7 Fundamentals for Correctly Interpreting PR in the PVSyst Manual

Table of Contents

• What does PR indicate as a metric?

• How to think about baseline power generation before reviewing PR

• Why you shouldn't judge based solely on the annual PR

• Seasonal variations to check in monthly PR

• The Importance of Reading the Loss Breakdown and PR Together

• Points to consider when comparing measured and simulated values

• How to use PRs to drive design improvements

• Misconceptions readers of the PVSyst manual should avoid

• Summary

What does the PR metric indicate?

When checking PR in the PVSyst manual, the first point to understand is that PR is not simply a number indicating whether generation is high or low. PR generally stands for Performance Ratio and is a representative indicator used to assess how efficiently a photovoltaic installation converts the solar irradiance it receives into electrical energy. It is used to facilitate comparisons of the equipment's operational condition and the magnitude of losses even when plant size, location, tilt angle, and irradiance conditions differ.

For example, even for the same 1 MW solar power plant, annual solar irradiance and temperature conditions differ greatly between Hokkaido and Kyushu, coastal and mountainous areas, and rooftop and ground-mounted installations. If you look only at annual generation, regions with higher irradiance will appear advantageous. However, you cannot determine from generation alone whether that output is reasonable relative to the plant’s capacity, whether losses are excessive, or whether the system is operating as assumed in the design. By looking at PR, you can to some extent normalize differences in irradiance conditions and more easily interpret the plant’s actual performance and loss situation.

In PVSyst reports, PR is an important metric for evaluating simulation results. In particular, when comparing multiple options in the design phase and when checking the validity of PCS capacity, module layout, orientation/tilt, shading conditions, and loss settings, PR serves as one of the decision-making criteria. However, PR is not a simple indicator where higher is always better. Even if PR is high, the assumed irradiance may have been underestimated, loss settings may have been too lenient, or the treatment of output curtailment and high-temperature losses may be out of step with reality.

Also, PR is not a number that directly represents a power plant's profitability. Electricity selling price, self-consumption rate, grid constraints, maintenance costs, land conditions, installed capacity, and so on cannot be evaluated by PR alone. PR is, at its core, a performance metric that indicates how much of the incident solar energy was converted into electrical energy. When looking at PR in the PVSyst manual, grasping this positioning up front makes it less likely to misinterpret the meaning of the figure.

How to think about reference energy generation before looking at PR

To read PR correctly, you first need to understand the concept of the reference generation. PR is calculated by comparing the actual or simulated output energy to a reference value that would ideally be obtained. The important point here is that the reference value is not a “perfect generation with no losses,” but a theoretical reference based on solar irradiance and system capacity.

When reading PVSyst results, it is important not only to look at the final annual energy production but also to check at which stages and in what amounts energy is being calculated. In photovoltaic simulations, there is first horizontal plane irradiance and tilted plane irradiance, from which the energy incident on the module surface is determined. After that, losses such as shading, reflection, temperature, module quality, mismatch, DC wiring, PCS conversion, and AC wiring are applied, and the final energy on the grid side or the user side is calculated.

PR is an indicator used in this workflow to assess how much of the final output remains compared to the reference. Therefore, if the settings for the reference solar irradiance or installed capacity are inappropriate, the way PR appears will change. For example, if the selected meteorological data do not match local conditions, PR may look high even though the actual expected power generation is not necessarily reliable. Conversely, if the solar irradiance is overestimated, PR may appear low.

A point that is easy to confuse here is that projects with high energy output are not necessarily the same as projects with a high PR. In regions with very high solar irradiance, the energy output itself tends to be large, but if high-temperature losses and output limitations are significant, the PR may not increase. Conversely, even in regions where solar irradiance is not particularly high, if temperature conditions are favorable and there is little shading or other losses, the PR can be relatively high.

When checking PR in the PVSyst manual, you should not look at PR in isolation; you need to check it together with the relationships among irradiance, plane-of-array irradiance, module capacity, PCS capacity, and final energy production. PR is useful as a summary of results, but unless you understand the reference values behind it, it can be difficult to tell whether a design is good or whether the difference is simply due to different assumptions.

Why You Shouldn't Judge Only by Annual PR

PVSyst reports make the annual PR stand out, so it's easy to be tempted to judge the quality of a design solely by that figure. However, judging based only on the annual PR is risky. The annual PR is the result of averaging losses and seasonal variations over a year, and it does not show in detail when problems occur or which losses are dominant.

For example, even if the annual PR is the same, the underlying situations can be completely different. In one design, high-temperature losses in summer may be large, while generation in winter is relatively good. In another design, shading losses in the mornings and evenings may be large, causing generation to drop during specific hours throughout the year. In yet another design, the PCS capacity may be small, leading to frequent peak clipping on clear days. Because the annual PR aggregates these into a single number, the detailed causes become harder to see.

