he more sun solar panels receive, the more electricity they produce, it’s as simple as that. So naturally, every solar owner wants their panels to be installed in a way that would allow them to collect maximum sunlight. But as they say, you can't manage what you can't measure, so determining the amount of sunshine is the first step in planning absolutely any photovoltaic system. Read ahead for a full explainer on what solar radiation is and how it is calculated.
The nature of solar radiation
Before we jump into calculating solar intensity, let’s quickly go over some basics about the Sun’s energy. Like any other star, the Sun is a gigantic gas ball (about 100 times bigger than Earth) composed chiefly of hydrogen and helium. It produces energy during a process called nuclear fusion, which occurs when two or more nuclei of hydrogen atoms collide in the sun’s core to form a helium atom.
Nuclear fusion releases enormous amounts of energy in the form of electromagnetic waves and particles (photons) constantly flowing out from the Sun. This solar radiation travels through space and arrives at the top of the Earth's atmosphere, but not all of it eventually reaches the Earth's surface – around 30% is reflected back to space.
Solar radiation and solar insolation: what is the difference?
Now let’s discuss a few terms and acronyms to avoid confusion. The first and easiest one is solar radiation – the electromagnetic radiant energy emitted by the Sun, which we discussed in the previous section.
The second term you might have come across is solar irradiance (sometimes called solar irradiation), which is the amount of radiation reaching an area or object. It comes in three types:
- Direct Normal Irradiation (DNI) is the amount of solar radiation received per unit area by a surface perpendicular to the sun's rays. In other words, it is the amount of sunlight that reaches the Earth directly, without being diffused through clouds or haze.
- Diffuse Horizontal Irradiation (DHI) represents solar radiation that doesn’t come in a direct path from the Sun but has been scattered by clouds and arrives evenly from all directions. Basically, DHI is all the irradiation that isn’t direct.
- Global Horizontal Irradiation (GHI) is defined as the total radiation (direct and diffuse) by a surface horizontal to the ground.
- Global Tilted Irradiation (GTI) is solar radiation that falls on a tilted surface. This value is usually calculated for photovoltaic applications since it is essential to estimate the output of solar panels.
Finally, we come to the third important term, which is solar insolation – the amount of radiation cumulated on a specific surface area over some time interval. In the solar industry, it is expressed in kilowatts per square meter (kW/ m²), which is also referred to as sun-hour.
Ways to calculate the solar irradiation
Manual calculations of solar radiation
The density of radiation can be established through a range of mathematical calculations. First, one must determine the position of the Sun in the sky (altitude angle and the azimuth angle). Based on the Sun's position throughout the year, the amount of irradiance and insolation on a tilted surface can be expressed as a function of latitude and day of the year. However, this isn’t enough when it comes to photovoltaic system design, as the equations mentioned above do not take into account local weather conditions.
Solar professionals use special tools that help calculate irradiation in the context of photovoltaic installations. One of them is a pyrheliometer - a device used to quantify direct radiation from the sun, otherwise known as direct beam irradiation. It has a thermopile detector and temperature sensors. When the sun’s rays hit the detector, it takes solar thermal energy and turns it into electric signals. Pyrheliometers can show how efficient your PV modules will be in a particular location.
These devices are commonly used in solar tracking systems that allow ground-mounted panels to follow the sun's path as it moves across the sky throughout the day.
Pyranometer is another device used to measure solar irradiation. But whereas pyrheliometers measure DNI, pyranometers measure DHI and GTI.
The devices come in both digital and analog varieties and show visible and non-visible parts of the solar spectrum. Pyranometers are ideally suited for determining irradiation of a specific location and, therefore, are commonly used by PV panel installers.
Pyrheliometers and pyranometers can be used separately depending on which values one needs to calculate. Nevertheless, they can also be used in combination to get a comprehensive picture of how much sun exposure a given location receives.
Sunshine recorders are simpler and older tools used to measure solar irradiation. Also known as heliographs, these instruments measure the amount of sunshine a particular area gets throughout the day. The sun’s rays pass through a glass sphere which concentrates them onto a recording paper card. The rays burn a trace on it as long as the direct irradiation lasts, and the length of the trail shows the sunshine duration.
Today, heliographs are rarely used for photovoltaic applications because their measurement accuracy is significantly inferior to modern alternatives. Besides, sunshine recorders help calculate only the visible light, that is, not the total amount of irradiance a PV panel would receive. Solar cells are indeed manufactured of materials that capture primarily visible spectrum, but they can also absorb a certain portion of light in the infrared and ultraviolet ranges.
Calculating the amount of solar irradiation your site receives
So suppose you want to figure out how much sunlight PV panels can potentially receive on your site. If you’re really good with math and science, you can calculate it manually with a rather complicated set of equations.
In case you are not in the mood for complex calculations, you can use one of the devices listed in the previous section. But be prepared for the fact that they are rather pricey: a high-quality pyranometer, for example, will cost you from $2,000 to $4,000. Generally, buying special devices is unnecessary since your installer will have all the tools to measure solar irradiation accurately.
But still, you will have to roughly determine the amount of sun in your location to find out whether it is suitable for solar in the first place. There are plenty of online sunlight calculators that can help you with that, all you have to do is enter your zip code. You can also use more complex resources such as Global Solar Atlas, which summarizes solar power potential globally.