What Does PVSyst Predict? An Introduction to Power Generation Simulation

PVSyst is a representative energy-yield simulation software used to estimate in advance the electricity production of photovoltaic (PV) power systems. For practitioners who will be involved in the design, financial assessment, pre-construction verification, and operation planning of solar power plants, it is important to understand what PVSyst predicts, how much those predictions can be trusted, and how to interpret the results. The true purpose of generation simulation is not merely to look at an annual energy production figure, but to comprehensively organize factors such as irradiance, system conditions, shading, temperature, conversion losses, wiring losses, degradation, and outage causes, so that the plant’s plan can be explained with numbers.

Table of Contents

• What is PVSyst used for as an energy-yield simulation?

• The primary quantity PVSyst predicts is annual energy yield

• Understand the flow from solar irradiance to energy production

• Break down losses to confirm the basis for the predicted energy yield

• Input conditions required for the simulation

• Key indicators to check on the results screen

• How to use PVSyst's prediction results in practice

• Precautions to avoid overreliance on predicted values

• On-site data and positioning accuracy support energy-yield prediction

• Summary

What is PVSyst used for in power generation simulation?

PVSyst is an energy-yield simulation used at the planning stage of solar power generation systems to quantify how much electricity can be expected. Even with the same installed capacity, actual energy production can vary greatly depending on the installation region, orientation, tilt angle, presence or absence of shading, temperature conditions, equipment configuration, wiring design, and maintenance condition. Therefore, it is not possible to simply judge expected generation by installed capacity alone. PVSyst organizes these multiple conditions and calculates the process by which solar irradiation reaches the photovoltaic cells, is generated as direct-current power, and, after passing through conversion equipment, is output as alternating-current power.

The purpose of power generation simulations is not to predict the future perfectly. What matters in practice is to clarify the assumptions, produce reasonable forecasts based on consistent calculation rules, and present them in a form that can be used for design and business decisions. For example, you can compare how generation changes when the layout on the same site is altered, how much shading impacts output over the year, and how losses change when the equipment configuration is modified. In other words, it's easier to understand PVSyst if you think of it not as a tool that delivers a single answer but as a tool that creates a common basis for comparing multiple design proposals.

Many practitioners who search for "What is PVSyst" have heard the name but are unsure what to input, what will be output, and which numbers to focus on. PVSyst does not deal only with generation figures. It treats, as a continuous process, meteorological data, sun position, irradiance on the array plane, equipment performance, temperature effects, shading effects, various losses, and the final output energy. Therefore, to interpret the results correctly, you need to look not only at the generation numbers but also at the calculation process that leads to those numbers.

The primary quantity that PVSyst predicts is annual energy production

The most important item PVSyst predicts is the annual energy production. In practice, how much energy can be obtained annually is directly linked to equipment planning, financial feasibility studies, power consumption planning, grid interconnection studies, and maintenance planning. Annual energy production is a central metric that indicates how much electricity a solar power installation generates over the course of a year and is an indispensable figure for assessing the value of a power plant. However, annual energy production does not exist as a single fixed value; it varies depending on the combination of input conditions and assumptions.

In PVSyst, energy production is estimated based on the meteorological conditions at the site, taking into account monthly solar irradiance and temperature. Because solar power’s irradiance conditions change with the seasons, looking only at the annual total makes it difficult to see when generation is high and when it is low. Even in regions with high irradiance in summer, if output loss due to high temperatures is large, energy production may not increase as much as expected. Conversely, generation efficiency can be higher in seasons with lower temperatures and more stable irradiance conditions. With PVSyst results, examining monthly variations as well as annual values allows you to grasp the plant’s characteristics more concretely.

Predictions of annual energy production are used not only to compare the size of power plants but also to evaluate generation efficiency per unit of installed capacity. While it is natural that larger installations produce more energy, in practice it is necessary to check how efficiently they generate power for the same capacity. When assessing whether azimuth and tilt are appropriate, whether shading has too great an impact, or whether the equipment configuration is feasible, looking at energy production relative to installed capacity is more useful than simple energy output. PVSyst organizes the metrics needed for these comparisons and makes it easier to understand the differences between design proposals.

