5 Ways to Read Irradiation in PVSyst｜Organizing Solar Radiation-related Information

Table of Contents

• PVSyst's Irradiation is the starting point for power generation

• How to read 1

• How to read 2

• How to read 3

• How to read 4

• How to read 5

• Terms commonly confused when reading Irradiation

• What to check when the irradiation looks low

• How to read monthly irradiation trends

• Useful views for explaining PVSyst results

• Perspectives to connect site verification with irradiation conditions

• Summary

The Irradiation in PVSyst is the starting point for energy production.

When reading PVSyst results, one of the first items you should check is Irradiation. Irradiation is an important indicator that shows the solar radiation energy entering a photovoltaic plant. PVSyst's energy production is not determined solely by module capacity or PCS capacity. How much solar irradiance is available at the site in the first place, the angle at which that irradiance reaches the array surface, and how much is deducted as losses can greatly change the annual and monthly energy production.

In solar power simulations, solar irradiation conditions are the starting point for all calculations. No matter how high-performance the modules are, if the input irradiation is low the energy yield will not increase. Conversely, with the same system capacity, regions with higher irradiation or designs with appropriate tilt and azimuth angles will achieve higher annual energy production. Therefore, when reviewing PVSyst results it is important to read the Irradiation as the prior stage leading to the final energy output, not just look at the final generation figure.

In PVSyst reports, the word "Irradiation" does not appear alone; instead, multiple solar radiation–related items appear, such as Global horizontal irradiation, Global incident in collector plane, Effective global irradiation, and Diffuse irradiation. If you read these without distinguishing between them, you can misinterpret why energy production is high, why losses are large, or where the conditions are changing.

This article organizes five perspectives to keep in mind when reading PVSyst's Irradiation, presented in a form that is practical for real-world use. Rather than simply translating the English items, it explains how they should be used when checking PVSyst results, conducting design reviews, giving internal briefings, and explaining to customers.

How to read 1

When viewing Irradiation in PVSyst, the first thing you should check is the horizontal plane irradiation. Horizontal plane irradiation represents the solar energy incident on a surface that is horizontal to the ground. In PVSyst it is often indicated by a label such as Global horizontal irradiation and is treated as a basic parameter of the meteorological data.

Horizontal irradiance is an input value that represents the local meteorological conditions at a site. When assessing the solar potential of different regions, this value serves as the benchmark. For example, Hokkaido, Tohoku, Kanto, Kyushu, and Okinawa differ in their annual irradiance and monthly trends. When comparing design conditions for a power plant, first checking the horizontal irradiance lets you determine whether a site is generally well endowed with sunlight, tends to drop in winter, or is strong in summer.

However, the irradiance on a horizontal plane is not the same as the irradiance that reaches a solar panel. Solar panels are typically installed with a specific tilt and azimuth. In other words, the irradiance on a horizontal surface differs from the irradiance on the actual array surface. If you assess the reasonableness of the energy production in PVSyst by looking only at horizontal-plane irradiance, you will overlook the effects of the design tilt and azimuth.

When reading horizontal irradiance, it is more practical to regard it not as a value that directly explains power output itself but as a value for verifying the assumptions of the meteorological data. Horizontal irradiance changes depending on whether the meteorological data used are Meteonorm, SolarGIS, or on-site observational data. When comparing multiple simulation results, even if the system conditions are the same, different meteorological data will result in different power output. Use horizontal irradiance as an entry point to explain those differences.

When explaining to customers or colleagues, it’s easier to convey by describing horizontal irradiance as the basic solar resource that falls on the site — the meteorological assumption made before beginning power plant design. It may appear to be directly linked to the final energy output, but in reality generation is calculated after subsequent factors are applied, such as conversion to the tilted surface, shading, reflection, IAM, soiling, temperature, and other conditions.

How to Read 2

The next important parameter is the array-plane irradiance. This represents the irradiance incident on the receiving surface of the solar panels. In PVSyst it may be listed as an item such as Global incident in collector plane. While horizontal-plane irradiance is the solar irradiance on the ground, array-plane irradiance is the irradiance actually incident on the panel surface.

In photovoltaic power generation, the irradiance on the array surface is extremely important, because it is close to the solar energy received by the modules. When considering power generation, array-surface irradiance is more directly meaningful than horizontal-plane irradiance. The irradiance converted from the horizontal plane to the array surface varies with tilt angle, azimuth, the solar altitude at the installation site, and seasonal variations.

