What is PVSyst? 7 checkpoints for interpreting simulation results

Table of Contents

• What is PVSyst software used to verify?

• Check 1: Don’t judge based only on annual energy production

• Check 2: Confirm the assumptions for irradiance and weather data

• Check 3: Confirm system capacity and layout conditions

• Check 4: Interpret the breakdown of loss components

• Check 5: Confirm the impact of shading and time-of-day declines

• Check 6: Confirm performance ratio and monthly variations

• Check 7: Align the assumptions used when comparing to actual performance

• Precautions when using PVSyst results in internal documents or proposals

• Summary: Interpret simulation results by examining their assumptions and losses

What is PVSyst software used to verify?

PVSyst is simulation software for estimating in advance the energy production of photovoltaic systems and for checking how design parameters and loss conditions affect the generation. The official notation is PVsyst, but because it is often written as PVSyst in Japanese, this article will also use the PVSyst spelling since it is more likely to be used in searches.

In the planning stage of a solar power plant, many factors affect power generation, including the installation site, orientation, tilt angle, the capacity of photovoltaic modules, the capacity of power conditioners, wiring conditions, shading, temperature, soiling, and downtime. If these factors are evaluated based only on intuition, discrepancies between expected and actual power generation are likely to occur. Therefore, it is important to organize the basis for generation forecasts and verify them in a form that can be used for design and business viability decisions.

The purpose of using PVSyst is not simply to produce a number for annual energy production. It is also important to interpret what kinds of losses occur under the entered conditions, which months see increased energy production, and which conditions have a large impact on the results. In practice, people sometimes judge performance solely by the absolute value of energy production, but that can lead to overlooking assumptions and risks. Simulation results only become meaningful after reviewing them together with the input parameters, meteorological data, design conditions, loss settings, shading assessment, month-by-month trends, performance ratio, and so on.

Especially for those responsible for the design and proposal of solar power plants, the ability to read PVSyst results is important. Energy production forecasts are used in various situations—explaining to customers, internal approvals, supplementary materials for investment decisions, pre-construction design checks, and comparisons with actual performance after commissioning. What is required is not to accept the simulation figures at face value, but to be able to explain the assumptions under which the figures were calculated. Because results change when conditions change, treating the results in isolation can easily lead to misunderstandings.

PVSyst reports display annual energy production, monthly energy production, irradiance, temperature, losses, performance ratio, system configuration, and an overview of the input data. However, the displayed items and their names may vary depending on the version, system type, and settings. Because there are many items, personnel seeing it for the first time can easily be unsure where to start, but by deciding the order in which to review them you can make sure you don’t miss any important points. In this article, after covering what PVSyst is, we organize seven perspectives that practitioners should check when reading simulation results.

Check 1: Do not judge solely by annual power generation

When viewing PVSyst results, the annual generation is often the first thing that catches the eye. Annual generation is an important figure for understanding the overall outlook of a power plant. It serves as the central figure in business planning, estimating revenue from electricity sales, and evaluating capital investments. However, you should avoid making decisions based solely on the annual generation, because the same annual figure can mean very different things depending on its breakdown and the assumptions behind it.

For example, even if the estimated annual energy production looks high, if the weather data are overly optimistic, the loss assumptions are insufficient, or the impact of shading is not fully accounted for, actual operation may fall short of expectations. Conversely, if calculations are made under conservative conditions, an apparently modest annual production figure may actually be a realistic estimate that incorporates risk. You cannot determine whether a projection is reasonable based solely on the size of the numbers.

When reviewing annual energy production, check whether the generation is excessively high or low relative to the installed capacity. It is important to determine whether the production falls within a range that can be explained after taking into account the site’s solar irradiation conditions, orientation, tilt, shading, temperature environment, and loss settings. If results appear unusually good, verify whether any input conditions have been set more favorably than reality. If results appear unusually poor, check whether shading, losses, orientation, or equipment configuration settings include overly adverse conditions.

Also, annual generation needs to be viewed together with monthly generation. Even if the annual figures appear reasonable, the monthly data may reveal that generation is unnaturally high or low in specific seasons. On sites with significant winter shading, in regions with large summer temperature increases, or in areas prone to the effects of the rainy season or snowfall, monthly variation becomes an important factor in decision making. Relying solely on annual values makes it easy to overlook these seasonal fluctuations.