Annual energy production also serves as an important basis for business planning. When installing solar power generation equipment, decisions about system size, installation timing, and operational policy are made based on how much electricity can be expected. The forecast values from PVSyst serve as the starting point for that assessment. However, those forecasts are merely calculation results based on the conditions set. Actual generation will vary due to that year’s weather, maintenance condition, changes in the surrounding environment, equipment aging, and other factors. Therefore, it is important to treat PVSyst’s annual production figures not as definitive future numbers but as reasonable reference values for explaining the design conditions.

Understanding the flow from solar irradiance to power generation

A key point in understanding PVSyst is that solar irradiance does not directly become generated electricity. In photovoltaic power generation, you first consider how much solar energy reaches the site. Then you convert that into the irradiance on the surface actually received by the solar cells, and further subtract the effects of cell temperature and performance, shading, soiling, wiring, and power conversion equipment to determine the final output energy. PVSyst performs these calculations sequentially and makes it clear at which stage and by how much energy has been lost.

The first important point is the difference between the solar irradiance reaching a horizontal plane and the irradiance reaching the surface of a solar panel. Solar panels are often installed with a fixed tilt angle and orientation. Because the sun’s elevation and azimuth change with the season and time of day, the irradiance reaching a horizontal plane does not match the irradiance incident on an inclined surface. PVSyst estimates the irradiance on the surface actually used for power generation by taking into account the installation surface’s orientation and angle. At this stage, it becomes clear that the orientation and tilt settings have a large impact on the amount of power generated.

Next, consider the process by which a solar panel converts the incident solar irradiance into DC power. Solar cells generate power when exposed to irradiance, but their output is influenced by ambient temperature and cell temperature. In general, solar cells tend to lose output as temperature rises, so even during periods of strong irradiance, losses due to temperature can be significant. PVSyst reflects temperature-induced output reductions by taking into account ambient temperature, wind, mounting configuration, and other factors. This makes it apparent that locations with simply higher irradiance are not always more advantageous.

Furthermore, the DC power is converted to AC power through a power conversion device. Losses due to conversion efficiency also occur during this process. If wiring is long or design conditions are not appropriate, wiring losses and mismatch losses can increase. PVSyst aggregates these losses at each stage and calculates the final amount of electricity that can be extracted from the power plant. When reading the results, it is important not to look only at the final value but to trace the flow from solar irradiance to the final output. Once you can understand that flow, it becomes easier to explain why generation is high or low and where there is room for improvement.

Break down the losses to verify the basis for power generation

A major feature of PVSyst is that it can break down and examine the losses that occur on the way to the final energy production. In solar power forecasting, looking only at the final annual energy yield often makes it difficult to judge whether that figure is reasonable. When the generated energy is lower than expected, you must distinguish whether the cause lies in the irradiation conditions, shading, temperature, or equipment configuration in order to make design improvements. PVSyst enables you to inspect the results in detail by showing at which stages the energy was lost.

Representative losses include losses due to shading, losses from angle of incidence, soiling losses, temperature-related losses, losses from variability between pieces of equipment, wiring losses, losses in power conversion equipment, and losses related to downtime and operational constraints. Each of these may appear small in percentage, but when they accumulate they can create large differences in annual energy production. In particular, the effect of shading can have a major impact on specific circuits even for short periods. Surrounding buildings, trees, terrain, and the spacing between racking rows are factors that should be considered when forecasting energy production.

Temperature losses are also important when verifying the basis for expected energy production. While photovoltaic power generation tends to increase with stronger solar irradiance, there is also the aspect that output falls as the temperature of the solar cells rises. Heat dissipation conditions can differ between rooftop installations and ground-mounted installations. Estimates of temperature losses change depending on the installation method and how ventilation conditions are set. Viewing temperature losses in PVSyst makes it easier to understand how meteorological conditions and the mounting structure relate to energy production.