For example, an array facing south with an appropriate tilt angle can have higher irradiance on its surface than on a horizontal plane. Especially in winter, when the solar altitude is lower, panels with a fixed tilt can receive sunlight more readily than a horizontal plane. On the other hand, with east–west orientation, low tilt, or complex terrain conditions, the increase in irradiance on the array surface may be limited.

When reviewing PVSyst results, check how much the irradiance changes when converting from horizontal-plane irradiance to plane-of-array irradiance. This difference helps assess the validity of the design conditions. If the tilt and azimuth angles are appropriate, the plane-of-array irradiance will show a consistent benefit throughout the year. Conversely, if the plane-of-array irradiance is not as high as expected, you should check the azimuth, tilt, terrain, shading, or meteorological data conditions.

One thing to note here is that high plane-of-array irradiance does not necessarily lead to higher energy production. Plane-of-array irradiance is a value close to the input for energy production, but after that it is subject to reflection losses, IAM losses, shading losses, temperature losses, mismatch, wiring losses, PCS losses, and so on. Therefore, if plane-of-array irradiance is high but energy production does not increase, you need to examine loss items other than irradiance.

In a design review, first look at the horizontal-plane irradiance to confirm the meteorological assumptions, then look at the plane-of-array irradiance to check how effectively the design conditions receive solar irradiance. Reading the results in this order makes PVSyst's results considerably easier to organize.

Reading 3

When reading the Irradiation in PVSyst, another important item is Effective irradiation. This is a concept that approximates the portion of solar radiation incident on the array plane that is actually used effectively in power generation calculations. In PVSyst it may be presented in contexts such as "Effective global irradiation after optical losses."

Array-plane irradiance represents the solar irradiance incident on the surface of a solar panel. However, not all of that irradiance effectively reaches the cells. At large angles of incidence, reflection at the glass surface increases. If there are nearby obstacles or mutual shading between arrays, some areas will not receive light. Soiling, snow cover, terrain shading, and the optical properties of the module surface also have an impact. The irradiance after accounting for these losses is closer to the value understood as Effective irradiation.

Reading this item reveals not simply whether solar irradiation is high or low, but how effectively the received solar irradiation is being used. For example, if Global incident in collector plane is high but Effective irradiation drops significantly, strong optical losses or shading effects may be present. Conversely, if this difference is small, the irradiation incident on the array plane can be considered to be used relatively efficiently.

Confirming Effective irradiation is especially important for projects with complex terrain, in mountainous areas, for sites with trees or buildings nearby, and for projects with narrow racking spacing. Looking only at annual energy production makes it difficult to determine which losses are affecting performance, but by comparing the array-plane irradiance and Effective irradiation, you can understand how much solar irradiance is being reduced before it contributes to power generation.

When explaining a PVSyst report to a client, Effective irradiation is also useful. For example, if the energy output appears lower than expected, simply explaining the system capacity or PCS may not be convincing. In that case, explaining the sequence of incoming solar irradiation, conversion to the array plane, and irradiation losses due to shading and reflection lets you present a logical explanation of why the generation has decreased.

Effective irradiation is also important when reading PVSyst’s loss diagram. In the Loss Diagram, solar irradiance is converted from the horizontal plane to the array plane, then subjected to losses such as IAM, shading, and soiling, before proceeding to the module output. Understanding this flow makes it easier to track at which stage in the loss diagram the power generation is being reduced.

How to Read 4

When reading "Irradiation", it is necessary to grasp the differences between direct, diffuse, and reflected solar radiation. The solar irradiance that reaches a solar panel is not only the light coming directly from the sun. It also includes light scattered in the atmosphere and light reflected from the ground and surrounding objects. In PVSyst, these components are taken into account in the calculations and are reflected in the plane-of-array irradiance and the effective irradiance.

Direct solar radiation is the radiation that reaches panels directly from the sun. It is strong on clear days and is highly influenced by the sun’s position, azimuth, and tilt angle. If the tilt and azimuth angles are properly designed, panels can receive direct solar radiation efficiently. However, because it is also easily affected by surrounding shadows and terrain shading, proper consideration of direct solar radiation is important in shading assessments.

