What Is Solar Irradiance? 10 Things (2024) You Must Know

In the past several years, the cost of solar panels has decreased significantly, but there is still one factor that you must consider: solar irradiance.

Solar irradiance is the measure of solar power at a location.

It quantifies the amount of solar energy that arrives in a particular area at a given moment.

Understanding the concept of solar irradiance can help you to recognize how much solar energy will be available to your solar installation over the course of the year.

Read on to learn more!

1. What is solar irradiance?

Solar irradiance is a measure of how much solar power you are getting at your location.

Depending on the time of year and the position of the sun during the day, the irradiance will vary.

When discussing solar irradiance, you may also hear the term “solar insolation.”

This is the measure of solar irradiance over a period of time (typically a single day).

2. What are the different types of solar irradiance?

There are several measure types of solar irradiance.

They include:

bulletTotal solar irradiance (TSI): This is the measure of the solar power over all wavelengths per unit area on the Earth’s upper atmosphere at the mean Earth-Sun distance (the astronomical unit, AU).

TSI is measured perpendicular to the incoming sunlight.

bulletDirect normal irradiance (DNI): Also called beam radiation, DNI is measured at the surface of the Earth at a given location.

It excludes diffuse solar radiation which is scattered or reflected by atmospheric components.

It is also measured perpendicular to the incoming sunlight.

bulletDiffuse horizontal irradiance (DHI): DHI is also called Diffuse Sky Radiation, and it’s the radiation at the Earth’s surface from light scattered by the Earth’s atmosphere.

It’s measured on a horizontal surface with radiation coming from all points in the sky excluding circumsolar radiation, which is radiation coming from the sun disk.

There would be almost no DHI in the absence of atmosphere.

bulletGlobal horizontal irradiance (GHI): GHI is the total irradiance from the sun on a horizontal surface on Earth.

It’s the sum of direct irradiance and diffuse horizontal irradiance.

Formula: GHI = DHI + DNI x cos(z)

bulletGlobal tilted irradiance (GHI): This is the total radiation received on a surface with a defined tilt and azimuth.

GTI can be measured or modeled from GHI, DNI, or DHI.

bulletGlobal normal irradiance (GNI): This is the total irradiance from the sun at the surface of the Earth at a given location with a surface element perpendicular to the Sun.

3. What unit is used to measure solar irradiance?

Solar irradiance is measured in watt per square meter (W/m2 = Wm−2).

An alternative unit of measure is the Langley per time.

4. How does solar irradiance factor into solar energy and solar panel systems?

If you’re considering getting solar panels, then you may wonder how solar irradiance plays a role.

This all links back to insolation.

Insolation is the measure of solar irradiance over a period (typically a day).

This is important to understand because not all of the solar energy that reaches Earth actually reaches its surface.

About 30 percent of the energy is actually reflected back into space.

Some factors that may help determine how much sunlight reaches a given area include sun angle, air mass, day length, cloud cover, and pollution levels.

How does this come into play with solar panels?

When you have values for insolation at certain positions on the earth, it helps you determine the size and output of solar power systems.

Thus, values for insolation can help to determine the expected output for your solar panels.

5. How do you calculate the annual solar energy output of a photovoltaic system?

To calculate the annual solar energy output of a photovoltaic system, use the following formula:

E = A * r * H * PR

E = Energy

A = Total solar panel area (m2)

r = Solar panel yield or efficiency (%)

This is the yield of the solar panel given by the ratio of electrical power (in kWp) of one solar panel divided by the area of one panel.

H = Annual average solar irradiation on tilted panels (shadings not included)

PR = Performance ration, coefficient for losses (range between 0.5 and 0.9, default value = 0.75)

This is a very important value to evaluate the quality of a photovoltaic installation because it quantifies the installation performance independently of the orientation and inclination of the panel.

It also includes all losses.

6. What’s the difference between solar radiation, solar irradiance and solar insolation?

Solar radiation is the radiant energy emitted from the sun.

Solar irradiance is the total solar radiation that reaches the Earth’s surface.

It’s important to remember that not all the solar energy that is emitted from the sun reaches Earth, and that not all the solar energy that reaches Earth actually reaches the surface.

As noted above, the factors that determine how much light reaches a given area are sun angle, air mass, day length, cloud coverage, and pollution levels.

Insolation is the average amount of solar radiation hitting an area over a set period of time.

Insolation is generally expressed in either kilowatt-hour per square meter (kWh/m2) per day or wats per square meter (W/m2).

The first represents the average amount of energy hitting an area each day while the second represents the average amount of power hitting an area over an entire year.

7. How do solar panels work?

Understanding how solar panels work can be instrumental to ensuring that you harness optimal energy when you install them at your home.