Also, the way annual PR is evaluated depends on the project's objectives. In feed-in projects, annual power generation and revenue from electricity sales are prioritized, whereas in self-consumption projects, alignment with the demand curve is important. Whether generation matches daytime peak demand, whether there is excessive surplus, and whether it pairs well with battery storage and load control are aspects that cannot be judged adequately by annual PR alone.

Also, be careful when the annual PR is too high. In general, if losses are set realistically, some losses will inevitably be reflected in the design. If temperature losses, soiling, wiring, PCS conversion, mismatch, degradation, downtime rate, etc., are underestimated, the PR will appear artificially high. In other words, a high PR may look attractive, but you must confirm whether it is based on realistic settings.

When viewing the annual PR in the PVSyst manual, it is practical to first grasp the overall level, then proceed to the monthly results, the loss diagram, and the various report items. The annual PR is an entry point, not a conclusion. Whether the figures are higher or lower than expected, you should take an approach of breaking them down and verifying why that is the case.

Seasonal variations to check in monthly PR

Checking monthly PR is critically important for correctly understanding PR. Variations that are averaged out and thus obscured in annual PR become clear when viewed on a monthly basis. In particular, in regions where seasonal changes affect solar irradiance conditions, temperature, solar altitude, snowfall, soiling, and the way shadows appear, examining monthly PR makes it easier to identify design and operational challenges.

A typical reason for PR declining in summer is the rise in module temperature. While photovoltaic modules tend to produce more power as irradiance increases, conversion efficiency falls when temperatures are high. Therefore, even though solar irradiance is greater in summer, PR is not necessarily higher. If PVSyst results show a lower PR in summer, it is necessary to assess how much temperature-related losses are affecting it.

If PR fluctuates in winter, it is necessary to consider the effects of solar incidence angle, snow cover, surrounding shading, and reduced insolation hours. In winter the sun’s elevation is lower, so shadows from nearby buildings, trees, rows of racking, and terrain become longer. For ground-mounted installations, shading from front rows onto rear rows can be influential, while for rooftop installations, upstands and adjacent structures can cause shading. In snowy regions, how snow coverage and reflection are treated can have a significant effect on the results.

PR can sometimes appear relatively high in spring and autumn. This is because temperatures are moderate, solar radiation is at a certain level, and thermal losses are not as large as in summer. However, if there are seasonal contaminants such as yellow sand, pollen, fallen leaves, or dust around agricultural fields, measured PR may be lower than expected. If the soiling loss in simulations is set uniformly across the year, it may not adequately represent real seasonal variations.

When looking at monthly PR, you should not focus only on months with low PR; you also need to consider how much those months affect generation. Even if PR drops in winter, when generation is low, the impact on annual generation may be limited. On the other hand, if PR declines from spring through summer, when generation is high, it can have a significant impact on annual financial performance.

When reading monthly results using the PVSyst manual, it is important to check not only variations in PR but also to correlate irradiance, temperature losses, shading losses, PCS limits, grid output, and utilized energy. Monthly PR is not merely a number in a table; it is a clue for interpreting the system's seasonal behavior.

The Importance of Reading the Loss Breakdown and PR Together

To interpret PR correctly, it is essential to check it together with the loss breakdown. In PVSyst, various loss items are organized as part of the simulation results. If PR is low, unless you identify whether the cause is shading, temperature, wiring, the PCS, or soiling, you cannot make improvements to design or operation.

A common loss is shading loss. When modules are shaded by surrounding buildings, trees, terrain, rows of racking, rooftop equipment, or similar objects, power generation decreases. Shadows do not necessarily reduce output only in proportion to the shaded area; the impact can be amplified by string configuration and the behavior of bypass diodes. Therefore, if the PR is lower than expected, it is necessary to check whether the shading analysis and 3D model settings are appropriate.

Temperature losses also have a major impact on PR. Module temperature varies not only with ambient air temperature but also with ventilation, mounting method, roof material, racking height, and rear-side heat dissipation conditions. Installations that are in contact with the roof tend to run hotter, so conditions differ from well-ventilated ground-mounted systems. If the temperature loss settings do not reflect the actual conditions, the PR assessment will be skewed.

Soiling losses are an item that is easily overlooked. The way surfaces get dirty varies greatly depending on local conditions: sand and dust, bird droppings, pollen, volcanic ash, dust from agricultural land, dust in industrial areas, and so on. In some regions rain naturally washes them away, while in others soiling accumulates over long periods. Whether a cleaning plan is in place also affects the actual PR.

Wiring losses and mismatch losses also need to be checked. When DC wiring is long, the cable cross-sectional area is insufficient, or there are large differences in conditions between strings, losses can increase. PCS conversion losses and peak clipping due to PCS capacity also affect PR. Setting a high DC-to-AC ratio brings benefits in efficiency and equipment utilization under low irradiance, but makes output curtailment more likely on sunny days.