Diffuse solar radiation is solar radiation that reaches the surface after being scattered by clouds and particles in the atmosphere. The reason photovoltaic systems can generate electricity even on cloudy days is because of this diffuse radiation. Because diffuse solar radiation is not as directional as direct solar radiation, the effects of azimuth and tilt angles are comparatively mild. When examining solar irradiance conditions in regions with frequent cloud cover or during winter, the proportion of diffuse radiation influences power generation trends.

Reflected solar irradiance is solar radiation that is reflected from the ground and surrounding surfaces and reaches the panels. In PVSyst, the albedo setting is relevant. If the ground is in states such as soil, grassland, concrete, or snow, which have different reflectance, the contribution of reflected irradiance also changes. In particular, in snowy regions, reflection from the snow surface can increase the irradiance on the array plane during winter. However, if snow covers the modules it will impede power generation, so the increase due to reflection and the losses due to snow cover need to be considered separately.

If you understand these three components, PVSyst's monthly results become easier to interpret. For example, in summer, even if horizontal irradiance is high, a rise in module temperature can reduce generation efficiency. In winter, although sunshine duration is shorter, tilted surfaces can receive more irradiance because of the solar elevation, and there may also be contributions from snow reflection. Rather than simply reading it as high in summer and low in winter, it is important to view the results as a combination of irradiance components and design conditions.

When reading PVSyst's Irradiation in practice, it is not necessary to quantify and explain every component—direct, diffuse, and reflected irradiance—in detail. However, when explaining differences in energy production, understanding which components of irradiance are likely contributing will improve the accuracy of your explanation.

How to Read 5

Finally, what is important is the perspective for reading monthly Irradiation. Looking only at annual Irradiation allows you to grasp the general trends of a power plant. However, in practice it is essential to examine month-by-month changes. Monthly solar irradiation trends are important for solar power revenue, output control, O&M, performance evaluation, and PR verification.

In PVSyst's monthly table, irradiation, energy production, PR, temperature, losses, and so on are listed for each month. What you should check here is whether months with high irradiation also show a corresponding increase in energy production; whether there are months with high irradiation but no corresponding increase in production; and whether PR changes abnormally in months with low irradiation.

For example, even if power output increases in summer when solar irradiance is high, PR can decline. This may be due to a reduction in module efficiency caused by high temperatures. Conversely, in winter, even with low solar irradiance, module efficiency can improve because of lower temperatures, causing PR to appear relatively high. Therefore, rather than reading solar irradiance and PR as moving in the same direction, it is important to separate them: treat irradiance as the input and PR as an indicator closer to conversion efficiency.

When looking at monthly Irradiation, we check not only the correspondence with power generation but also the seasonal trends in the meteorological data. If irradiation for a particular month is extremely low or high, it is necessary to check the annual representativeness of the meteorological data, monthly corrections, missing-data imputation, and differences with nearby observation points. Because simulations often use standard-year data, they may not match the weather of the measurement year. If you do not understand this difference, you may draw incorrect conclusions when comparing with actual power generation.

How you interpret monthly data also affects business planning. For estimates of revenue from electricity sales, charge/discharge planning when batteries are installed, overlap with demand in self-consumption projects, and assessment of output control risks, annual values alone are insufficient. By reading PVSyst's Irradiation on a monthly basis, you can identify which seasons tend to have higher generation, which seasons tend to see generation drop, and which months concentrate risk.

In practice, situations often call for month-by-month explanations rather than an explanation of annual energy production. Why is generation lower in this particular month, why does the PR drop in summer, and why doesn't energy production increase in winter even though irradiation is at a certain level? To answer these questions, you need to learn how to read monthly Irradiation.

Terms Easily Confused When Reading Irradiation

When reading the Irradiation in PVSyst, it is easy to confuse Irradiation with Irradiance. Irradiation refers to the solar energy accumulated over a certain period. The unit kWh/m² (kWh/ft²) is often used, and it is used when reading monthly or annual total solar irradiation. On the other hand, Irradiance refers to the instantaneous irradiance and is expressed in W/m² (W/ft²).

When looking at PVSyst result tables or annual energy production, what is most often used is Irradiation. This is to determine how much solar energy has been received over the year and how much irradiation occurred each month. On the other hand, when considering instantaneous output, time-based simulations, or peak output on clear days, the concept of Irradiance is relevant.

