【How to Interpret PVSyst PR? 5 Key Points to Check When Reviewing Results】

Table of Contents

• Basics you should understand before looking at PVSyst's PR

• Why you shouldn't judge good or bad based solely on PR

• Check item 1: Check consistency between annual generation and PR

• Check item 2: Read the factors lowering PR from the loss diagram

• Check item 3: Confirm monthly PR variability and seasonal fluctuations

• Check item 4: Review the validity of input conditions

• Check item 5: Be able to explain deviations between site conditions and design conditions

• How to proceed when checking PR in practice

• To improve the accuracy of PR checks, the quality of on-site data is important

• Summary

Basics You Should Understand Before Looking at PVSyst's PR

One of the indicators that frequently appears when reviewing PVSyst results is PR. PR stands for Performance Ratio, and in Japanese it is sometimes called 性能比 or システム性能比. It is a representative metric used to check how efficiently a solar power installation produces electrical energy relative to the incident solar irradiation. When you are starting to learn how to use PVSyst, you tend to focus on annual or monthly energy production, but in practice PR is an item you should always verify.

PR is not simply a number for judging whether power generation is high or low. Power generation is greatly influenced by installed capacity, the solar irradiance at the installation site, orientation, tilt angle, shading, temperature, and loss conditions. On the other hand, PR is an indicator that evens out differences in solar irradiance conditions to some extent, making it easier to compare the overall efficiency of a system. Therefore, it is useful when comparing regions with different solar irradiance despite having the same installed capacity, or when checking how system efficiency changes when design conditions are changed.

However, PR is not a simple number where higher is always better and lower is always worse. In solar power generation systems, generation output is reduced by various factors such as output reduction due to temperature rise, wiring losses, conversion losses, mismatch, soiling, shading, snow accumulation, and downtime. Because PR is determined as a result that reflects these losses, you must always check PR together with a breakdown of its components.

In PVSyst's result screens and reports, PR is displayed not only as an annual value but can also be read as monthly values and within the flow of the loss diagram. What practitioners should grasp first is to treat PR not as a single evaluation metric, but as a checkpoint that connects energy yield, irradiation, losses, input conditions, and site conditions. PR is a summary of results, but at the same time it serves as an entry point indicating where to dig deeper.

Why you shouldn't judge whether something is good or bad based only on PR

When you run a simulation in PVSyst, the report shows the annual energy production and the PR. Someone seeing the results for the first time might be tempted to judge the plan as "good" or "bad" based solely on the PR number. However, in practical work it is dangerous to draw conclusions from PR alone. PR is an important indicator, but it is not an all-purpose evaluation metric that represents the entire design.

For example, in locations where shading has a significant impact, the PR may read low. In this case, the cause of the low PR might not be the system’s own performance but could lie in surrounding terrain or structures, inter-row spacing, racking arrangement, and so on. Also, in regions prone to high temperatures, PR can decrease due to temperature-related losses; this is not necessarily a design error of the equipment but a factor that is difficult to avoid as a meteorological condition.

Conversely, a high PR may not be reassuring. In cases where the input loss assumptions are too optimistic, shading settings are insufficient, soiling or downtime rates are not taken into account, or the terrain’s slope and elevation differences are not adequately reflected, PR can appear better than reality. In other words, because PR is strongly influenced by the input conditions, you need to check not only how the numbers look but also the assumptions under which those numbers were calculated.

Also, although PR and annual energy generation may seem similar, their roles are different. Annual energy generation is important when considering the project's viability and the expected amount of electricity. PR, on the other hand, is an indicator of how effectively the equipment converts solar irradiance into electrical energy. In regions with very high solar irradiance, annual energy generation can be large even if PR is somewhat low. Conversely, in regions with low solar irradiance, annual energy generation can be limited even if PR is high.

Therefore, when reviewing PVSyst results, it is important not to look at PR first but to consider it together with energy production, solar irradiation, the loss diagram, monthly variations, and input conditions. PR is a starting point for result verification, but relying on it as the basis for a final judgment requires consistency with the surrounding information.

