What is PVSyst? Understand the relationship between solar irradiation, losses, and electricity generation in 5 minutes

Table of Contents

• What PVSyst is a tool for checking

• Understanding solar irradiation changes how you view power generation

• Losses are a framework for organizing factors that reduce power generation

• Grasp the flow from solar irradiation to power generation

• Points to note when reviewing simulation results

• What to clarify before using PVSyst in practice

• Summary

What is PVSyst used to check?

PVSyst is one of the software tools used for design studies of photovoltaic power systems, generation simulations, and verification of loss factors. When planning a solar power plant, annual energy production is not determined simply by choosing the panel capacity. Many conditions affect the energy yield: the solar irradiation conditions at the installation site, the panel orientation and tilt, equipment specifications, temperature rise, shading effects, wiring losses, outages and output limitations, and more. PVSyst is used to input these conditions and to organize and verify how irradiance is converted into generated energy and what losses occur along the way.

"Many practitioners who search 'What is PVSyst' may have a general understanding that it is used to calculate energy production, but the connection between solar irradiance, losses, and energy output may still be unclear. In practice, simply looking at the annual energy production figure is not sufficient. If you cannot grasp why that level of energy production occurred, which conditions would change the results, or which losses are the largest, it becomes difficult to translate the findings into design improvements or the preparation of explanatory materials."

What’s important when reading PVSyst results is not to look at the final energy yield alone, but to examine the conversion process from solar irradiance to generated energy. Energy arrives from the sun, is incident on the installation surface, is converted to DC power by the solar cells, is transformed and transmitted within the system, and is finally evaluated as AC energy output. Along this chain, losses occur — shading, reflection, temperature effects, equipment conversion losses, wiring losses, downtime, and so on. In other words, PVSyst is easier to understand if you view it not only as providing the “answer” for energy production but also as a tool to check the “breakdown” that leads to that answer.

In evaluating photovoltaic power generation, even with the same installed capacity, energy output varies depending on the region and installation conditions. Even in areas with high solar irradiation, generation can fall short of expectations if the installation angle does not align with the plan, shading has a large effect, or the system is susceptible to temperature increases. Conversely, in locations that are average in terms of irradiation, if orientation and tilt, minimal shading, and assumptions about loss management are appropriate, relatively stable generation can be expected. In this way, the purpose of using PVSyst is not to “make the generation look larger,” but to estimate a reasonable generation based on conditions and to identify design checkpoints.

What practitioners should be aware of is that PVSyst's results strongly depend on the input conditions. If solar irradiance data, installation conditions, equipment specifications, loss rates, shading settings, and so on differ from reality, the predicted energy production will also deviate. Even a sophisticated analysis tool cannot provide reliable assessments if the inputs remain ambiguous. PVSyst is not a tool that, given correct inputs, will automatically produce a single correct answer; it is a practical tool for quantifying design conditions and evaluating the plausibility of the predicted energy production.

Understanding Solar Irradiance Changes How You View Power Generation

The starting point of solar power generation is solar irradiance. Solar irradiance is the concept that indicates the amount of energy arriving from the sun at the Earth's surface or on the surface of a panel. When considering energy production, many people tend to focus on system capacity or the number of panels, but without the energy coming in as sunlight in the first place, you cannot generate electricity. Therefore, to understand energy production in PVSyst, you must first grasp how solar irradiance is handled.

There are several ways to view solar radiation: radiation arriving on a horizontal plane, radiation arriving on an inclined panel surface, direct solar radiation, and diffuse (scattered) solar radiation. In practice, what matters most is how much radiation ultimately reaches the surface of the solar cells. Even with the same regional solar conditions, the amount of radiation received changes when panel orientation and tilt angle change. A near-south-facing layout and an east–west-oriented layout differ not only in annual power generation but also in generation patterns in the mornings, evenings, and daytime. Changing the tilt angle also alters the incidence conditions by season.