By looking at PVSyst's loss diagram, you can check in order how much energy is lost at each stage. What matters here is not to regard the existence of losses themselves as a problem, but to see whether those losses are realistic and reasonable. Solar power installations always have losses. The question is whether you have overlooked losses that should have been assumed, conversely estimated them too high, or left any losses that can still be reduced.

PR is an indicator that summarizes results, and the loss breakdown is the material that explains the reasons. When learning PR from the PVSyst manual, it is important in practice not to memorize only the PR formulas and definitions, but to make a habit of reading them together with the loss diagram.

Precautions when comparing measured and simulated values

The PR calculated by PVSyst is useful during the design phase and for preliminary evaluations. However, when comparing it with measured PR after operation begins, there are several points to be aware of. Because the assumptions behind the simulation values and the measured values do not fully match in the first place, simply comparing the numbers can lead to incorrect conclusions.

The first thing to check is the location and accuracy of the solar irradiance data used for measurements. When evaluating PR with measured data, it is important how you measure the solar irradiance actually received at the plant. Whether you use nearby meteorological data, an on-site pyranometer, measure tilted-plane irradiance, or convert horizontal-plane irradiance will affect the PR value. You also need to check pyranometer soiling, installation angle, calibration status, and the effects of shading.

Next, the point at which generation data are collected is also important. PR will vary depending on which point's energy you use—PCS output, point of connection, export meter, remaining energy after self-consumption, etc. If you do not match which output point the PVSyst simulation results represent with which point the measured data correspond to, the comparison will not be valid.

Also, in actual operation stoppages and control actions occur. Shutdowns for inspection, communication malfunctions, PCS failures, output curtailment by the grid, restrictions due to voltage rise, remote-control interventions, protective actions, and so on reduce the measured PR. The meaning of comparison results changes depending on the extent to which these are considered in the simulation. Even if the measured PR is lower than the design PR, the cause may be grid constraints or downtime rather than equipment performance issues.

Furthermore, the effects of aging must be taken into account. Modules experience a gradual decline in output over time. Comparing the PR immediately after commissioning with the PR several years later against the same benchmark can lead to mistakenly classifying the degradation as an anomaly. For long-term operation, it is necessary to evaluate yearly solar irradiance conditions, temperature, soiling, downtime, and degradation rate separately.

When comparing simulated values with measured values, it is important not simply to ask "does it match PVSyst's PR," but to examine, after aligning the assumptions, which discrepancies can be explained and which cannot. Discrepancies that can be explained can be attributed to meteorological conditions, downtime, or measurement conditions. On the other hand, if unexplained discrepancies continue to occur, there may be issues in design, construction, equipment, measurement, or maintenance.

How to Use PRs to Inform Design Improvements

PR should not be seen merely as the evaluation value listed in a report, but can be used as a clue for design improvements. The purpose of using the PVSyst manual in practice is not just to read numbers. It is to compare multiple design proposals and determine which conditions to change to improve power generation performance and project viability.

First, we check how PR changes with differences in orientation and tilt. The optimal orientation and tilt depend on the region, installation site, roof shape, land shape, and the demand curve. A near-due-south arrangement can be advantageous, but for self-consumption systems, east- or west-facing arrangements may be considered to match morning or afternoon demand. Configurations that appear disadvantageous if you look only at PR can be effective when you consider the timing of power use.

Next, we examine the relationship between module layout and shading. Increasing row spacing tends to reduce shading losses, but it can reduce the capacity that can be installed on the same land. Conversely, packing in too much capacity may increase annual energy yield but can cause the PR to decline. Here, it is necessary to consider PR, annual energy yield, installed capacity, and land-use efficiency comprehensively.

Setting the PCS capacity is also important. If the PCS capacity is reduced relative to the DC-side capacity, it may be possible to secure energy production while lowering equipment costs. However, if it is made too small, peak clipping increases and PR decreases. By creating multiple cases in PVSyst and changing the DC/AC ratio while observing changes in PR and annual energy production, you can evaluate the balance between overdesign and underdesign.

Wiring plans and string configurations are also targets for PR improvement. String length, mixed orientations, how shading occurs, and how PCS inputs are divided can affect mismatches and operational instability. Especially for rooftop installations and projects on complex terrain, results can vary depending on the string configuration even with the same capacity.

Maintenance planning also affects PR. Soiling, shading from vegetation, snow handling, routine inspections, and recovery procedures in the event of failures all influence long-term PR. Even if the design stage indicates a high PR, the measured PR will decline if cleaning and weed control are inadequate during operation. It is important to connect the PVSyst settings with the on-site operational plan.