If this distinction is blurred, explanations of energy production become confusing. For example, a value of 1,000 W/m² for a certain period is the instantaneous solar irradiance and does not directly represent annual energy production. An annual value of 1,400 kWh/m² is the solar energy accumulated over that year, not an instantaneous intensity. When reading PVSyst reports, you can tell whether the value you are looking at is an instantaneous value or an accumulated one just by checking the units.

Also, terms such as Global, Diffuse, Beam, Incident, and Effective are parts that are easily confused. Global often refers to the total solar irradiance including direct and scattered components, Diffuse refers to scattered solar radiation, and Beam is used in a sense close to direct solar radiation. Incident means that it is incident on a surface, and Effective can be understood to mean the portion that is effectively used after losses.

You don't need to memorize all the terminology. In practice, if you can clarify whether the solar irradiance is on the horizontal plane or on the array plane, whether values are before or after losses, and whether they are instantaneous or cumulative, your ability to read PVSyst will be greatly improved.

What to check when solar irradiance appears low

When PVSyst results indicate low Irradiation, it's premature to immediately judge the design as poor. The first thing to check is the meteorological data you are using. Meteonorm, SolarGIS, satellite data, nearby observation stations, on-site measured data, etc., all produce different irradiation values depending on the data source. Even for the same site, differences in the database or in the time period can cause variations in annual irradiation.

Next, what I want to check is the location information. We will verify whether the latitude, longitude, elevation, time zone, and selection of nearby observation stations are appropriate. If the location is misaligned, it can affect meteorological data and solar position calculations. In particular, in mountainous or coastal areas, even a slight shift in position can change weather trends.

The next things to check are the tilt angle and the azimuth. If the horizontal-plane irradiance is fine but the plane-of-array irradiance is lower than expected, the azimuth or tilt conditions may be affecting it. If the orientation is significantly off south, or if the tilt angle is extremely low or high, the annual plane-of-array irradiance will change.

Additionally, check near shading and terrain shading. Shading from surrounding mountains, trees, buildings, and between racking rows reduces the solar irradiation reaching the array surface. In PVSyst, the Near Shadings and Horizon settings affect energy production. If losses at the Irradiation stage are large, verify whether these settings are overestimated or whether they accurately reflect the site conditions.

Low solar radiation does not have a single cause. Meteorological data, site, tilt and azimuth, shading, reflections, soiling, snow, and other factors interact in combination. Therefore, in PVSyst it is important not to look only at the final energy production but to check, step by step, from the incoming solar radiation.

How to Read Monthly Solar Radiation Trends

When reading monthly irradiation, consider regional characteristics separately from seasonal characteristics. In Japan, conditions that affect power generation vary by region — for example, the rainy season, typhoons, snowfall, insufficient winter sunlight, and high summer temperatures. When looking at PVSyst's monthly table, don't just compare the numerical values; verify that the seasonal variations are characteristic of that region.

For example, in regions where solar irradiance decreases during the rainy season, electricity generation in June and July may be lower than expected. In snowy regions, how winter solar irradiance, albedo, and snow-related losses are handled becomes important. Along coastal areas, temperature, humidity, and cloud cover can also have an impact. In mountainous areas, morning and evening solar irradiance can be reduced by topographic shading.

When you look at monthly Irradiation and power generation side by side, you can see how closely the power output follows irradiation. If irradiation is increasing but generation is not rising as expected, check temperature losses, PCS limits, output curtailment, wiring losses, mismatch, soiling, and so on. If PR is abnormally high or low in months with low irradiation, you should also check the loss items and the data conditions.

When reading month-by-month data, the important thing is to detect anomalies that are hidden in the annual values. Even if the annual generation looks reasonable, if a particular month is extremely low there may be a configuration error or a bias in the data. Conversely, if the monthly variability is reasonable as a regional characteristic, it becomes easier to justify the reliability of the annual results.

Easy-to-use Views for Explaining PVSyst Results

When explaining PVSyst's Irradiation, it's easier to understand if you describe it as the process by which solar irradiance is converted into energy output, rather than listing numbers right away. First, there is the horizontal plane irradiance at the site. Next, it is converted to the plane-of-array irradiance depending on the tilt and azimuth. After that, the effective irradiance is determined by shading, reflections, soiling, etc., and then, after module and PCS losses, the final energy production is obtained. Explaining it in this flow makes PVSyst's results easier to understand.