Furthermore, annual energy production is a convenient figure for proposal documents, but in internal reviews you need to break down its underlying basis. Organizing which meteorological data were used, whether system capacity is based on the DC side or the AC side, the extent of losses that have been incorporated, and which method was used to evaluate shading will make comparisons easier if conditions change later. For PVSyst results, it is important not only to look at the final numbers but to read the process by which those numbers were derived.

Checkpoint 2: Verify assumptions for solar irradiance and weather data

In solar power generation simulations, assumptions about solar irradiance have a large impact on energy output. When reading PVSyst results, first check which location’s meteorological data are being used, whether the distance to the planned installation site or the elevation difference is significant, and whether regional characteristics are reflected. Solar power generation conditions vary even within the same prefecture—coastal areas, mountainous areas, basins, urban areas, and heavy-snow areas. Even if the data are from a nearby site, differences in the surrounding environment can lead to discrepancies with the actual solar irradiance conditions.

Meteorological data include elements such as solar irradiance, ambient temperature, and wind speed. Solar irradiance tends to be most directly related to power generation, but temperature is also important. Because photovoltaic modules lose output at high temperatures, power generation varies depending on air temperature and installation conditions. Even in regions with high solar irradiance, if summer temperature rises are large, temperature-related losses can be significant. Therefore, it is important not only to consider whether solar irradiance is high or low, but also to check it together with temperature conditions and installation methods.

Attention must also be paid to the types of solar irradiance. Depending on which values are used and how—horizontal plane irradiance, inclined plane irradiance, direct irradiance, diffuse irradiance, etc.—the meaning of the calculation results changes. When practitioners read a report, they do not need to understand all the detailed formulas, but they should be aware of how the irradiance reaching the installation surface is being estimated. In particular, if the tilt angle or azimuth is changed, the irradiance incident on the inclined surface changes, so the power generation will also change.

Meteorological data are generally based on historical statistics and representative-year data. Therefore, simulation results do not guarantee future power generation. Some years have many sunny days, while other years may experience prolonged unfavorable weather. Because actual power generation fluctuates from year to year, PVSyst results should be treated as forecasts based on certain assumptions. In proposal materials and internal documents, it is safer to make it clear that these are forecasts and to avoid overly definitive wording.

When you can compare multiple meteorological datasets, it is also useful to examine the differences in the results. Checking how much the annual generation varies depending on the type of data makes it easier to explain the uncertainty in the forecast. If generation changes significantly due to differences in meteorological data, it should be treated as a sensitivity in the business plan. When reading PVSyst results, always verify not only the generation figures but also the underlying assumptions about solar irradiance and meteorological data.

Checkpoint 3: Confirm equipment capacity and layout conditions

When reading PVSyst simulation results, verifying equipment capacity and layout conditions is essential. In a solar power plant, the total capacity of PV modules, inverter capacity, string configuration, installation orientation, tilt angle, row spacing, and racking height all affect energy production. If these conditions deviate from the actual design, the simulated energy output will not match reality.

First, what I want to confirm is the relationship between DC-side capacity and AC-side capacity. In solar power generation, the capacity on the photovoltaic module side and the capacity of the power conversion equipment do not always match. In designs that make the DC-side capacity larger than the AC-side capacity, clipping can occur in which part of the peak is cut off by the power conditioner's rated limit during periods of strong solar irradiance. This should be distinguished and confirmed as a phenomenon separate from output control by utilities. On the other hand, designs that provide a certain DC/AC ratio may be adopted to account for operation in the morning, evening, and cloudy conditions. When reviewing PVSyst results, check that this capacity ratio aligns with the design intent.

Next, check the orientation and tilt angle. For fixed installations in Japan, there is a tendency for annual power generation to be higher the closer the system faces south, but the approach to layout changes depending on land shape, site preparation conditions, connection conditions, and constructability. The tilt angle is also determined by whether you prioritize annual generation, winter generation, or wind load and constructability. A high simulation result does not necessarily mean that the layout is easy to construct on site. Cross-check the design drawings with the input conditions to confirm they match the actual layout.