Also, breaking down losses is useful when preparing materials to explain energy generation. In internal briefings and discussions with stakeholders, simply saying "the annual generation is about this" may be insufficient. It is important to be able to explain why that generation occurred, which losses are large, and whether changes to the design could improve it. By reading PVSyst’s loss breakdown, you can treat the generation figure not as a mere prediction but as a number with a documented decision-making process. The practical value of generation simulation lies in this explainability.

Input Conditions Required for the Simulation

To predict power generation with PVSyst, organizing the input conditions is indispensable. No matter how advanced the calculations, if the input conditions are ambiguous, the reliability of the results will not improve. In power generation simulations, you set the site's location information, meteorological data, installation azimuth, tilt angle, system capacity, photovoltaic panel specifications, power conversion equipment specifications, circuit configuration, wiring conditions, shading conditions, and operational loss conditions, among others. These conditions are also basic information that should be collected during the power plant planning stage.

Location information is fundamental for calculating solar irradiation conditions and the sun’s position. Even within the same region, generation conditions vary depending on site elevation, surrounding topography, and nearby obstructions. If the power plant’s location is not correctly identified, mismatches can occur when correlating meteorological data and evaluating shading. Moreover, the installation azimuth and tilt angle are important factors that determine how solar panels receive sunlight. If the azimuth shown on design drawings differs from the actual site bearing, simulation results and actual generation trends may diverge.

Equipment specifications are also important. Solar panels have rated power, temperature characteristics, efficiency, and electrical characteristics. Power conversion equipment has input range, conversion efficiency, capacity, and control conditions. If these combinations are not appropriate, output limiting or conversion losses can be significant even when solar irradiance is sufficient. In PVSyst, energy production is calculated based on equipment specifications, so the validity of the specification values entered directly affects the results. In practice, it is necessary to verify during the design phase that the equipment information being used matches the latest project plan.

Do not overlook the input of shading conditions. Around a power plant there may be elements that cast shadows, such as buildings, trees, utility poles, slopes, mountains, and adjacent equipment. For large-scale ground-mounted installations, mutual shading between rows also needs to be considered. Because shading changes with the seasons and time of day, it cannot be judged solely based on impressions from a site visit. When taking shading into account in PVSyst, how accurately the site’s topography and obstacles can be represented is important. The more accurate the input conditions, the greater the explanatory power of the energy yield prediction.

Main indicators to check on the results screen

When reviewing PVSyst results, it is easier to understand if you first check the annual energy production, then look at monthly production, irradiance, losses, and performance indicators in that order. The annual energy production is the figure that shows the overall picture, but in practice monthly imbalances are also important. Whether production is concentrated in a particular season or stable throughout the year affects how you approach electricity utilization planning and maintenance planning. By looking at monthly production, you can confirm how weather conditions and changes in solar altitude are reflected in the results.

Next, check the solar irradiance on the array plane. If the irradiance reaching the solar panels is insufficient, the final energy yield will not increase. However, if irradiance is high yet energy yield is low, you should suspect other factors such as temperature losses, shading, equipment configuration, or output limitations. Use PVSyst’s results not to view irradiance and energy yield in isolation, but to trace how irradiance is converted into electrical energy. Mastering this perspective allows you to make deeper assessments than simple comparisons of energy yield.

Performance indicators are also important. When comparing the performance of power plants, you need to look not only at the installed capacity but also at how efficiently they converted the incident solar radiation into electricity. If performance indicators are low, there may be room for improvement in shading, temperature, equipment configuration, wiring, shutdown conditions, and so on. Conversely, if performance indicators appear excessively high, you should check whether the input conditions are overly optimistic. In power generation simulations, a good number does not mean it is correct; what matters is whether that number is derived from realistic assumptions.

