What Are Solar Radiation and Solar Flux? Explore the Role of Solar Radiation in PV Systems

Solar radiation is the energy that the sun sends out into space. It travels in the form of electromagnetic waves, such as ultraviolet (UV) light, visible light, and infrared radiation. This energy is what we simply call sunlight or solar energy.

Solar radiation is measured in watts or joules, and it reaches the Earth in three ways:

• Direct solar radiation: It comes straight from the sun
• Diffuse solar radiation: It’s scattered by clouds or air particles
• Reflected solar radiation: It bounces off surfaces like the ground, buildings, or water

Together, these three types of solar radiation make up the total sunlight received at any location on Earth.

One of the most useful applications of solar radiation is in solar photovoltaic (PV) systems that convert sunlight into solar electricity. The most common PV systems in India are on-grid solar systems, off-grid solar systems, and hybrid solar systems. These systems use solar panels that are made of photovoltaic cells that capture photons in sunlight and convert them to electricity. 

While often used interchangeably, solar radiation and solar flux differ. Solar radiation is the total energy that the sun releases, and solar flux is the total solar radiation that falls on per unit area. Solar flux and solar irradiance, on the other hand, are the same thing. While solar installers usually call it solar irradiance, physicists prefer using the term solar flux. They’re both measured in watts per square meter (W/m²).

In this blog, we’ll walk you through the meaning of solar radiation, its types, how it differs from solar flux, why it is important for Earth, and its applications. We will also walk you through the reasons why on-grid rooftop solar panel systems for homes are most popular, how they exactly convert solar radiation to electricity, and their cost and ROI.

TL;DR Summary Box: What is Solar Radiation and its Role in PV?

Solar radiation refers to the electromagnetic waves the sun releases in the form of visible light, UV light, and infrared light. In layman’s terms, solar radiation is known as sunlight or solar renewable energy. Photovoltaic technology uses PV cells, mostly made of silicon, to capture the visible light from solar radiation and convert it to electricity, which can be used to power homes, housing societies, and commercial or industrial buildings.

While solar thermal systems capture heat (infrared rays) from solar energy, it’s the PV technology that’s stable enough, efficient enough, and affordable enough to be deployed on a mass scale. It can tackle the problem of rising pollution and climate change since PV solar panels use solar energy, not polluting fossil fuels, to produce power.  

Here are the main topics we’ve discussed in this blog in detail:

What is Meant by Solar Radiation?

Solar radiation is the energy the Sun emits in the form of light and heat. It travels through space and reaches Earth from almost 93 million miles away. This energy comes in different forms, mostly ultraviolet radiation (UV), visible light spectrum, and infrared radiation.

When solar radiation reaches the outer edge of Earth’s atmosphere, it measures about 1,361 watts per square meter (W/m²). This is called the solar constant. However, not all of this energy reaches the Earth’s surface. Some of it gets absorbed, scattered, or reflected by clouds, dust, and gases in the atmosphere. That’s why the actual amount of sunlight on the ground is often much less.

How is Solar Radiation Measured?

The main tool used to measure solar radiation is a pyranometer. It measures the total sunlight (direct + diffuse) falling on a flat surface.

The amount of solar energy falling on a surface is measured in watts per square meter (W/m²), which simply means how much solar power is hitting each square meter of land. On a clear sunny day, this can go up to 1,000 W/m². On a cloudy day, it can drop to 200-300 W/m², or even lower.

Here are some other instruments that scientists sometimes use besides a pyranometer to measure solar radiation:

• Pyrheliometer: It tracks the sun and measures only direct sunlight, known as Direct Normal Irradiance or DNI.
• Pyranometer with a shadow band: It blocks direct sunlight to help measure just the scattered or diffuse sunlight. 
• Solar irradiance meters: These are portable instruments used for field measurements and commissioning.
• Data loggers: These record solar radiation readings continuously for long-term monitoring.

Solar Radiation vs Solar Flux: What’s the Difference?

Solar radiation is the total electromagnetic energy the Sun releases. Solar flux, on the other hand, is the amount of that energy that actually falls on a specific area. In even simpler words, solar radiation is the total heat and light the Sun produces, and solar flux is the amount of heat and light that actually lands on the Earth’s surface. 

Here’s a tabulated snapshot of the main differences between the two:

What Are the Three Types of Solar Radiation?

When sunlight reaches Earth, it doesn’t always come straight from the Sun. It takes three main paths: some of it comes directly, some gets scattered in the sky, and some bounces off surfaces. Based on this, solar radiation is usually explained in three types: direct, diffuse, and reflected. Together, they make up the sunlight that hits the Earth’s surface every day.

Here’s a tabulated snapshot of how these three types of solar radiation differ:

Now, let’s check out all three types in detail.

#1. Direct Normal Irradiance or DNI

Direct Normal Irradiance, or DNI, is solar radiation that comes straight from the Sun to the Earth’s surface without being scattered. It is the strongest form of solar radiation and is most predictable under clear skies.

DNI can range from 0 W/m² on a cloudy day to over 1,000 W/m² on a clear sunny day.