Inter-row spacing is also important. If rows of photovoltaic modules are too close, shadows from the front row can more easily fall on the rear rows during periods of low solar altitude. This effect can be especially significant in winter, when the sun’s altitude is lower. In PVSyst results, this influence appears as losses due to shading and in the monthly energy production. Comparing how much shading effects change with layout modifications makes it easier to judge the balance between land-use efficiency and power generation.

String configuration and combinations of equipment also need to be checked when interpreting the results. If the number of modules, circuit configuration, input voltage range, temperature conditions, etc. are not appropriate, they will affect simulated losses and operating conditions. Detailed electrical design requires verification by a specialist, but as a practitioner it is important to confirm that the design drawings, single-line wiring diagrams, equipment specifications, and PVSyst input conditions are based on the same assumptions. If the drawings and the simulation versions are misaligned, the basis for the estimated energy production becomes unclear.

Layout conditions affect not only energy production but also constructability and maintainability. If you focus solely on energy production without considering walkway widths, inspection access routes, mowing, drainage, nearby structures, snowfall, wind effects, and so on, problems can arise during the operational phase. PVSyst is a useful tool for assessing energy production, but it does not automatically evaluate all site conditions. When reviewing simulation results, verify that the input layout does not conflict with actual construction and maintenance conditions.

Checkpoint 4: Interpreting the Breakdown of Loss Items

What is particularly important in PVSyst results is the breakdown of losses. In solar power generation, the irradiance that reaches the PV modules does not directly become electrical energy. Generation is reduced by various factors such as reflection, temperature, shading, wiring, conversion, equipment characteristics, soiling, mismatch, and outages. By looking at the breakdown of losses, you can identify which factors have the greatest impact on the results.

When examining losses, first check the largest items. Rather than treating all losses with equal weight, it is important to prioritize those that have a large impact on power generation. For example, if shading losses are large, it may be necessary to reconsider the layout and surrounding obstacles. If temperature losses are large, check the installation method, ventilation conditions, and regional characteristics. If wiring losses are large, it may be necessary to consider cable length, conductor size, and the layout of junction boxes and equipment.

Temperature losses are an item often checked in photovoltaic power generation simulations. Photovoltaic modules heat up when exposed to solar irradiance, and their output decreases as temperature rises. Temperature conditions vary depending on roof-mounted or ground-mounted installation, rack height, backside ventilation, and the surrounding environment. If PVSyst results show large temperature losses, check not only the local ambient temperature but also whether the installation method matches the input conditions. If you enter conditions with better ventilation than reality, the estimated energy generation may be higher than actual.

Losses due to soiling are another factor that’s easy to overlook. Depending on the environment around the power plant, impacts from sand dust, pollen, bird damage, fallen leaves, dust from farmland, and particulates from factories and roads will vary. If soiling loss is set too low, actual generation during operation may fall below forecasts. Conversely, if it is set excessively high, the expected generation can become overly conservative. Take cleaning plans and the surrounding environment into account, and verify that the setting can be explained.

Wiring losses and conversion losses are factors related to electrical design. Wiring losses can increase under conditions such as long cables, high currents, or distant equipment placement. Conversion losses occur when converting DC power to AC power. These losses cannot be eliminated entirely, but they can sometimes be mitigated through design. When reading PVSyst results, verify that the losses are not inconsistent with the design conditions and are not estimated unrealistically low.

Mismatch losses and losses due to equipment characteristics are also important in practice. Solar cell modules have individual variations, and even with the same nameplate rating they do not produce exactly the same output. In addition, differences in irradiance conditions and how shading affects each string cause variations in energy yield. Simulation results change depending on how much of this variability you assume. When reviewing loss items, it is important to confirm that each value matches the site conditions and design intent, and that you can explain them.

Checkpoint 5: Verify shadow effects and time-of-day reductions

In solar power generation simulations, shading is a factor that has a significant impact on energy production. Causes of shading include surrounding buildings, trees, utility poles, fences, mountains, slopes, rows of mounting racks, and equipment. When reading the results from PVSyst, check how much shading loss is reflected. If the impact of shading is evaluated as smaller than in reality, the actual energy production may be lower than predicted.

