5 Items to Learn About Illuminance and Solar Irradiance Settings in the PVSyst Manual

Table of Contents

• Fundamentals to Grasp Before Studying Illuminance and Solar Irradiance in PVSyst

• Item 1: Do not confuse illuminance with solar irradiance

• Item 2: Confirm the types of meteorological data and their input sources

• Item 3: Understand the difference between solar irradiance on a horizontal plane and on an inclined plane

• Item 4: Separate and examine the direct, scattered, and reflected components

• Item 5: Verify the validity of settings using the variables on the results screen

• Points to Note When Reading the PVSyst Manual for Practical Use

• Summary

Fundamentals to Grasp Before Learning Illuminance and Solar Irradiance in PVSyst

When learning illuminance and irradiance settings in the PVSyst manual, the first thing to be aware of is that, in photovoltaic simulations, what matters is how much energy reaches the photovoltaic surface rather than how bright it appears. In Japanese, the terms "照度", "日射", "日射量", and "日射強度" are sometimes used interchangeably in on-site conversations, but when evaluating energy production in PVSyst you mainly need to follow, in order, the horizontal plane irradiance, the diffuse irradiance, the direct irradiance, and the irradiance incident on the tilted surface as meteorological data.

In the official PVSyst documentation, representative meteorological and solar-radiation variables are organized, such as GlobHor, corresponding to global horizontal irradiance; DiffHor, corresponding to diffuse horizontal irradiance; BeamNorm, corresponding to direct normal irradiance; and BeamHor, corresponding to direct horizontal irradiance. In other words, when reading the PVSyst manual, rather than simply looking for the "screen to set illuminance," a shortcut is to understand the flow of solar radiation data used in energy-yield calculations.

If you proceed with simulations without this understanding, it can lead to issues such as generation being too high despite the same system capacity, monthly generation trends not matching intuition, and an inability to explain differences when changing azimuth or tilt. In particular, the way solar irradiance is received changes even with slight changes in conditions—roof-mounted, ground-mounted, agrivoltaic, snowy regions, high-temperature regions, and so on. The purpose of reading the PVSyst manual is not merely to learn how to operate the interface; it is to understand how the irradiance data you input is converted to a tilted surface and how it is reflected in losses and energy production.

When learning illuminance and solar radiation settings, it is easiest to organize the process by first looking in the order of "Input Data", "Conversion Model", "Component Breakdown", and "Result Verification". For input data, confirm which location's meteorological data will be used. In the conversion model, see how solar radiation on the horizontal plane is converted to the solar panel's mounting surface. In the component breakdown, verify how direct radiation, diffuse radiation, and reflected radiation are handled. Finally, on the results screen check the variables and monthly values to determine whether the settings are reasonable.

Because PVSyst has many configuration items, it can be difficult for beginners to know which numbers to look at. However, if you have a set of checks focused on illuminance and irradiance, reading the manual becomes simpler. This article narrows the PVSyst manual’s illuminance and irradiance settings down to the five items that are especially important in practice and explains them.

Item 1: Do not confuse illuminance with solar irradiance

The first point is not to confuse illuminance with solar irradiance. Illuminance is generally used to describe a perception of brightness by the human eye. By contrast, what matters in photovoltaic simulations is the solar energy reaching the solar cells. When evaluating energy production in PVSyst, you don’t look at whether it’s “bright or dark” as in lighting design, but instead check which surface receives how much solar irradiance and at what times of day.

When a search user looks up "PVSyst manual illuminance", they are likely troubled by not knowing how to input solar radiation, not understanding the meaning of the meteo data, or not knowing how to interpret the solar radiation conditions that affect power generation. In that case, what you should first look for in the manual is not illuminance itself but entries such as meteo data, solar radiation, irradiance, irradiation, GlobHor, DiffHor, etc. In PVSyst's simulation variables, GlobHor is described as the global horizontal irradiation read from the meteo data file, DiffHor as the diffuse horizontal irradiation, and BeamHor as the direct horizontal irradiation obtained by subtracting DiffHor from GlobHor.