Check Item 1: Verify the consistency between annual energy generation and PR

When checking PR in PVSyst, the first thing to look at is the consistency with the annual energy production. PR is an indicator of system efficiency, but what is ultimately prioritized in practice is how much electrical energy the planned installation will generate. Therefore, rather than looking at PR alone, you should simultaneously verify whether the annual energy production is reasonable relative to the design capacity and solar irradiation conditions.

If annual power generation is lower than expected, it is necessary to determine whether the PR is also low or whether the solar irradiation itself is low. If the site has low solar irradiation, annual power generation will be low even if the PR is standard. On the other hand, if solar irradiation is sufficient but annual power generation does not increase and the PR is low, there may be significant system losses. The important point here is to check the relationship between solar irradiation and power generation before looking at the PR figures.

It is also important to have a sense of annual power generation relative to installed capacity. If past projects or internal standards are available, compare with projects in similar regions, with similar orientations, and similar tilt angles. Even when there are no comparable cases, take into account the site’s solar irradiation conditions, the panel surface tilt, the presence or absence of shading, and the system’s oversizing conditions, and confirm whether the generation is not unnaturally too high or too low.

When PR and annual energy output are high, things may look good at first glance, but even then you need to verify the input conditions. Check whether loss rates have been set too low, whether surrounding shading has been accounted for, whether the temperature assumptions are appropriate, and whether the downtime and soiling assumptions are realistic. In particular, in early-stage assessments the conditions are often simplified, which can lead to PR being overestimated.

If PR is low and annual energy production is also low, the next step is to determine which losses are large. Depending on whether it is shading, temperature, wiring, the capacity setting of power conversion equipment, or unfavorable azimuth or tilt, the countermeasures will differ. A low PR result alone does not determine the corrective measures. From a practical standpoint, it is advisable to verify consistency with the annual energy production and then proceed to the loss diagram.

Checklist Item 2: Identify the factors that are lowering PR from the loss diagram

The most important thing when checking PR in PVSyst is reading the loss diagram. The loss diagram is a flow that shows how much is reduced at each stage from when solar irradiance enters the photovoltaic system until it becomes the final output. If you feel the PR is low, looking at the loss diagram makes it easier to trace which factors are affecting the figures.

Losses can be broadly classified into irradiance-side losses, module-side losses, and system-side losses. On the irradiance side, factors include incident conditions determined by azimuth and tilt, shading from nearby obstacles, shading from terrain and between rows, and losses due to reflection and angle of incidence. On the module side, factors include output reduction due to temperature rise, module characteristics, mismatch, and degradation over time. On the system side, factors include DC wiring, conversion equipment, AC wiring, and downtime.

One of the most noticeable factors that lowers PR is temperature loss. While solar power generation systems tend to produce more power with stronger solar irradiance, module output decreases as module temperature rises. Therefore, even in regions with high irradiance, PR can be reduced by temperature conditions. When temperature loss is large, the installation method, ventilation conditions, racking height, and surrounding environment should also be checked.

Losses from shading also have a major impact on PR. If there are surrounding buildings, trees, slopes, or inter-row shading within the installation, annual energy production will decrease and PR will drop. If shading is not modeled in detail, PR can appear higher than the actual value, so caution is required. This is especially true in locations with undulating terrain or nearby obstacles, where simplifying shading reduces the reliability of the results.

Wiring losses and conversion losses may each look small as individual numbers, but when added together they can have a non-negligible impact. If cable length, voltage conditions, equipment capacity, or circuit configuration are not appropriate, they can cause the PR to decrease. When you are not yet familiar with using PVSyst, you may end up calculating with default or provisional values, but in practice it is necessary to review them to match the design conditions.

When looking at a loss plot, it is important not only to identify which loss is largest but also to consider whether that loss is realistic. A large loss is not necessarily a problem if it is reasonable given local conditions. Conversely, if a loss is too small, you may have overlooked necessary conditions. In PR reviews, it is essential to use the loss plot to check not only the "reasons it is low" but also the "reasons it is too high."