When checking shadows, it's important to consider not only annual losses but also seasons and times of day. In winter the sun's elevation is low and shadows tend to extend farther. As a result, shadows that seem small on an annual basis can have a large impact on winter mornings and evenings. Conversely, in summer the sun's elevation is high, and the same obstacles may have less of an effect from shading. Checking monthly power generation and the seasonal variation in shading losses makes it easier to see when shadows have the greatest impact.

Shading at different times of day is also important. If morning shading is significant, the morning power generation ramp-up will be delayed. If evening shading is significant, afternoon power generation will end earlier. When shading occurs around noon, it tends to have a larger impact on energy production. Especially for large-scale projects, even small losses can translate into an annual energy difference that is difficult to ignore. When reviewing PVSyst results, check how much weight shading losses carry relative to energy production.

Shading is not simply a matter of area. Photovoltaic modules are composed of multiple cells, and even partial shading can affect the output of the entire circuit. The impact on power generation varies depending on where, at what times, and to what extent the shading occurs. Therefore, shading assessments must be verified together with on-site conditions and design drawings. It is not sufficient to be reassured just because shading has been included in simulations; it is important to check whether the height and position of obstacles are accurately reflected.

It is also necessary to consider the possibility that shading conditions may change in the future. Trees grow. In locations where buildings or structures may be erected nearby, future shading risks must be taken into account. After site development, slopes, retaining walls, drainage facilities, fences, and surveillance equipment may be added. Even if shading appears minimal at the initial design stage, changes in equipment locations on the final drawings can alter the shading impact. When using PVSyst results, be clear which layout (at what point in time) the shading assessment was based on.

The purpose of checking the impact of shading is not to make the estimated power generation look higher, even slightly. It is to identify in advance the causes of reduced power generation and to use that information as input for design decisions. Whether the option to maximize land use to increase capacity or to increase inter-row spacing to reduce shading is more advantageous depends on the project. By comparing PVSyst results, it becomes easier to evaluate the balance among capacity, shading, constructability, maintainability, and project viability.

Checkpoint 6: Confirm the performance ratio and monthly variations

In PVSyst results, the performance ratio is also an important parameter to check. The performance ratio is an indicator of how efficiently a photovoltaic (PV) system converts the solar irradiance it receives into electrical energy. For grid‑connected systems, it is evaluated based on factors such as the irradiance incident on the collector surface, the nominal capacity of the PV array, and the electrical energy delivered to the grid. Although the absolute value of generated energy is strongly influenced by irradiance, examining the performance ratio makes it easier to identify system losses and the impact of operating conditions.

When the performance ratio is extremely high, check whether loss settings are insufficient or whether input conditions are overly optimistic. Conversely, when the performance ratio is extremely low, check whether losses such as shading, temperature, wiring, conversion, soiling, and downtime have been set excessively large. However, installation conditions and regional differences also affect the performance ratio. In high-temperature regions, temperature losses tend to be larger, and in areas with a lot of shading the performance ratio tends to decrease. Rather than making a uniform judgment based only on the numerical values, it is important to interpret them together with the on-site conditions.

We also check monthly variations. Because solar power generation experiences seasonal changes in irradiance and solar elevation, monthly generation differs. There are also region-specific variations such as the rainy season, typhoon season, winter, and periods of snow accumulation. By looking at PVSyst’s monthly results, you can see which months have higher generation and which have lower. Annual values are often used in business planning, but monthly values are important for operations management. When comparing with actual results, you must evaluate them while taking month-to-month weather differences into account.

When looking at monthly performance ratios, trends that are not apparent from generation amounts alone become visible. For example, in summer, even with high solar irradiance, performance ratios can fall due to temperature losses. In winter, although solar irradiance is low, equipment efficiency tends to increase because of lower temperatures, while the low solar altitude can make the system more susceptible to shading. Spring and autumn often offer a favorable balance of temperature and irradiance, making them relatively conducive to power generation. Understanding these seasonal characteristics makes it easier to explain the results.