In PVSyst, the solar radiation incident on the installation surface is calculated based on meteorological data. What is crucial here is the quality and representativeness of the meteorological data. If data from a location distant from the actual planned construction site are used, differences in elevation, distance from the coast, propensity for cloud formation, tendencies for snowfall or fog, and other factors can affect the expected power generation. Even if there appears to be little difference in the annual average, differences may emerge when viewed on a monthly basis. Therefore, solar radiation should be considered not merely as an initial input but as the foundation of the entire power generation simulation.

When looking at solar irradiance, it is important not to simply assume that "more irradiance necessarily means more power generation." In solar power generation, higher irradiance increases generation opportunities, but it is also affected by ambient temperature and module temperature. In environments with strong irradiance, panel temperatures tend to rise, causing output reductions due to the temperature increase. In other words, while irradiance is a primary factor that boosts generation, it needs to be considered together with other factors such as temperature losses.

Also, solar irradiation affects not only annual energy production but also how a facility is operated and assessed commercially. For example, even if total annual generation is similar, a system that concentrates generation in summer and one that produces relatively steadily throughout the year have different operational implications. When considering power usage conditions, feed-in/tariff conditions, and the effects of output curtailment, monthly and time-of-day generation patterns cannot be ignored. When interpreting PVSyst results, checking not only annual values but also monthly trends and any generation bias makes the output more useful for design decisions.

Understanding solar irradiance is also useful for explaining why generation may deviate upward or downward. If generation appears lower than expected, you need to separate whether it is caused by the irradiance data, shading effects, equipment losses, or temperature conditions. Looking only at losses without checking the assumptions about irradiance can lead to incorrect conclusions about the cause. Think of PVSyst as a tool that visualizes the flow from irradiance to energy generation to help make that distinction.

Losses: a way of organizing the factors that reduce power generation

Understanding losses is as important as understanding solar irradiance when using PVSyst. In solar power simulations, not all the solar radiation that reaches the installation surface is converted into electricity. Generated energy is reduced by various factors—such as the solar cell conversion process, temperature conditions, shading, soiling, reflection, equipment conversion, wiring, equipment downtime, output limitations, etc. These collected factors are what we call losses.

When you see the word "losses" you might feel it implies something bad. However, in practice, losses are not so much failures to be avoided as a way to appropriately anticipate the differences that occur in real equipment. Even the best-performing power plants experience conversion losses, temperature losses, and wiring losses. The important thing is not to make losses zero, but to identify which losses occur and to what extent, and whether there is room to improve them through design, construction, or operation.

One common type of loss is temperature-related loss. Solar cells generate electricity from light, but generally their output decreases as panel temperature rises. While stronger solar irradiance makes generation easier, conditions that tend to raise temperature also cause output reductions. The effect of temperature also varies with differences in rooftop versus ground-mounted installations and with ventilation conditions. When reviewing temperature losses in PVSyst, it is necessary not only to look at the magnitude of the numbers but also to verify that they are consistent with the installation environment and racking conditions.

Losses due to shading are also important. When shadows are cast by nearby buildings, trees, terrain, structures, or adjacent panel rows, the generation during those times is reduced. The impact of shading is not determined solely by the shaded area. Depending on the circuit configuration of the solar cells and the operating conditions of the equipment, partial shading can have a large effect on output. Therefore, in shadow analysis it is important to confirm not only whether shading occurs, but also when it occurs, over what area, and to what extent.

Losses due to soiling are also an item that is easy to overlook in practice. If dust, pollen, bird droppings, fallen leaves, salt, or residues after snowmelt remain on the panel surface, the solar irradiance that can be received is reduced. The degree of soiling varies depending on the region, installation tilt, rainfall conditions, and maintenance frequency. If underestimated, expected power generation can become overly optimistic; if overestimated, the plan may become excessively conservative. When actual performance data are available, it is desirable to use assumptions that are close to the on-site conditions.