Check Item 3: Confirm monthly PR variability and seasonal fluctuations

PR is better understood by examining monthly variations as well as the annual value. Even if the annual PR looks typical, large month-to-month deviations may indicate that seasonal factors or input conditions require attention. When reviewing PVSyst results, after checking the annual values, examine the monthly PR and monthly energy production to see if there are any unnatural variations confined to specific seasons.

One of the main reasons monthly PR fluctuates is differences in solar altitude. In winter, the solar altitude is lower, making the system more susceptible to the influence of surrounding obstacles and inter-row shading. Therefore, if PR is lower in winter, it is necessary to check the shading settings and layout conditions. Especially during periods with a lot of low-angle sunlight, even slight obstacles can have a large impact through shading.

In summer, while solar irradiance increases, temperature-related losses also tend to become larger. Because module temperature rises reduce power output, if PR decreases in summer, temperature losses may be influencing it. At such times, if you look only at monthly generation, summer may appear to have higher generation. However, when viewed by PR, the efficiency relative to the received solar irradiance may be lower.

In regions with a rainy season or a snowy period, monthly PR fluctuations can be even larger. If the handling of meteorological data, settings for snow accumulation and soiling, or operational shutdown conditions are not properly reflected, the monthly results may seem off. Checking monthly PR is effective for identifying seasonal issues that are hard to see in annual values.

When looking at monthly PR, simply comparing the highest and lowest months is insufficient. Read it in conjunction with irradiance, ambient temperature, shading, and the breakdown of the loss diagram. If PR is extremely low for a particular month, suspect that month’s shading, temperature, meteorological data, shutdown conditions, or input errors. Conversely, if PR is extremely high for only one month, also check whether the condition settings are unnatural.

In practice, it is important to be able to explain monthly PR. When asked in a report or internal review “why did the PR drop only in this month?”, being able to explain it from the perspectives of temperature losses, shading, solar irradiance conditions, snow, soiling, downtime rate, and so on increases confidence in the simulation results. Checking monthly trends rather than stopping at annual PR is the key to bringing PVSyst result verification closer to an operational level.

Checklist Item 4: Review the validity of input conditions

PVSyst's PR is strongly affected by the input conditions. In other words, whether the PR figure is correct is determined not only by the results screen but also by whether the input assumptions match reality. Because the simulation software calculates based on the entered conditions, if the assumptions are inaccurate the results will also be inaccurate.

First, what I want to check are the site information and the meteorological data. Confirm whether the installation location is set correctly, whether the latitude, longitude, and elevation match the actual site, and whether the meteorological data being used is appropriate for the purpose of the analysis. Even a slight misplacement of the site can result in different terrain or solar radiation conditions. Differences in elevation and meteorological conditions affect the assumed temperature and solar radiation, and consequently influence the PR.

Next, check the azimuth and tilt angle. Azimuth and tilt are fundamental conditions that directly affect the incident solar irradiance. Whether the orientation is close to south or east-west, and whether the tilt is large or small, will change the monthly power generation and PR trends. It is important to confirm that the values on the design drawings match the values entered into PVSyst and that they have not been left as provisional entries.

Equipment capacity and circuit configuration are also important. If the number of modules, the number of series strings, the number of parallel strings, the capacity of conversion equipment, and the oversizing conditions are not appropriate, the apparent clipping and conversion losses will change. When PR is low, it may not simply be that losses are large; the capacity design conditions may not be appropriate. Conversely, if PR is too high, it could mean that constraints that should occur in reality have not been sufficiently accounted for.

We always review the settings for loss conditions. Dirt, wiring, mismatch, downtime rate, degradation over time, shading, and temperature conditions directly affect PR. During the initial study phase, general values may be used, but in detailed design and project feasibility studies they must be updated to match local conditions and design specifications. If the company has internal standard values, verify that they comply with those standards.

