What Is Solar Irradiance? Types, Measurement & More

The output of energy that is generated by the system after receiving at an area on the Earth is known as solar irradiance. Solar irradiance is measured as electromagnetic radiation in W/m� (Watts per meter squared).  

The energy released from the sun is the primary energy source for Earth; it affects everything from plant metabolism to climate change. Small changes in output can have dramatic effects on Earth. Consequently, it is an important variable to measure for studies in agriculture, climate, and power generation.  

The agricultural sector keeps track of received solar energy and historical data to better plan growing cycles. The role of climate modelers is to obtain precise outputs from their models that help make accurate predictions.  

The measurement of solar irradiance can be done on Earth or in space. On Earth, measurements are affected by atmospheric conditions; it depends on the position of the sun in the sky and the angle of the measuring surface. In space, primarily distance from the sun affects solar irradiance. There is a significant contribution to the sun�s solar cycle.  

To large  solar farms, solar irradiance is of particular interest. Solar irradiance data facilitates insights into PV panel performance by comparing the expected outputs with the actual ones. The solar insolation data can determine optimal sites so that the building of new solar farms and optimized panel orientation can occur.  

Solar irradiance on Earth is affected by various factors, including atmospheric changes, weather events, and local �obstacles� like mountains or trees. The usage of multiple sensors is done to measure all solar irradiance types. Here below, we have mentioned some of the types of solar irradiance.  

Diffuse Horizontal Radiation (DIF)  

Diffuse Horizontal Radiation is a type of solar irradiance in which the particles and molecules in the Earth�s atmosphere scatter the sun's radiation. DIF occurs when radiation interacts with tiny particles in the atmosphere. Shorter wavelengths are scattered, which is why the sky is blue.  

Mie scatter and Non-Selective scatter have a more significant effect on diffuse horizontal irradiance. Mie occurs when the radiation wavelength is similar to atmospheric particles, such as pollution, dust, and pollen. Non-Selective scatter occurs when the particles are larger than the wavelength of the radiation, spreading visible and infrared spectrums. Clouds are the primary contributor.  

Pyranometers are used to measure DIF. A shading ball or ring fitted to a pyranometer blocks the DNI leaving only diffuse. After tilting the pyranometer at an angle, it can measure tilted global irradiance.  

Global Tilted Irradiance (GTI)  

GTI is an approximate value for the energy yield calculation of fixed-installed tilted PV panels. It represents irradiation that falls on an inclined surface. A sloped surface also receives a small amount of ground-reflected radiation.  

Direct Normal Irradiance (DNI)  

DNI represents the quantity of radiation received per unit area on a surface perpendicular to the sun. A pyrheliometer is used to measure DNI, typically with a sun tracker like SOLYS2. The pyrheliometer does not measure diffuse radiation. DNI is essential for concentrated solar power stations.  

Global Horizontal Irradiance (GHI)  

GHI refers to the total radiation absorbed on a horizontal surface. It supports both Direct and Diffuse Horizontal Irradiance. A pyranometer positioned horizontally is used to measure GHI.  

Diffuse Horizontal Irradiance (DHI)  

DHI is the irradiance received by a flat surface scattered by the atmosphere. It does not include direct sunlight. The sun is blocked using a ball or disc to eliminate the beam component, and a tracker is used to shade only the sensor. A correction factor is applied based on location and time.  

What Is Solar Irradiance?  

Irradiance refers to the rate of solar power received per area [W/m�], while insolation refers to the total solar energy received over time.  

How To Measure Solar Irradiance  

To measure solar radiation accurately, ensure proper site selection, instrument mounting, and data logger setup. Quality control and regular maintenance are essential for reliability.  

What needs to be measured?  

‍ Solar radiation includes direct sunlight, diffuse sky radiation, and reflected ground radiation. GHI is the total radiation on a surface.  

Choose an instrument    

Choose tools like pyranometers, pyrheliometers, or albedo meters based on your measurement needs. Use ventilation units in dew-prone areas.  

Install your sensor    

Install instruments properly to avoid misalignment. Keep the sensor�s view unobstructed.  

 Datalogger    

Choose a high-resolution datalogger compatible with low voltage signals for your measurement instruments.  

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FAQ:

Q: What is 1000 W/m² solar radiation?

A: When people mention “1000 W/m²,” they’re referring to a standard measurement of solar irradiance—about 1 kW of sunlight hitting each square meter on a clear, sunny day at sea level with the sun directly overhead. This value serves as a baseline—often called “one sun”—for rating and testing solar panels.

Q: What is the difference between solar energy and solar irradiance?

A: Solar irradiance is an instantaneous measure—the power density (in W/m²) hitting a surface at a particular moment. Solar energy (or insolation), on the other hand, refers to the total amount of sunlight accumulated over time, often given in kWh/m².

Q: What is irradiance for PV?

A: For photovoltaics, irradiance refers to the sunlight power per unit area the panels receive—usually measured in W/m². These values (like Global Horizontal Irradiance or Plane-of-Array Irradiance) are crucial for predicting solar panel performance and system output.

Q: How to measure solar irradiance?

A: Start by selecting the right sensor (like a pyranometer or pyrheliometer), install it correctly (level and unobstructed), and connect it to a data logger. Regular calibration and upkeep ensure your readings stay accurate over time.

Q: What tool is used to measure solar irradiance?

A: The go-to tool is a pyranometer, which measures total (both direct + diffuse) sunlight on a flat surface. For direct-only readings, a pyrheliometer mounted on a sun-tracker is used.