Understanding this difference will greatly change how you read the manual. For example, if the power output at a given location is lower than expected, simply describing it as "poor sunlight" does not narrow down the cause. You need to distinguish whether the solar irradiance on the horizontal plane is low, whether the proportion of diffuse solar radiation is high, whether the conversion to the tilted surface is unfavorable, or whether shadows from nearby obstacles or the terrain are having an effect.

Also, terms related to solar radiation that frequently appear in PVSyst can refer either to the concept of instantaneous intensity or to the concept of energy accumulated over a given period. In practice, during the early stages of design people often look at annual or monthly values, while in detailed studies they check hourly data and even the incident irradiation on tilted surfaces. When reading the PVSyst manual, it is important to look at the units and variable names and distinguish whether they are input values, intermediate calculation values, or output results.

If illuminance and solar radiation are confused, errors will occur in the selection of meteorological data and the interpretation of results. In particular, even for the same location, different databases may show different annual solar radiation totals and monthly trends. Before dismissing those differences as "software configuration errors," it is necessary to verify which solar radiation values are being input, which components are being used, and what conversions have been applied.

Item 2: Confirm the Types of Meteorological Data and Their Sources

The second item is to check the type of meteorological data and its source. In PVSyst's power generation simulation, the solar irradiance setting is a prerequisite for the energy production. No matter how carefully you configure the modules and power conditioners, if the meteorological data do not match the project conditions, the reliability of the results will decrease.

PVSyst can generate hourly or sub-hourly meteorological data from monthly values saved in the site information. The official documentation explains that PVsyst can generate hourly and sub-hourly meteorological data from monthly values stored for a geographic site, and that the generation uses the Meteonorm 9.0 algorithm included in PVsyst.

If you understand this mechanism, the difference between monthly values and hourly values becomes easier to see. Monthly values are convenient for grasping a region’s general solar irradiation trends, but actual power generation calculations involve solar altitude, time of day, weather variability, shading, temperature, and other factors. Therefore, in PVSyst, rather than simply substituting monthly values directly for annual generation, the workflow expands them into hourly data and performs the calculations.

On the other hand, when importing external meteorological data, it is necessary to verify the data source, the period, the concept of the representative year, whether the values are measured or estimated, and whether missing data handling has been applied. PVSyst's meteorological data management tutorial indicates that it covers managing meteorological data and importing from external sources. In other words, when reading the PVSyst manual, it is important to be able to explain not only "which screen to load the data on" but also "the reasons why it is acceptable to use that data."

In practice, there may be no measurement station near the candidate site, or detailed data may not be available in the early stages of a project. In such cases, estimates are made using built-in data or external data, and the data source may later be changed to assess sensitivity. What is important here is not to treat the differences in power output between multiple cases as mere differences in results, but to interpret them as differences in the input solar radiation.

When selecting meteorological data, what you particularly need to look at is not just the annual total. You also need to check for monthly biases. It is natural for values to be lower in winter and higher in summer, but the trend can change due to regional characteristics, snowfall, the rainy season, typhoon periods, cloud cover in mountainous areas, and so on. Even if the annual power generation is similar, if the monthly peaks and troughs differ greatly from the actual conditions at the project site, it becomes difficult to explain the simulation results.

Also, meteorological data include not only solar irradiance but also ambient temperature and wind speed. In high-temperature regions, rises in module temperature tend to reduce output, and wind speed conditions also affect temperature calculations. Therefore, when learning illuminance and irradiance settings, it is important not to view irradiance in isolation but to check it as part of the overall meteorological conditions involved in power generation calculations.