When validating input conditions, it is effective to review the results first and then return to the assumptions. Rather than adjusting settings because the PR is low, check why it is low using loss diagrams and monthly values, and review the input conditions related to the cause. The way to use PVSyst that is trusted in practice is not to change conditions to make the results look better, but to adjust conditions to make them closer to reality.

Checklist item 5: Be able to explain discrepancies between on-site conditions and design conditions

The last thing to confirm when checking PR is the discrepancy between site conditions and design conditions. In PVSyst, you input and calculate factors such as terrain, shading, orientation, tilt, equipment layout, and loss conditions. However, on actual sites there may be locations where construction cannot be carried out exactly as drawn, areas where terrain undulation is greater than expected, or places where the surrounding environment changes. Such discrepancies can lead to differences between the simulated PR and actual operational results.

For example, on sites with large ground slopes or elevation differences, the height of the panel surface and the shadows between rows can differ from the design assumptions. Even if the drawings appear to show sufficient spacing, on-site undulations can cause shadows to lengthen in winter. If such conditions are not adequately reflected in PVSyst, the PR may appear higher than it actually is.

Surrounding buildings, trees, site grading, slopes/embankments, and the placement of equipment and devices also affect PR. Especially in projects ranging from low-voltage to medium-scale and high-voltage scale, multiple small obstructions may exist. Because the impact of shading influences both annual power generation and PR, it is important to consider how much of the information obtained from on-site surveys has been reflected in the input.

Discrepancies between the design conditions and actual site conditions also occur during the construction phase. The positions of mounting racks may shift slightly, layouts may be changed to secure walkway widths, equipment locations may be adjusted, and the ground elevation after earthworks may differ from the initial assumptions. If these changes are not reflected in the power generation simulation, there will be discrepancies in the PR explanation.

In practice, when explaining PR figures, it is important to be able to clearly state, "Under the simulation conditions, the calculations were performed based on these assumptions." Even if the actual site conditions are not fully reflected, knowing which aspects were simplified and which conditions were assumed makes it less likely that the results will be mishandled.

PR is a numerical result of simulations, but its reliability is supported by an understanding of on-site conditions. The closer the desk-based design is to the actual site conditions, the greater PR's explanatory power. To gain practical proficiency in using PVSyst, you need to be mindful not only of the on-screen settings but also of how to reflect terrain and location information obtained on site into the design conditions.

How to proceed when reviewing PRs in practice

When checking PR in PVSyst, it's efficient to decide the order in which you will review things. First, check the annual energy production and the annual PR to grasp the overall level. Next, look at the relationship with irradiance to roughly distinguish whether the low energy production is due to irradiance conditions or system losses. Then check the loss diagram and identify the main factors that are depressing the PR.

Next, we examine PR by month. We check for seasonal variations that are not visible from annual values alone—whether there are temperature losses in summer, whether shading effects are stronger in winter, or whether any specific month shows anomalous values. If monthly variability is large, we review that month’s solar irradiance, temperature, shading, weather conditions, and loss settings.

After that, return to the input conditions. Check the site, meteorological data, azimuth, tilt angle, system capacity, circuit configuration, shading, loss rates, downtime rate, and so on, and verify that the results are not inconsistent with the assumptions. In particular, when moving from an initial assessment to a detailed study, it is important to confirm that no provisional conditions remain. If a PR produced under provisional conditions is treated as the final evaluation value, it will be difficult to explain later.

In practice, keeping a record of PR checks is also helpful. Recording under what conditions the calculations were made, which losses were large, which input values were changed, and how PR changed after those changes makes internal reviews and explanations to clients easier. When comparing multiple proposals, organizing the differences in conditions and the differences in results lets you explain why a particular proposal was chosen.

If you want to improve PR, don’t target numbers alone; instead examine the potential for improvement for each loss factor. If shading is significant, review the layout and row spacing; if thermal losses are large, check the mounting method and ventilation conditions; if wiring losses are large, check cable lengths and circuit configuration. If the capacity conditions of the conversion equipment are having an impact, review the capacity ratios and operating range.