To use PR for design improvements, it is effective to compare multiple cases with different conditions rather than looking at a single finalized plan. However, if you change too many conditions at once, it becomes difficult to tell what caused the change in PR. By changing one factor at a time—orientation, tilt, row spacing, PCS capacity, loss settings, etc.—and examining the results, it becomes easier to understand the effect of each improvement.

Misconceptions to Avoid When Reading the PVSyst Manual

One misconception that people learning about PR from the PVSyst manual should avoid is treating PR as an absolute pass/fail criterion. PR is an important indicator, but it should not be applied uniformly without regard to project-specific conditions. The reasonable range of PR changes depending on the region, mounting method, weather conditions, temperature conditions, shading conditions, grid constraints, and system configuration.

For example, rooftop installations can suffer from poor ventilation, resulting in greater temperature-related losses. Even for ground-mounted systems, the shape of the land may prevent sufficient row spacing. In mountainous areas, terrain shading in the morning and evening can be significant. In snowy regions, a reduction in power generation during winter must be anticipated. It is dangerous to judge a system as poor simply because its PR is low, or good simply because its PR is high, without taking these conditions into account.

Another misconception is to assume that a higher PR will always produce the maximum annual generation. If you limit the installed capacity, losses can appear smaller and PR may look higher. However, from a business perspective, increasing installed capacity can sometimes yield greater annual generation and revenue even if PR is slightly lower. Conversely, if capacity is increased too much, peak clipping and shading losses will increase, reducing the cost-effectiveness of the equipment.

Also, caution is needed when comparing PR with other projects. If the meteorological data, loss settings, output points, calculation conditions, or measurement methods differ, the same PR can have a different meaning. When comparing multiple projects within the company, it is important to align the configuration rules. When comparing with PRs from external sources, numbers whose underlying assumptions are not clearly stated should be treated only as indicative.

Furthermore, it is necessary to avoid the misconception of treating PVSyst results as if they were real guaranteed values. Simulations are predictions based on input conditions. They do not fully reproduce everything, such as interannual weather variability, the actual performance characteristics of equipment, construction quality, downtime, maintenance condition, and grid conditions. PVSyst is a powerful analysis tool, but it must be used with the understanding of the differences between the input conditions and the on-site conditions.

To properly interpret PR, it's important not only to look at the value itself but to be able to explain why that value was produced. By checking why the PR is what it is, which losses are significant, which settings could be changed to improve it, and how much of the difference can be explained when compared with measured data, you can make PVSyst's results useful in practice.

Summary

To correctly interpret PR in the PVSyst manual, it is important not to treat PR merely as a score of power generation performance, but to read it in context of solar irradiance, system capacity, losses, seasonal variations, and output conditions. PR is a useful indicator of how effectively a photovoltaic system converted the solar irradiance it received into electrical energy, but it alone cannot be used to judge all aspects of design and operation.

The basic thing to understand first is the definition and role of PR. PR is not the generation amount itself, but shows how performance appears relative to reference solar irradiance and installed capacity. Therefore, if meteorological data or the settings of reference values change, the interpretation of PR also changes. A project with large annual energy production is not necessarily the same as a project with a high PR.

Another important point is not to judge solely by the annual PR. The annual PR is useful as an entry point for grasping the overall picture, but it does not reveal the root causes of problems. By examining monthly PR, you can more easily identify summer temperature losses, winter shading or snow cover, and seasonal soiling. In particular, if PR declines during periods of high generation, it can have a larger impact on annual energy production, so caution is needed.

Furthermore, verification against the loss breakdown is indispensable. There are many factors that affect PR, including shading, temperature, soiling, wiring, mismatches, PCS conversion, peak cut, and downtime rate. In practice, the key point is whether you can explain the reasons from the loss diagram or report items, whether PR is low or high.

When comparing measured values with PVSyst simulation values, you need to align and verify factors such as the pyranometer location, the points where generation is measured, downtime, output curtailment, degradation over time, and cleaning status. If you compare only PR while conditions differ, you may mistakenly judge differences that are not equipment problems to be anomalies.

To leverage PR for design improvements, it is effective to compare multiple cases while varying azimuth and tilt, row spacing, PCS capacity, string configuration, and loss settings. However, a scheme with a higher PR is not always the best. It is necessary to judge in combination with annual energy production, equipment cost, land use, self-consumption rate, maintainability, and grid conditions.

The purpose of learning PR in the PVSyst manual is not to memorize numbers, but to develop an ability to interpret them for design and operational decision-making. If you can explain, when looking at PR figures, why a value is what it is, which losses are affecting it, and which settings should be reviewed, the simulation results become more useful for practical decision-making. Correctly interpreting PR is fundamental to improving the design quality, profitability, and operational performance of solar power projects.