On the results screen, also check the magnitude and order of the losses. If a single loss is disproportionately large, that item may represent a design issue. For example, if shading losses are large, there may be room to review the layout or how obstacles are handled. If temperature losses are large, it is necessary to check the installation method and ventilation conditions. If conversion losses or wiring losses are large, you should reconsider the equipment configuration and wiring plan. The ability to read PVSyst results is the ability to trace back from the final numbers to the causes and link that to the necessary verification work.

How to Use PVSyst's Forecast Results in Practice

PVSyst's prediction results can be widely used in power plant design studies. In the early stages, they can be used to compare the generation potential of candidate sites. Even on sites where the same system capacity can be installed, the amount of energy generated will vary if azimuth, tilt, surrounding environment, or shading conditions differ. By performing generation simulations, the advantages and disadvantages of sites can be quantified, making it easier to determine project priorities. Its practical value lies in enabling comparisons based on defined assumptions rather than on intuitive judgment.

At the detailed design stage, it can be used to evaluate layout and equipment configurations. Changing the orientation and tilt of the solar panels, row spacing, circuit configuration, and the capacity ratios of power conversion equipment alters the energy yield and losses. By using PVSyst, you can compare differences between design proposals and examine which option offers the best balance of energy yield, constructability, and maintainability. The proposal with the highest energy yield is not always the optimal choice. It is necessary to make judgments that also take into account site conditions, construction constraints, maintenance access routes, and ease of future management. PVSyst's predicted results serve as one of the inputs for that judgment.

They can also be used as explanatory materials for stakeholders. In power plant planning, people from multiple roles are involved, such as design, construction, business, and maintenance representatives. If each does not share the same assumptions, their perceptions of the expected power generation will diverge. By using PVSyst results, you can explain which meteorological conditions were used, under which equipment conditions the calculations were performed, and which losses were anticipated. This makes the power generation figures shared not as mere wishes but as the results of an analysis based on stated assumptions.

Simulation results remain useful even after operations begin. By comparing actual generation with predicted generation, you can verify whether the system is operating as expected. However, when evaluating deviations from the predictions, you need to take into account that year’s weather variations. Lower-than-predicted generation does not necessarily indicate equipment failure immediately; solar irradiance may be lower than the long-term average. Conversely, if irradiance is sufficient but generation does not increase, you should check for soiling, shading, equipment faults, output curtailment, measurement errors, and so on. PVSyst’s predicted results can also be used as a benchmark for post-operation anomaly detection and improvement considerations.

Precautions to Avoid Overreliance on Predicted Values

PVSyst is a useful software tool for forecasting power generation, but it is risky to treat the calculation results as absolute answers. Power generation simulations are estimates based on input conditions and calculation models. They do not fully predict future weather, nor do they automatically reflect all changes in the local environment. Therefore, when using PVSyst results, you need to check the validity of the input conditions, the conservativeness of the assumptions, and the interpretation of the results together.

Particular attention should be paid to how meteorological data are handled. Solar power output is heavily influenced by solar irradiance, but the characteristics of meteorological data vary by location and period. When using data from nearby sites, discrepancies with the actual site conditions can arise. In mountainous areas, along coastlines, in urban areas, and in snow-prone regions, solar irradiance and temperature trends can differ even over short distances. For power generation forecasts, it is important to check not only the source of the meteorological data but also its consistency with the target site.

Care must also be taken with shading inputs. If on-site obstructions are not fully reflected, shading losses can be underestimated. Even if there are no nearby shading objects at the planning stage, trees may grow or the use of adjacent land may change in the future. In particular, during periods of low solar altitude, shadows can be much longer than expected. Underestimating the impact of shading can easily lead to discrepancies between predicted and actual values.

Also, if loss conditions are set too optimistically, the estimated power generation will be overstated. Dirt, wiring, equipment variability, downtime, and degradation may each appear small on their own but become non-negligible differences over a year. In power generation simulations, it is important not to manufacture the numbers you want to see, but to realistically reflect conditions that can occur on site. To improve the accuracy of forecasts, you need to determine input conditions not only through software operation but also by understanding site surveys, design drawings, construction conditions, and maintenance policies.