Here are the main characteristics of DNI:

• Highest energy density: It provides maximum power per unit area. Concentrated solar plants and tracking solar panels depend heavily on DNI.
• Weather dependent: It gets dramatically reduced by clouds, fog, or pollution.
• Geographic variation: It’s highest in deserts and dry regions with clear skies.
• Time sensitivity: It peaks around noon when the Sun is highest.

# 2. Diffuse Horizontal Irradiance or DHI

Diffuse Horizontal Irradiance or DHI is solar radiation that gets scattered by molecules, dust, and clouds before it reaches the ground. This is why even on a cloudy day, the sky still looks bright.

Here are the key characteristics of DHI:

• Atmospheric scattering: It’s caused when air molecules and dust particles bend/scatter sunlight.
• Stable energy source: It’s significantly more consistent throughout the day compared to direct radiation.
• Cloud role: On overcast days, DHI makes up almost 100% of the sunlight that falls on Earth. 

#3. Reflected or Albedo Radiation

Reflected solar radiation, also known as Albedo radiation, is sunlight that bounces off surfaces like the ground, water, buildings, or snow, and then reaches solar panels.

Here are the main characteristics of reflected radiation:

• Surface dependency: It varies based on whether or not there are reflective surrounding materials like snow, sand, water, or concrete.
• Seasonal variation: Snow cover can double the reflected radiation values.
• Bifacial advantage: Bifacial solar panels capture both direct and reflected radiation.
• Urban effects: Concrete and building surfaces also increase the albedo contribution.
• Installation optimization: The solar panel’s tilt angle and height can also affect the albedo capture. It’s extremely useful to maximize the electricity generation from solar systems.

When you add up direct (DNI), diffuse (DHI), and reflected (albedo) solar radiation, you get Global Horizontal Irradiance (GHI). GHI is the total sunlight available on a flat surface. This is the most important value for designing solar PV systems.

How Does Solar Radiation Affect Earth?

Solar radiation drives almost every natural process on Earth. Plants use it to grow, and animals (including humans) depend on it for survival. Without sunlight, Earth would simply not be habitable.

Here are some of the key ways sunlight affects our planet every single day:

• Temperature regulation: Sunlight controls global and regional temperatures. The uneven heating of land, oceans, and air creates warm and cool zones across Earth.
• Atmospheric circulation: It powers winds, jet streams, and pressure systems that move air around the planet.
• Water cycle activation: Sunlight provides the energy for evaporation, cloud formation, and rainfall.
• Seasons and weather: Different parts of the planet get different amounts of sunlight because of the Earth’s tilt. That’s the reason we have different seasons on Earth.
• Photosynthesis: Plants use solar energy to make food. Needless to mention, plants are the base of every food chain.
• Oxygen production: Plants and trees absorb carbon dioxide and release oxygen, which keeps the air on Earth breathable. 
• Vitamin D synthesis: Humans can’t synthesize vitamin D on their own. It’s the UV-B radiation that lets the human body produce vitamin D. 
• Skin health effects: Sunlight has both beneficial and harmful effects on the skin, depending on exposure levels.

Why is Solar Radiation Important?

Besides the fact that solar radiation is important for all kinds of life on Earth to exist, it’s also the most abundant, sustainable, and non-polluting source of energy with the potential to meet all human energy needs while reducing greenhouse gas emissions. 

Let’s understand what makes it so important: 

• Renewable energy potential: Sunlight is an unlimited and clean energy source for producing electricity.
• Climate stability: It maintains Earth’s temperature within the range that’s habitable and allows all life forms to exist.
• Agricultural and food security: Without solar radiation, crops and plants would not grow, and food chains would collapse.
• Economic opportunities: Solar and renewable energy industries are creating millions of jobs worldwide.
• Energy security and self-reliance: Solar radiation provides every country a chance to become self-reliant and produce electricity without the need for expensive imported oil and fossil fuels. 

What Are the Applications of Solar Radiation?

The most significant applications of solar radiation include the production of electricity through on-grid solar systems that reduce electricity bills by at least 90% in urban areas and heating systems for homes and industries in rural areas. Off-grid solar is another important application as it uses sunlight to light up homes in villages where the grid supply hasn’t even reached yet.

Let’s check out all the uses of solar radiation in detail:

• Rooftop solar systems: Homeowners and housing societies can install solar panel systems on their roofs to generate their own electricity. This reduces monthly bills and carbon emissions significantly. Even commercial rooftop systems are great applications as they use sunlight to meet all energy needs at offices and industries. 
• Solar water heaters: Solar thermal systems heat water for homes, hotels, and swimming pools by using sunlight to warm water flowing through special collectors. 
• Large solar farms: These are massive installations with thousands of solar panels that are used to generate electricity for entire cities and regions. 
• Solar heating for buildings: Passive solar design in BIPV uses building orientation and window placement to heat buildings without any mechanical systems.
• Industrial process heat: Factories use concentrated solar systems to generate high-temperature heat for manufacturing processes, steam production, and other industrial applications.
• Off-grid solar systems: Off-grid standalone PV systems are used to produce electricity in rural and remote areas with no grid supply. 
• Street lighting: Solar-powered LED streetlights, traffic signs, and bus stops can operate independently without connection to the electrical grid. These are perfect solutions for remote village areas.
• Portable solar battery chargers: You can carry these chargers to remote locations while hiking or camping to charge devices like smartphones and flashlights. Even smaller wattage solar panels like 10-watt solar panels or 20-watt solar panels can be used for charging small devices in remote locations.