However, it is not possible to make all losses zero. Photovoltaic systems have losses that are physically unavoidable. What matters in practice is not forcing the PR higher, but producing a reasonable PR under realistic conditions and being able to explain the reasons for it. When checking PVSyst results, the mindset of creating numbers you can explain, rather than numbers that merely look good, is important.

The quality of on-site data is essential for improving the accuracy of PR verification

Checking PR in PVSyst cannot be completed by software operations alone. To improve the accuracy of power generation simulations, the quality of the site data entered is important. In particular, terrain, elevation, layout, obstacles, and shading conditions tend to be oversimplified when on-site information is insufficient. As a result, the PR may deviate from actual conditions.

When on-site data are coarse, design conditions become coarse as well. For example, if the terrain is treated as flat but in reality has significant elevation changes, assessments of inter-row shading and the area available for placement can change. If the heights and positions of surrounding obstacles are ambiguous, it is also difficult to correctly estimate the impact of shadows. Because PR is an indicator sensitive to loss conditions, insufficient understanding of on-site conditions reduces the explanatory power of the results.

Also, in planning solar power generation facilities, the required data granularity changes between the initial design, detailed design, pre-construction, and post-construction stages. In the initial phase, estimates may be sufficient, but in detailed examinations, more accurate positional and elevation information is required. For post-construction verification, it is necessary to confirm whether the actual arrangement and heights match the design. In such situations, the ability to accurately acquire on-site positional information affects the reliability of simulation results.

LRTK, as an iPhone-mounted GNSS high-precision positioning device, can acquire location information on site and be used for surveying, point cloud acquisition, geotagged photo records, and other applications. For planning and verification of photovoltaic power generation facilities, obtaining on-site data to prepare the prerequisites for input into PVSyst is important—such as understanding the terrain of candidate installation sites, recording the positions of obstacles, managing the locations of on-site photos, and post-construction verification. If site conditions that are difficult to grasp through desk-based simulations alone can be organized together with location information, PR explanations also become easier.

To strengthen your ability to assess PR in PVSyst, it is essential not only to know how to read the results screen but also to reinforce the basis for the input conditions. If you can incorporate on-site coordinates, terrain information, and photo records obtained in the field into the design review, it becomes easier to make judgments about shading, layout, and elevation differences. By using high-precision positioning devices such as LRTK, you can organize the on-site information that underlies the simulation and thereby improve the accuracy of PR verification.

Summary

The PR in PVSyst is an important indicator for checking the system efficiency of photovoltaic power generation facilities. However, it is not appropriate to judge the quality of a facility by PR alone. By checking annual energy production, solar irradiance, the loss diagram, monthly variations, input conditions, and site conditions together, you can correctly interpret the meaning of PR.

The first thing to check when reviewing results is the consistency between the annual energy generation and the PR. By distinguishing whether low generation is due to solar irradiance conditions or to losses, the next points to verify become clear. Next, review the loss chart to determine which factors—temperature, shading, wiring, conversion, soiling, downtime rate, etc.—are affecting the PR. Furthermore, examining the monthly PR makes it easier to notice seasonal fluctuations and anomalies in specific months.

Reviewing the input conditions is also essential. If the site, meteorological data, orientation, tilt angle, installed capacity, circuit configuration, loss rates, and so on do not match reality, the PR figures will also diverge from the actual situation. What matters in using PVSyst is not producing nice-looking results, but producing explainable results based on realistic assumptions.

And to improve the accuracy of PR verification, understanding site conditions is important. If terrain, elevation, obstacles, layout, and post-construction conditions can be accurately determined, the reliability of the conditions entered into PVSyst increases. By leveraging LRTK, high-precision positional information and records obtained on site can be more easily applied to design evaluations. Combining the work of checking PR in PVSyst with the collection of accurate on-site data makes the planning, design, and construction verification of photovoltaic power generation facilities more reliable.