Item 3: Understand the difference between horizontal-surface solar irradiance and inclined-surface solar irradiance

The third item is to understand the difference between solar irradiance on a horizontal plane and on a tilted surface. Solar irradiance provided as meteorological data is often referenced to the horizontal plane. However, photovoltaic modules are not always installed horizontally. The irradiance received by the PV surface changes depending on roof pitch, racking tilt, azimuth, tracking method, and so on.

In PVSyst, the concept of converting horizontal-plane irradiance data into irradiance incident on a tilted plane is important. The official documentation explains that transposition is the calculation of irradiance incident on a tilted plane from horizontal-plane irradiance data. PVSyst also provides the Hay or Hay-Davies model and the Perez model, and explains that the direct, diffuse, and albedo components are treated separately.

If you look at the results without understanding this transformation, you won't be able to explain why power generation increases or decreases when you change the orientation or tilt. For example, roofs that are close to south-facing tend to have better annual irradiance conditions, whereas east- and west-facing roofs change the proportion of generation in the morning and evening. Increasing the tilt angle can make it easier to receive winter solar radiation, but it is not necessarily advantageous at the high solar altitude in summer. When interpreting PVSyst results, it's important not to assume that the irradiance on the installation surface will be the same even if the irradiance on the horizontal plane is the same.

What becomes important here are the results related to incident irradiance on inclined surfaces, such as those from GlobInc. If the irradiance on a horizontal plane is the starting point of the inputs, the irradiance incident on the inclined surface is an important intermediate result after applying the design conditions. When power generation is lower than expected, first distinguishing whether the horizontal-plane meteorological data are low or whether the conversion to the inclined surface is disadvantageous makes it easier to explain the cause.

Also, the PVSyst manual emphasizes that understanding what a model converts is more important than memorizing model names. Hay-type models and Perez-type models differ in their approach to handling diffuse irradiance. The official documentation also explains that the Perez model is a more sophisticated model that requires well-measured horizontal-plane data, while the Hay model is robust even when knowledge of diffuse irradiance is incomplete.

In practical work, you look not only at which model to choose but also at whether the quality of the source data is sufficient. Choosing a high-precision model does not always produce better results. If the input data are coarse, using a complex model does not necessarily increase the reliability of the results. When reading the PVSyst manual, be aware that you should evaluate models together with data quality rather than judge models on their own.

Item 4: Separate and Examine the Direct, Scattered, and Reflected Components

The fourth item is to separate the direct, diffuse (scattered), and reflected components. Sunlight is not composed solely of the direct component that arrives straight from the sun. There are components that arrive after being scattered by clouds and the atmosphere, and components that arrive after being reflected by the ground and surrounding surfaces. This decomposition of components is essential for understanding PVSyst's illuminance and irradiance settings.

PVSyst's documentation explains that when converting to a tilted surface the direct component, diffuse component, and albedo component are treated separately. The direct component is handled mainly by geometric transformation, the diffuse component is treated differently depending on the model, and the albedo component is evaluated as reflection from the ground surface.

Direct irradiance is the component that arrives directly from the sun under clear-sky conditions. Because it is strongly affected by shading, nearby buildings, trees, mountains, and mutual shading between rows of mounting racks can greatly influence power generation. Diffuse irradiance is the component that reaches the site by being scattered from the whole sky. Under cloudy conditions or when humidity is high, the proportion of the diffuse component can increase. The reflected component is the portion of light reflected from the ground and surrounding surfaces that reaches the modules, and it can be non-negligible in snowy areas or on bright ground surfaces.

If you consider these three separately, it becomes easier to understand the meaning of the settings and results screens in PVSyst. For example, in snowy regions the ground surface reflectance can become strong, but at the same time snow cover and operational losses due to snow also need to be considered. For ground-mounted installations, the reflected component and the effects of mutual shading change depending on racking height, row spacing, and ground conditions. For rooftop installations, in addition to the roof surface's orientation and tilt, you need to check the impact of shadows from surrounding obstacles on the direct component.