Losses in wiring and conversion also affect power generation. The direct current (DC) power produced by solar cells travels through wiring, is converted to alternating current (AC) by conversion equipment, and, if necessary, undergoes voltage transformation and power collection before being evaluated. During this process, losses occur due to electrical resistance and conversion efficiency. If wiring runs are long, currents are high, cable selection does not match the plan, or equipment operating ranges do not match the design conditions, losses can exceed expectations.

Additionally, there are losses due to system mismatch. Even with the same specifications, solar cells do not produce identical outputs because of unit-to-unit variation, aging, and differences in irradiance on each mounting surface. When multiple circuits are operated together, some conditions can affect the overall output. Such mismatches are important to consider when designing circuit grouping, mixed orientations and tilts, and the way shading occurs.

When reviewing the loss display in PVSyst, it is important not only to look at individual loss items but also to interpret them in the context of design decisions. If temperature losses are large, check ventilation and installation conditions; if shading losses are large, review the layout, spacing, and obstacle conditions. If wiring losses are large, check wiring routes and cable conditions. Losses are not merely negative factors but clues pointing to areas for improvement.

Understand the process from solar irradiance to power generation

A shortcut to understanding PVSyst is to view the flow from solar irradiance to electricity production as a single conversion process. First, solar irradiance from the sun reaches the installation site. Next, it is determined how much of that irradiance strikes the panel mounting surface. From there, the effective irradiance reaching the solar cells is reduced by shading, reflection, soiling, and similar effects. After that, the solar cells convert the irradiance into DC power and are affected by temperature and electrical mismatch. Finally, after conversion equipment, wiring, and overall system constraints, it becomes the energy production that is evaluated.

Understanding this sequence will change how you interpret simulation results. For example, if the annual power generation is lower than expected, instead of immediately suspecting installed capacity or equipment performance, you can see at which stage the energy is being lost. By checking sequentially whether the irradiance on the installation surface is low, shading has a large impact, temperature losses are significant, or conversion and wiring losses are substantial, it becomes easier to identify the cause.

When considering energy production, the relationship with installed capacity is also important. Larger installed capacity tends to increase generation, but you cannot make simple comparisons unless irradiance and loss conditions are the same. Even systems with large capacity will have low generation per unit of capacity if shading or output limitations are significant. Conversely, small-capacity systems can generate efficiently if installation conditions are good and losses are small. Therefore, in PVSyst results, you need to look not only at the total annual generation but also at generation efficiency relative to installed capacity.

There is also a difference between the DC side and the AC side when it comes to power generation. The electricity produced by solar cells is direct current (DC), whereas the electricity that is actually used or tied to the grid is treated as alternating current (AC). Losses occur during the process of converting DC to AC. Furthermore, depending on the combination of DC capacity and the capacity of the conversion equipment, output can reach its limit during periods of strong solar irradiance, causing some generation opportunities to be curtailed. This limitation does not necessarily indicate poor design, but if excessive it can be a factor in reduced power generation.

In power generation simulations, monthly variations must not be overlooked. Even if the annual generation looks reasonable, a particular month may be extremely low. Possible causes include winter solar altitude, snowfall, the rainy season or cloudy weather, lengthening shadows, temperature conditions, and output curtailment. Problems that are invisible from annual values alone are easier to detect by examining monthly data. If you use PVSyst results in practice, it is important to check the annual values, monthly values, and the breakdown of losses together.

A simple way to express the relationship between irradiance, losses, and energy production is that the energy produced is determined by the incoming solar energy minus the losses at each stage, together with the conversion capacity of the equipment. With this mindset, even if many items are listed on PVSyst screens or in reports, it becomes less confusing to know what to look at. When tracing the meaning of numbers, it is easiest to organize them by checking from upstream to downstream: irradiance, installation surface, solar cells, DC, AC, and final energy output.