Performance ratio and monthly variability are also useful for monitoring after commissioning. By comparing the monthly values from the simulation with actual results, you can verify whether the power plant is operating as expected. However, when comparing actual results you need to take into account that year’s weather, downtime, output curtailment, snowfall, failures, cleaning status, and so on. Simply being lower than the simulated value does not necessarily indicate a problem. Conversely, even if it exceeds the simulated value for a short period, over the long term it may approach the average.

When using the performance ratio in internal documents or customer explanations, it is important to explain the meaning of the metric in an easy-to-understand way. If you explain it only with technical terms, the difference between it and energy production can be hard to convey. In practice, it is easier to understand if you describe it as an indicator of how efficiently the system is expected to generate electricity under solar irradiance conditions. When reading PVSyst results, view annual energy production, monthly energy production, and the performance ratio side by side to comprehensively grasp the plant's characteristics.

Checkpoint 7: Ensure consistent assumptions for performance comparisons

PVSyst simulation results are also used for comparing actual performance after the plant begins operation. However, when comparing simulated and actual values, it is extremely important to align the assumptions. If you compare them with differing assumptions, you cannot determine whether there is a problem with the plant, simply different weather conditions, or the effects of outages or control actions. From the stage of reading PVSyst results, it is important to organize the conditions so they can be used for future performance comparisons.

First, align the periods being compared. Whether you compare annual values, monthly values, or daily values changes the meaning of the evaluation. For daily or monthly comparisons, weather has a large influence and short-term differences are more likely to appear. Looking at annual values evens out those variations, but differences can still occur if the weather in a particular year deviates from the long-term average. Do not judge the validity of a simulation based only on short-term results; you need to be aware of the length of the period and the weather conditions.

Next, align the measurement points for generated energy. Confirm which point's electrical energy the simulation output represents. Depending on whether it is DC-side generation, AC-side output, or a value close to the point of common coupling, the appropriate comparison target in the actual data will differ. Measured data are also affected by factors such as the instrument installation location, data acquisition methods, missing data, corrections, and communication failures. If the definitions of the values being compared differ, an accurate evaluation cannot be made.

It is also necessary to take downtime and output curtailment into account. If a plant is shut down due to inspections, faults, communication failures, grid-side constraints, maintenance work, or the like, actual generation will naturally decrease. Simulations often assume normal operation, so it should be clarified whether the effects of downtime are excluded when evaluating performance or included as availability in the project’s business plan. In regions or projects where output curtailment occurs, how it is treated is also important. Care must be taken not to confuse reductions caused by curtailment with a decline in plant performance.

Also, when actual solar irradiance measurements are available, it is useful to compare while taking irradiance conditions into account. If the actual weather is worse than normal, it is natural for energy production to be lower than the simulation. Conversely, if there are more sunny days, production may exceed the simulation. Comparing only generation without considering irradiance can lead to incorrect judgments about the condition of the equipment. In practice, it is desirable to check power generation, solar irradiance, performance ratio, and outage information together.

When using PVSyst results for comparison with actual performance, version control of the simulations is also important. Layouts and equipment configurations can change between early design, pre-construction, and completion stages. If you use simulations from an initial proposal for performance comparisons, they may not match the actual installed conditions. Save the simulation results based on the final design and make it clear under which conditions they were created. Organizing file names, creation dates, input parameters, drawing numbers, and equipment configurations will make later verification easier.

The purpose of performance comparison is not to assign blame, but to determine whether the power plant is operating as expected and to detect abnormalities early. By using simulation results as a baseline and evaluating them in light of solar irradiance conditions and downtime information, it becomes easier to spot signs of soiling, faults, increased shading, equipment degradation, and measurement anomalies. It is important to organize PVSyst results as information that can be used not only in the planning stage but also during the operational stage.

Points to note when using PVSyst results in internal documents and proposals

The results from PVSyst are often used in internal documents and proposals, but care is required in how they are handled. Simulation results are merely predicted values based on the input conditions. They do not guarantee future power generation. Therefore, in materials avoid definitive expressions such as "will definitely generate" or "will certainly be achieved," and make clear that the figures are projections based on the stated assumptions. In practice, it is important not to confuse forecasts with guarantees.