Care is also needed when handling diffuse irradiance. PVSyst’s official documentation explains that if diffuse irradiance is not directly measured, it is estimated by models from global horizontal irradiance (GHI) or irradiance on a tilted plane. Because satellite meteorological data and ground measurements from a single pyranometer measuring global irradiance can have the diffuse component modeled, it is important to check how much of the input data consists of actual measured values.

This point also concerns the accountability of simulation results. When assessing power generation projects, it is necessary to explain not only the annual generation figures but also why those figures were obtained. If solar irradiance is high but generation does not increase, it is necessary to distinguish whether direct irradiance is being lost to shading, whether temperature losses are large, or whether a high proportion of diffuse irradiance makes it difficult for inclined surfaces to receive more light.

When reading the PVSyst manual, it is important not to learn direct, diffuse, and reflected irradiance as mere technical terms, but to grasp them as a perspective for analyzing the causes of results. In particular, when comparing design proposals, checking not only the system capacity and number of panels but also how the components of solar irradiance reaching the tilted surface have changed will clarify the basis for the proposals.

Item 5: Verify the validity of settings using variables on the results screen

The fifth item is to verify the validity of the settings by checking the variables on the results screen. In PVSyst, it is not enough to stop at configuring the input screen. After the simulation, it is important to examine the solar radiation–related variables and confirm whether the input conditions and the conversion results are plausible.

First, what I want to check is the input values on the horizontal surface. I look to see whether GlobHor deviates substantially from regional expectations and whether the monthly trends look natural. Next, I check the breakdowns such as DiffHor and BeamHor. In PVSyst's result variables, GlobHor, DiffHor, BeamHor, and so on are organized as meteorological data and the irradiance components calculated from them.

Next, what we want to check is the incident solar radiation on inclined surfaces. By comparing the irradiance on the installation surface with the irradiance on a horizontal plane, it becomes easier to judge whether the azimuth and tilt settings are appropriate. The way inclined-surface irradiance behaves changes with design conditions—south-facing, east–west-facing, low tilt, high tilt, tracking systems, and so on. If you compare only the final power output without looking at this, it becomes difficult to identify which factors caused the differences.

Furthermore, checking the monthly results is also important. Even if the annual energy production appears reasonable, the monthly energy production can be inconsistent. For example, if winter solar irradiation looks excessively high, if the decline during the rainy season is not reflected, or if changing the azimuth or tilt hardly alters the monthly pattern, these are indications to review the meteorological data or the design conditions.

When reading the PVSyst manual, simply memorizing the variable names on the results screen is not sufficient. What matters is understanding the relationships between the variables. The solar irradiance on the horizontal plane is input, the diffuse and direct components are handled, conversion to the tilted surface is performed, and through shading, reflections, and various losses it progressively approaches the final energy production. Once you can follow this sequence, the order in which to check things when analysis results do not match becomes clear.

A common mistake in practice is to look only at the final power generation and overlook the reasonableness of the solar irradiance settings. When generation is too high, check whether the irradiance data are overestimated, whether the conversion to sloped surfaces is coming out overly favorable, and whether shading or other losses are being underestimated. When generation is too low, check whether the meteorological data are too conservative, whether the installation surface conditions are incorrect, and whether the handling of diffuse and direct irradiance matches the actual conditions at the project site.

As such, PVSyst's illuminance and solar irradiance settings are not completed on the input screen alone. Checking variables on the results screen and, when necessary, returning to the meteorological data or design conditions increases the simulation's credibility.

Points to note when reading the PVSyst manual for practical use

When reading the PVSyst manual in a practical context, you need to be conscious not only of replacing terms with Japanese but also of which process or stage is being described. Depending on whether the explanation covers meteorological data, a model for conversion to tilted surfaces, shading and losses, or output variables, the same term "solar radiation" can mean different things.