Precautions when viewing simulation results

When reviewing PVSyst simulation results, the thing you most want to avoid is judging quality solely by the final annual energy production. Annual energy production is an important metric, but that number alone cannot tell you whether the input conditions are reasonable, whether losses have been appropriately accounted for, or whether design risks are being concealed. In practice, you should verify both the final value and its breakdown.

First, what I want to confirm is the assumptions behind the solar radiation data. Verify which region’s meteorological conditions are being used, whether they are representative of the local conditions, and whether they can be regarded as a long-term average. Photovoltaic power generation is affected by year-to-year weather variations, so it is dangerous to treat the performance of a single year as a long-term forecast. Simulations do not guarantee future power generation; they are projections based on certain assumptions. Being able to explain these assumptions is also important when briefing the client and other stakeholders.

Next, confirm that the installation conditions match the actual plan. If the orientation, tilt angle, installation height, row spacing, layout, or terrain conditions are misaligned, the amount of solar radiation captured and the effects of shading will change. Preliminary design estimates may use provisional conditions, but when progressing to detailed analysis it is necessary to update the conditions to be as close as possible to the actual layout. Evaluating power generation based on an outdated layout proposal may lead to discrepancies later.

Loss rate settings are also an item that should be examined carefully. Simply using initial values or common defaults may not sufficiently reflect on-site realities. Soiling, snowfall, temperature, wiring, downtime, degradation, shading, output limits and other factors vary depending on the region, design, and operational policies. Overly optimistic loss settings can make a business plan look better than reality. Conversely, settings that are more conservative than necessary can disadvantageously affect plan evaluations. It is important to set them based on reasonable justification.

Shading settings require particular attention. The results change depending on how comprehensively surrounding obstructions, terrain, and shading between panel rows are modeled. If shading is simplified, that assumption should be clearly stated. If shading actually exists but is not modeled, the estimated power generation may be overstated. Conversely, applying overly strict obstruction conditions can produce an assessment lower than reality. Shading analysis should be considered together with on-site inspections and design drawings.

Also, attention must be paid to significant figures and rounding in the reported power generation results. Simulation outputs may display very detailed numerical values, but not all of those digits can be predicted with the same accuracy in practice. Considering interannual meteorological variability, construction tolerances, equipment unit differences, and changes in operating conditions, it is not appropriate to judge the superiority of designs based solely on small differences in decimal places. In comparative evaluations, it is necessary to verify why differences occurred and whether those differences are of a magnitude that affects practical decision-making.

When sharing PVSyst results internally or externally, it is important to present the underlying assumptions as well. If you include only the energy generation figures in the documentation, later you will not be able to understand "why this value was obtained" or "under what conditions it was calculated." Organizing the design proposal, solar irradiance data, loss settings, system capacity, layout conditions, and the time the analysis was conducted makes it easier to review and explain things in later stages. Energy production simulations are not something you create once and finish; they should be updated as the design progresses.

Points to clarify before using PVSyst in practical work

Before using PVSyst in practice, it is important to get organized before beginning data entry. If you start entering conditions directly on the screen, assumptions can become mixed in later and provisional conditions may be mistaken for finalized ones. This is especially true when multiple people are carrying out the design or analysis, as the management of input conditions can determine the quality of the energy production simulation.

First, you should clarify the purpose of the study. Whether it is a rough estimate in the initial stage, a comparison of layout proposals, the annual energy yield for a project feasibility assessment, or loss verification for detailed design, the required accuracy and the level of input detail will differ. Over-refining details at the conceptual stage reduces work efficiency, and conversely, keeping coarse assumptions during the detailed study stage is insufficient as a basis for decision-making. Before using PVSyst, you need to be clear about what you want to verify with this simulation.