When presenting in materials, showing not only annual power generation but also the main assumptions reduces misunderstandings. Organizing assumptions such as the installation location, system capacity, orientation (azimuth), tilt angle, the meteorological data used, the main loss settings, and whether shading was evaluated makes it easier to convey the basis for the results. You do not need to include every detail in the main text, but avoid omitting conditions that affect decision-making.

When comparing multiple options, make the comparison conditions consistent. For example, if you only want to compare layout options, you need to keep the meteorological data, equipment conditions, and loss settings the same. If meteorological data, equipment conditions, and loss settings differ when comparing, you won't be able to tell what caused the change in power output. In comparison materials, clearly indicate the conditions that were changed and those that were held fixed to make evaluation easier.

Also, be careful when rounding power generation figures. Showing too many digits can make the forecast appear more precise than it actually is. Although simulations perform detailed calculations, there is uncertainty in weather and operations. In documents, it is more important to clearly explain the assumptions and trends than to emphasize unnecessarily precise digits. Especially in customer-facing materials, wording that prevents the reader from being misled is required, not just numerical accuracy.

When design, sales, construction, and maintenance personnel within the company are looking at the same results, it is necessary to standardize their understanding of terminology. Terms such as DC capacity, AC capacity, annual energy generation, performance ratio, losses, output control, and downtime may be interpreted differently depending on the area of responsibility. When sharing PVSyst results, clarify which figures will be used for which purposes and provide supplementary explanations as needed.

Furthermore, when linking PVSyst results to site management and operations management, it is important not to rely solely on simulations. By combining them with site surveys, design drawings, construction records, equipment information, monitoring data, and inspection records, you can understand the power plant’s condition more accurately. Simulations are the foundation of planning, while actual performance data are the foundation for operational improvements. Managing and connecting both makes it easier to identify the causes of reduced power generation.

What matters when leveraging PVSyst results is not making the numbers look neat, but organizing them into information that can be used for decision-making. If you can explain why energy production is high, why it is low, why losses are large, the differences between comparison scenarios, and the causes of discrepancies with actual performance, the credibility of the documents increases. When looking ahead from planning through operation of a solar power plant, you need both the ability to interpret simulation results and the ability to organize field data.

Summary: Interpret simulation results in light of assumptions and losses

PVSyst is simulation software for predicting the power generation of photovoltaic systems and for verifying how design conditions and loss assumptions affect the results. However, you cannot fully rely on the reported annual generation as-is for practical use. When reading simulation results, it is important to check, in order, the assumptions for annual generation, irradiation, meteorological data, system capacity, layout, losses, shading, performance ratio, monthly variability, and comparisons with actual performance.

What’s especially important is being able to explain the background behind the numbers. If the annual energy production is high, check why it is high. If it is low, check which losses are having an effect. By sorting out whether shading is significant, temperature losses are large, wiring or conversion losses are large, or the meteorological data assumptions are conservative, you can link this to design improvements and risk explanation. Generation forecasts are not just numbers; they are material for design decision-making.

Also, PVSyst results are useful not only during the planning stage but also for comparing actual performance after the start of operation. By checking monthly energy production and the performance ratio, it becomes easier to identify deviations from actual results. However, when comparing with actuals, it is necessary to consider weather, outages, curtailment, measurement location, missing data, and so on. If the assumptions behind the simulation and the actuals are not aligned, a correct evaluation cannot be made.

When practitioners handle PVSyst results, it is more effective to decide on an order of checks to follow than to memorize overly technical formulas. First look at the annual energy production, then check the meteorological data, system conditions, loss breakdown, shading, performance ratio, and the assumptions for comparing with actual results. If you make this sequence a habit, you will reduce oversights in reports and improve the accuracy of explanations to internal stakeholders and customers.

A large amount of information is generated at the design, construction, and operation stages of a solar power plant. By correctly interpreting simulation results and connecting them with on-site information for management, it becomes easier to identify the causes of reduced power generation. If you want to use generation forecasts not only for business planning but also for operational improvement, it is important to organize PVSyst results, field data, monitoring data, and inspection records under the same assumptions. An approach that interprets not only the numerical results but also the assumptions and losses leads to improved planning accuracy and operational performance of solar power plants.