Especially beginners tend to treat all the numbers on the screen as equally important. However, what you should check first is whether the entered meteorological data are reasonable, whether the conversion from horizontal-plane irradiance to tilted-plane irradiance is appropriate, whether the handling of direct, diffuse, and reflected components matches the project conditions, and whether the monthly trends in the results can be explained. Simply following this order makes reading the manual much easier.

Also, PVSyst sometimes uses an approach that generates hourly values from monthly values. The official documentation explains that when only monthly meteorological data are available, hourly synthetic data for global horizontal irradiance and ambient temperature are generated. Therefore, rather than looking only at the monthly input values and concluding that "the settings are correct," you should verify how they are treated as hourly data and how they are reflected in the final results.

Comparing meteorological data is also important. Using multiple meteorological data sources can change annual energy production by several percent. Those differences can affect system design and project feasibility assessments. The purpose of using the PVSyst manual to learn illuminance and irradiance settings is not simply to press the right buttons, but to strengthen the rationale for the input data and the explanatory power of the results.

Furthermore, the aspects to prioritize vary by project type. For rooftop installations, orientation and tilt, roof shape, and shadows from surrounding obstacles are important. For ground-mounted projects, row spacing, ground reflection, racking tilt, and terrain conditions have an impact. For agrivoltaic projects, shadows from overhead structures and crop-side conditions also need to be considered. In snowy regions, both increased reflection and snow-related losses are examined. In high-temperature regions, even with high solar irradiance, temperature-related losses can depress power generation.

Illuminance and solar irradiance settings are not only the entry point for power generation simulations but also the foundation for explaining the results. If you proceed to detailed loss settings while leaving this unclear, it will become difficult to analyze the causes later. Conversely, if you have a handle on the flow of solar irradiance, you can organize PVSyst’s complex screens into the sequence "Input", "Conversion", "Losses", "Results".

Summary

When learning illuminance and solar irradiance settings in the PVSyst manual, it is first important not to confuse illuminance with irradiance. What directly affects energy production in PVSyst is not the brightness perceived by the human eye but the solar energy reaching the surface of the solar cells. Therefore, you need to understand irradiance-related variables such as GlobHor, DiffHor, BeamHor, and BeamNorm, and clarify which values are inputs, which are intermediate calculation values, and which are results.

Next, verify the source of the meteorological data. In some cases hourly data is generated from monthly values, while in others meteorological data from external sources is imported. Which data you use affects not only the annual power generation but also the monthly trends. Being able to explain the basis for the input data improves the credibility of the simulation.

Furthermore, it is essential to understand the difference between horizontal-plane irradiance and tilted-plane irradiance. The horizontal irradiance provided as meteorological data does not reach the PV surface unchanged. Factors such as azimuth, tilt, solar altitude, diffuse irradiance, and reflected irradiance are taken into account to evaluate the irradiance incident on the installation surface. Grasping PVSyst's transposition concept makes it easier to explain differences in energy yield between design options.

It is also important to separate the direct, diffuse (scattered), and reflected components. Direct solar radiation is strong in clear conditions, diffuse solar radiation affects output even on cloudy days, and solar radiation reflected from the ground surface influences power generation differently. In projects that involve shading, ground conditions, snow cover, or installation angle, understanding each component helps in interpreting the results.

Finally, verify the validity of the configuration using the variables on the results screen. PVSyst is not software where you simply enter the inputs and stop. After the simulation, it is important to check the horizontal-plane irradiance, the tilted-plane incident irradiance, the breakdown between diffuse and direct components, and the monthly results, and to be able to explain how the settings relate to the results.

The five items to learn about illuminance and irradiance settings in the PVSyst manual are: grasping the difference between illuminance and irradiance, checking the types and sources of meteorological data, understanding the conversion from a horizontal plane to a tilted plane, separating and examining the direct, diffuse, and reflected components, and verifying validity using the result variables. If you cover these five items in order, you will find it easier not only to operate the PVSyst interface but also to explain the rationale behind power generation simulations in practical work.