Next, organize the installation site and meteorological conditions. Check the candidate site's location, elevation, surrounding environment, presence or absence of snow accumulation and salt damage, and any structures that could cause shading. Solar radiation conditions, which have a major impact on power generation, are the starting point for inputs. If you perform calculations based on assumptions that differ from the actual site conditions, you will need to make corrections later. If possible, it is advisable to organize conditions related to solar radiation and shading early on, together with design drawings, on-site photographs, and surrounding information.

Equipment conditions are items you should also confirm in advance. The capacity of the solar panels, the number of panels, circuit configuration, the capacity of power conversion equipment, installation orientation, tilt, wiring routes, and connection conditions directly affect power generation. When the equipment has not yet been finalized, comparisons may be made using provisional conditions, but in that case it is necessary to clearly record that they are provisional. If the equipment specifications change later, not only the power generation but also the breakdown of losses may change.

Regarding loss settings, it is more stable to establish rules within the company and for each project. If values are changed based on the individual judgment of the person in charge each time, comparing projects becomes difficult. Organizing which items—dirt, wiring, temperature, downtime, degradation, shading, output limits, etc.—are set under which assumptions improves the explainability of the results. However, rather than fixing everything to constant values, there should be room to revise them according to site conditions. Balancing standardization and case-by-case judgment is essential.

File management and records of analysis history are also important in practice. Simulations generate multiple versions due to layout changes, equipment changes, changes in loss rates, and so on. If you can’t tell which is the latest, which is for submission, and which is for comparative review, there is a risk of using incorrect results. Simply including the project name, analysis date, design proposal, and main changes in the file name or review notes can reduce confusion in later stages.

Also, it is important not to place too much trust in PVSyst results. Simulations do not perfectly reproduce reality; they are a means of organizing conditions to estimate power generation. Actual power output fluctuates due to weather, equipment condition, cleaning status, failures, output control, changes in the surrounding environment, and other factors. Therefore, simulation results should be treated not as "definitive values" but as "estimates based on assumptions." When explaining to stakeholders, it is important to convey this premise without misunderstanding.

For practitioners, the value of using PVSyst is not simply in calculating energy production. It lies in comparing differences between design options, identifying the factors that cause the largest losses, and considering directions for improvement. If you understand the relationship between solar irradiance, losses, and energy output, you will be less likely to be swayed by the output figures and better able to use them in design decisions.

Summary

PVSyst is software for assessing the energy production of photovoltaic systems: it organizes solar irradiance, installation conditions, losses, and equipment parameters, and checks the flow that leads to the final energy production. The important point is not to look only at the final annual energy production, but to understand the assumptions from which that figure is derived. Solar irradiance is the starting point for generation and determines how much energy reaches the installation surface. From there, losses such as shading, soiling, reflection, temperature, wiring, conversion, downtime, and control determine the final energy production.

To correctly read PVSyst results, it is important to check the flow from solar irradiance to energy production sequentially from upstream. By verifying whether the irradiance conditions are reasonable, whether the installation azimuth and tilt match the plan, whether there are any omissions in the shading settings, and whether the loss rates are not excessively high or low for the site conditions, you can more easily increase the reliability of the results. Rather than judging solely by the annual energy production figure, looking at monthly trends and the breakdown of losses makes it easier to obtain information that can lead to design improvements.

In practice, organizing the conditions before using PVSyst is essential. By clarifying the study objectives, meteorological conditions, installation conditions, equipment specifications, loss settings, and file management, it becomes easier to explain the simulation results. Because power generation simulations form the basis for design and business decisions, proceeding while leaving input conditions ambiguous can lead to rework and misunderstandings in later stages.

The first step to mastering PVSyst is not to memorize all the complex settings, but to understand the relationship between solar irradiance, losses, and energy production. Once you understand this relationship, it becomes easier to interpret the figures shown in reports and to identify potential design improvements. When evaluating photovoltaic installations, it is important not only to consider calculated energy yields on paper but also to verify site conditions and operational stages. PVSyst results do not guarantee energy production; they should be treated as study material based on assumptions and assessed together with design drawings, on-site verification, and operational performance.