Particularly in customer-facing explanations, rather than using the technical term Irradiation as-is, it's easier to convey the meaning if you explain it by substituting phrases such as solar radiation conditions or the amount of solar radiation incident on the receiving surface. For technical audiences, use the item names Global horizontal irradiation, Global incident in collector plane, and Effective irradiation; for non-technical audiences, use the expressions horizontal-plane solar radiation, panel-plane solar radiation, and usable solar radiation.

Even when PVSyst results differ from other companies' analyses, comparing the Irradiation is useful. When there is a difference in energy production, first compare whether the solar radiation data are the same, whether the horizontal-plane irradiation is similar, or whether the array-plane irradiation is similar. If the difference is large here, it is likely that meteorological data or design conditions are the primary cause, rather than the modules or PCS. Conversely, if the Irradiation is similar but the energy production differs, check the loss settings, temperature model, wiring losses, PCS settings, output limits, and so on.

Thus, Irradiation is not simply an item of solar irradiance but a criterion for isolating the root causes of differences in analysis results. In PVSyst reviews, tracing the sequence starting from Irradiation, rather than working backwards from the final energy production to identify causes, leads to more consistent judgments.

A perspective connecting on-site inspection and solar radiation conditions

The irradiation conditions in PVSyst cannot be understood solely from the numbers displayed on the screen. In actual power plants, surrounding topography, trees, buildings, racking layout, panel orientation and tilt, ground surface conditions, snow accumulation, weeds, soiling, and other factors affect the irradiation. Therefore, when reading PVSyst's Irradiation, it is important to view it in light of the site conditions.

For example, even if shading is estimated lightly in PVSyst, mountain and tree shadows on site can extend far in the morning and evening. Conversely, even if the simulation models shading conservatively, in reality shadows can be reduced by tree removal or site development. Solar radiation-related items only become meaningful when cross-checked with actual site conditions.

In construction management and O&M fieldwork, smartphone-based positioning, AR checks, and situational awareness using point cloud data can be useful. For example, if you have an environment where you can use an iPhone together with a high-precision GNSS like LRTK to confirm the site location and cross-check it against drawings and point clouds, it becomes easier to verify racking positions, surrounding terrain, obstacles, and post-development conditions. Confirming whether the orientation, layout, and shading conditions set in PVSyst match the actual site also increases the explanatory power of the simulation results.

Especially at photovoltaic power plants, the terrain at the design stage may not perfectly match the terrain after construction. Earthworks, slopes, drainage facilities, surrounding structures, fences, and tree growth can alter the solar irradiance conditions. To correctly read PVSyst's Irradiation, it is important to consider the desk-based figures and the actual on-site conditions together rather than viewing them separately.

Summary

PVSystのIrradiationは、発電量を読むための出発点です。最終的な年間発電量やPRだけを見ても、なぜその結果になったのかは分かりません。日射がどれだけ入り、アレイ面でどのように受け取られ、影や反射などの損失を受けた後に、どれだけ有効に発電へ使われたのかを順番に見ることで、PVSystの結果は理解しやすくなります。

First, verify the meteorological data assumptions using horizontal irradiance. Next, examine the effects of tilt and azimuth using plane-of-array irradiance. Then, with Effective irradiation, check how much irradiance is being reduced by shading, reflection, IAM, soiling, and other factors. Furthermore, by understanding the differences among direct, diffuse, and reflected irradiance and by looking at monthly variations, you can also discern regional characteristics and seasonal variability.

The ability to read PVSyst's Irradiation is useful for design reviews, energy production comparisons, customer explanations, internal sharing, and O&M analysis. Being able to explain not only that production is high or low but also why, based on solar irradiation conditions, greatly increases the credibility of the simulation results.

In solar power analysis, solar irradiation is the most fundamental yet the most easily misunderstood item. When reviewing PVSyst results, it is important not to skim over "Irradiation" as merely an English label, but to carefully check it as the entry point of the process that leads to energy yield. From the horizontal plane to the array plane, from the array plane to the effective irradiation, and from the effective irradiation to the energy yield. Reading it in this order is the basic practice for mastering PVSyst's Irradiation in practical work.