What Device is Used to Capture Solar Energy?

Solar thermal collectors and photovoltaic solar cells are the primary two devices used to capture solar energy. Solar thermal collectors capture thermal energy (heat) from solar radiation to heat fluids for domestic hot water, space heating, and industrial processes. The PV cells, on the other hand, use sunlight to produce electricity to light up homes, housing societies, commercial buildings, and industries. 

Installing rooftop on-grid solar systems that use the photovoltaic effect is the most beneficial technology that generates literally free electricity. Once you’ve paid the initial investment cost to set up the solar system, that system uses nothing but sunlight to continue delivering free power to your home for 25 years of its life. 

How Do PV Systems Convert Solar Radiation to Electricity?  

Solar panels in a solar system capture photons present in the sunlight to generate DC (direct current). This DC then goes to solar inverters that convert it to AC (alternating current) that’s used to power appliances in homes.

Here’s a step-by-step breakdown of the entire process:

• Step 1 – Solar cells in solar panels capture photons: Solar cells, made of silicon, capture energy packets, known as photons, from sunlight when it strikes the surface of solar panels.
• Step 2 – Photons excite electrons present in the solar cells: Electrons absorb energy from photons and leave their places. The place they leave behind is the hole, which is positively charged.
• Step 3 – Built-in electric field in the solar cell keeps electrons and holes separated: The electric field pushes holes and electrons in the opposite direction, creating usable voltage.
• Step 4 – Electrons flow in one direction through an external circuit: This unidirectional flow of electrons is direct current.
• Step 5 – The DC power flows into the solar inverter: The inverter converts DC power to AC power which is used to keep home appliances running.
• Step 6 – Extra electricity is transferred to the grid in on-grid rooftop solar systems: Any excess energy generated by the system which is not used is transferred to the grid through the bi-directional net meter. This meter imports units at night to keep the load running. It records the unit exchange carefully. 

Why is the Rooftop On-Grid Solar PV System the Most Popular?

On-grid rooftop solar systems are most popular because they’re the cheapest of the three types. They are connected to the grid and do not use a lithium battery bank, which the other two types use to store solar energy. Lithium batteries are expensive to install, maintain, and replace. 

So, if you live in an urban area with a reliable grid connection, you don’t need off-grid or hybrid solar systems. In fact, on-grid solar systems have clear advantages over off-grid solar systems as the former earns homeowners and housing societies a subsidy under the PM Surya Ghar Muft Bijli Yojana, reducing the final solar system installation cost. 

Cost of Installing On-Grid Solar Systems vs the ROI in 25 Years of the System’s Life

Let’s give you a head-on comparison between the estimated cost of installing a rooftop on-grid solar system in Nagpur and the money this system will save in 25 years of its life:

*Please note: The above-mentioned solar plate price is indicative as of 25th August 2025 for the SolarSquare Blue 6ft variant. The final cost of installing an on-grid rooftop solar panel system at home depends on your DISCOM charges, product variant opted for, panel type, inverter type, mounting structure height, type of after-sales service, savings guarantee, roof height, etc. Prices are subject to change. Additionally, while calculating savings, we have considered the annual tariff escalation at 3% and the annual degradation at 1%. The actual final savings from solar panel installation depend on the types of solar panels you’ve installed and their efficiency, intensity of sunlight your rooftop receives, orientation of the panels and tilt angle, the pollution level and weather conditions in your city, the temperature, shadow on the roof, impact of dirt/dust, and how well you maintain your panels after installation.

Want to check out how much money an on-grid rooftop solar system can save in your city? Use SolarSquare’s free solar rooftop calculator to calculate your savings now.

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Conclusion

Solar radiation makes life on Earth possible. Besides supporting life, it’s also a great source of energy that’s inexhaustible and can be used to produce electricity without polluting the environment. The most credible use of solar radiation is using PV solar systems to produce electricity, which can power residences and industries. The savings on bills are phenomenal, and solar systems also reduce the carbon footprint. 

If you’re considering installing a solar system too, book a free solar consultation call with SolarSquare today.

FAQs

Q1. What is the SI unit of solar radiation?

Ans. The SI unit of solar radiation is watts per square meter (W/m²). It tells how much sunlight is falling on each square meter of a surface.

Q2. Which device is used to measure solar radiation?

Ans. Solar radiation is usually measured with a pyranometer, which records how much sunlight hits a surface. However, a pyrheliometer is used to measure direct sunlight.

Q3. What is the latest solar technology?

Ans. One of the latest solar technologies is perovskite solar cells. They are cheaper to make and more efficient than traditional panels. However, they’re still not ready for commercial use because their lifespan is only 1-2.5 years. Bifacial panels are the most widely used solar PV technology as of now. These are up to 22.5% efficient as they can capture sunlight from both front and rear ends.