On-site data and positioning accuracy support power generation forecasting

To improve the accuracy of energy yield simulations in PVSyst, the quality of on-site data is crucial. Predicting energy production may seem to be completed by entering conditions on the screen, but in reality it is supported by basic information such as the site's location, orientation, elevation differences, obstacles, site boundaries, and the available installation area. If there are discrepancies in this information, they will affect the installation angle, shadow estimates, and layout planning, and will ultimately lead to differences in the predicted energy yield. In particular, if a simulation is run while the site's orientation or obstacle locations are left ambiguous, the results may appear coherent on screen yet not match the conditions of the actual power plant.

In photovoltaic power generation facilities, slight elevation differences within the site and the positions of surrounding objects can influence the design. For ground-mounted installations, it is necessary to consider racking layout, row spacing, site grading conditions, drainage planning, and maintenance access paths. For rooftop installations, roof pitch, orientation, level differences, installed equipment, handrails, and shadows from surrounding buildings affect power generation. To accurately grasp this information, not only drawings but also positioning data and records obtained on site are useful. Bringing the assumptions of power generation simulations closer to actual site conditions is the first step to improving the reliability of the results.

Also, even at the stage of comparing predicted results with actual performance, on-site information is important. After operations begin, if power generation differs from expectations, it is necessary to determine whether the cause is weather conditions, shading, equipment, or misalignment during installation. In that case, if accurate location information and on-site records are retained, it becomes easier to isolate the cause. Simulation is not something that ends with desk calculations; it is connected to each stage of site survey, design, construction, and operation.

For acquiring such on-site data, a system that makes it easy to handle high-precision positional information is useful. In power plant planning and inspections, if you can accurately record the site, obstacles, equipment locations, and verification points, it becomes easier to improve the validity of the conditions entered into PVSyst. Not separating on-site surveys and power generation simulations, and linking them within the same planning documentation, leads to improved accuracy in practice.

Summary

PVSyst is a generation simulation tool for predicting the energy output of photovoltaic systems and for organizing the underlying solar irradiance, equipment conditions, shading, temperature, wiring, conversion, and various losses. The primary output is annual energy production, but in practice what matters is not just checking that number. It is important to understand the flow by which solar irradiance reaches the PV panels, becomes DC power, passes through conversion equipment, and ultimately becomes the final electrical energy, and to grasp at which stages and what kinds of losses occur.

Power generation forecasts can be used to compare design proposals, assess business plans, explain to stakeholders, and evaluate actual performance after operations begin. On the other hand, forecast values are heavily influenced by input conditions. If meteorological data, azimuth, tilt angle, equipment specifications, shading conditions, loss assumptions, or site location information are inaccurate, no matter how clear the report, the reliability of the results will not improve. To master PVSyst, it is essential not only to know how to operate the software but also to understand the meaning of the input parameters and how they correspond to the actual site.

Practitioners who will be using PVSyst should begin by correctly grasping "what the software is intended to predict." PVSyst is not meant to perfectly forecast future power generation; rather, it is a tool to explain generation based on realistic assumptions and to support design decisions. When generation figures are produced, it is important to check the underlying irradiance, losses, equipment conditions, and site conditions, and to revise input assumptions as necessary.

To improve the accuracy of power generation simulations, the quality of on-site location information and positioning data is also important. If site boundaries, obstacles, equipment locations, and verification points can be recorded accurately, the basis for the conditions entered into PVSyst becomes clear, making it easier to manage consistently from design through construction to post-operation verification. When prioritizing the efficiency of site surveys and high-precision location recording, utilizing LRTK (an iPhone-mounted GNSS high-precision positioning device) makes the on-site data required for solar power planning easier to handle and helps clarify the assumptions for power generation simulations.