI of 2,000 kWh/m²/year supports a large 500 kW installation with excellent yield, minimal shading, and high POA irradiance.

3. Utility Solar Plant in Spain

With GHI above 1,900 kWh/m²/year, a utility-scale developer uses this data to model a 50 MW single-axis tracking array with optimized east–west orientation.