Solar radiation is light, which is also known as electromagnetic radiation.

This is emitted by the sun to every location on Earth.

However, some parts of the Earth receive more sunlight over the year, and the amount of solar radiation in different spots on Earth’s surface varies.

Solar technologies like solar panels have the ability to capture this radiation and turn it into a useful form of energy.

The primary types of solar energy technologies include photovoltaics (PV) and concentrating thermal power (CSP).

Photovoltaics are utilized in solar panels.

The sun shines onto the panel, and energy from the sunlight is absorbed by the PV cells in the panel.

This energy creates electrical charges that move in response to an internal electric field in the cell, which causes electricity to flow.

Solar energy helps us in a variety of ways.

It allows us to reduce the cost of electricity, contribute to a resilient electrical grid, spur economic growth, create jobs, generate backup power for nighttime and power outages, and operate at similar efficiency on both small and large scales.

Solar panel systems come in a variety of shapes and sizes.

If you’re looking to install one at your home, then you’re seeking a residential system, which is often found on a rooftop.

Today, businesses and utilities are also interested in installing solar panels to save money or provide customers connected to the grid.

If you’re a business with large buildings or acres of land, you may want to offset some of your costs by installing solar panels.

8. Why is it important to understand solar irradiance and solar insolation?

It’s essential to understand solar irradiance and solar insolation at certain positions on Earth because these figures allow you to determine the size and output of solar power systems.

Insolation values, in particular, can help to determine the expected output for the solar panels and to understand where on Earth solar panels will be most effective.

If you want to install solar panels on your home or at your property, doing this type of analysis will ensure that your investment is as effective as it can be.

9. What other key solar energy terms should you know?

In addition to solar irradiance and insolation, here are some other terms that you should know as you’re researching your solar project.

bulletSolar access

This term is used to quantify how much sunlight is available for a solar array at a particular site.

It may also be called Solar Access Percentage or Solar Access Value.

This metric expresses the available energy as a percentage of what would be available in ideal (i.e., shade-free) conditions.

Here is the formula for calculating solar access:

To find solar access, you divide the actual solar energy present given shading at the site by the amount of solar energy you would have if there was no shading at the site.

This metric allows you to understand how significantly shade is reducing the available sunlight.

Even a small amount of shade can drastically reduce the power output of a solar array.

Therefore, determining the extent of shading is essential.

This should be your first step in understanding whether your property (or an area of your property) is feasible for a solar panel system.

bulletTilt and Orientation Factor (TOF)

Tilt and orientation factor (TOF) is a metric that considers the slope and direction of a given surface.

Similar to solar access, TOF is expressed as a percentage.

It indicates actual conditions compared to optimal conditions.

To find TOF, you divide the solar energy available at the actual tilt and orientation at the surface by what would be available at the optimal tilt and orientation.

bulletTotal Solar Resource Fraction (TSRF)

TSRF is the measure of available solar energy, accounting for the two other metrics above (solar access and TOF).

You can find TSRF by multiplying the solar access of the site by the TOF percentage.

TSRF provides a complete picture of how much solar energy will be available for the solar panels to convert into electricity.

It considers both the percentage of available solar energy at a site given shading (solar access) and how much of that energy will reach the surface where solar panels are mounted given the tilt and orientation (TOF).

10. Does solar irradiance mean radiation?

If you’re considering installing solar panels at your home, you may be concerned about radiation.

Could solar panels be harmful to your health?

While solar panels don’t emit electromagnetic fields (EMFs), their systems do.

Most of the EMF radiation results from solar panel systems that come with smart meters installed.

The radiation from the smart meter comes from radiofrequency, which is one of the three types of EMF radiation (the other two types are magnetic field and electric magnetic field).

Radiofrequency is emitted from devices that communicate or receive communication wirelessly.

Common sources of this type of radiation include cell phones, Wi-Fi routers, cell towers, etc.

This is because radio waves are sending packets of data between two wireless devices, but they also emit a type of non-ionizing radiation.

You may be able to opt-out of having a solar panel smart meter.

They’re often installed by power companies without the homeowner’s knowledge or consent.

If you aren’t able to opt-out, then you can get a smart meter shield that will block radiation or use EMF protection paint to help to prevent radiation from coming into your home.

However, it’s important to note that, according to the American Cancer Society, smart meters are unlikely to cause cancer.

Final Thoughts

As the cost has significantly decreased in the past few years, solar panels are all the rage with homeowners looking to decrease their energy costs and add value back to their homes.

If you choose to go this route, solar irradiance is just one of many factors you’ll need to consider if you want to install solar panels.

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Erika Gokce Capital

Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.


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