6 Ways to Interpret the Performance Ratio in PVSyst｜How to Think About PR

When reading a PVSyst report for a solar power plant, one of the metrics many people check first is the Performance Ratio, or PR. PR — called 性能比, 性能係数, or パフォーマンスレシオ in Japanese — is a very important figure for evaluating a plant’s design and simulation results.

However, although PR may at first glance appear to be a simple ratio, it is an indicator that is easily misunderstood in practice. If you simply conclude that a high PR always means a good power plant and a low PR always means a bad one, you can overlook design conditions, meteorological conditions, loss settings, output limits, snowfall, shading, temperature conditions, and other factors.

To correctly interpret the Performance Ratio in PVSyst, you should not look only at the PR number itself but also confirm under what conditions that number was calculated and which losses it represents. Especially for utility-scale solar, PR is used in many contexts—documents submitted to banks, EPC quotations, third-party evaluations, energy-yield guarantees, and O&M performance assessments—so misreading it can affect design decisions and revenue judgments.

This article organizes how to read the Performance Ratio in PVSyst from six perspectives. It is useful not only for those reading PVSyst results for the first time, but also for people who want to compare multiple options, cross-check reports from other companies, identify causes of low power generation, or use it for design reviews.

Table of Contents

• Performance Ratio is an indicator that measures a power plant's efficiency

• PR is not the generated energy itself but the percentage after losses

• Check the reference solar irradiance before looking at PR

• Annual PR and monthly PR have different meanings

• Read the causes of low PR from the Loss Diagram

• When comparing PRs, align the baseline conditions

• Precautions when using PR in practice

• How to utilize PVSyst's PR for design improvement

• Summary

Performance Ratio is an indicator used to assess the efficiency of a power plant

Performance Ratio is a performance indicator that shows, relative to the solar irradiation energy received by a photovoltaic power plant, how much electrical energy was ultimately delivered to the grid. In PVSyst, PR can be checked in the simulation results summary, the Result Sheet, and the Loss Diagram.

The basic concept of PR is that, relative to the energy a solar cell would generate under ideal conditions, actual energy is reduced by temperature loss, wiring loss, mismatch loss, PCS loss, shading loss, soiling loss, IAM loss, etc., and PR is the proportion remaining afterward.

For example, if a power plant's annual PR is 84%, it can be interpreted that, relative to the simulation reference, the energy equivalent to 84% was effectively utilized after accounting for various losses. However, you cannot judge whether this 84% figure is good or bad by itself: whether it is a cold area or a hot area, whether the tilt angle is appropriate, whether there is shading, whether the PCS capacity is somewhat small, whether snowfall is taken into account, or whether output curtailment is included will all affect what a reasonable PR is.

PVSyst's PR is not merely a measure of generation efficiency, but an overall performance indicator for the power plant that includes design conditions and loss conditions. Therefore, when looking at PR it is important to read it from the perspective of "how much theoretical generation capacity this plant has, and how much of that remains as final output."

One point to note is that PR is not an indicator of the absolute amount of power generation. Regions with high solar irradiation tend to produce more power, but that does not necessarily mean the PR will be higher. Conversely, in areas with low irradiation the PR can appear high if losses are small and the design is well optimized. In other words, PR is a measure of how efficiently the received solar irradiation was converted into electrical energy, rather than a measure of the site's quality.

PR is not the amount of electricity generated itself, but the proportion after losses.

The most common misunderstanding when reading PVSyst's Performance Ratio is to treat the PR as synonymous with the amount of energy produced. People tend to assume that projects with higher PRs will have higher annual energy production and those with lower PRs will have lower annual production, but this is not necessarily correct.

Annual power generation is greatly influenced by installed capacity, solar irradiance, temperature, tilt angle, orientation, shading, surrounding environment, and operating rate. On the other hand, PR is an indicator that, based on solar irradiance and installed capacity, shows how much output was obtained after losses. Therefore, in regions with very high solar irradiance, annual generation can be large even if PR is somewhat low. Conversely, in regions with low solar irradiance, the absolute amount of generation can be small even if PR is high.

For example, in regions like Hokkaido, where air temperatures are low and module temperature losses are small, PR tends to be higher. However, if losses from snow cover are strongly factored in, winter generation can fall sharply, affecting the annual PR as well. In regions with high solar radiation, such as Kyushu and the Kanto area, annual generation tends to be larger, but higher module temperatures in summer increase temperature-related losses and can cause PR to drop slightly.

Thus, PR should not be read as a direct indicator of a power plant's profitability, but rather as a metric summarizing its loss structure. When evaluating profitability, you should also check annual energy production, Specific Yield, electricity selling price, curtailment, auxiliary power consumption, grid constraints, degradation rate, and so on.

In PVSyst reports you can check items such as E_Grid, Specific Production, Available Energy, and Normalized Production alongside PR. Rather than extracting PR alone to make a judgment, looking at the generated energy, the energy produced per unit of installed capacity, and the various loss items together clarifies the meaning of the figures.

In practice, even a 1% difference in PR affects annual energy production and revenue from electricity sales, so it is important to pay attention to differences in PR. However, unless you identify which loss items are causing that 1% difference, you cannot turn it into design improvements. PR is a result, and the causes lie in the Loss Diagram and Detailed Losses.

Check the baseline solar irradiance before looking at PR

To read PVSyst's Performance Ratio correctly, you must first verify the assumptions about solar irradiance. Because PR is calculated based on solar irradiance, it is difficult to compare PR values when the meteorological data or irradiance conditions used are different.

In PVSyst, you can run simulations using various meteorological data such as Meteonorm, SolarGIS, satellite data, measured data, and nearby station data. Which meteorological data you adopt affects Global Horizontal Irradiation, Diffuse Irradiation, temperature, wind speed, and so on, and as a result the energy yield and the way losses manifest will also change.

PR is a metric normalized by solar irradiance, but that does not mean it is immune to the influence of meteorological data. For example, if the ratio of direct to diffuse irradiance changes, it affects IAM losses and the calculation of irradiance on a tilted surface. When air temperature changes, module temperature changes and temperature losses also change. If wind speed data differ, module cooling conditions change, which also affects temperature losses.

Therefore, when comparing PR in PVSyst, it is important to first check the type of meteorological data, the target year(s), the method used to create the representative year, the horizontal-plane irradiance, the tilted-plane irradiance, and the annual average temperature. In particular, when comparing with another company's report, if the meteorological data differ between your analysis and theirs, it becomes difficult to determine whether the PR difference is caused by the design or by the meteorological conditions.

In snowy regions, effective power generation in the winter months is more important than solar irradiation itself. The monthly PR during winter can vary greatly depending on how snow losses and Soiling Loss are configured in PVSyst. Reports that strongly account for snow loss and reports that barely account for it can show different annual PRs even for the same location.

Before looking at PR, make sure to check which plane's solar irradiance is being used as the reference. PVSyst converts horizontal-plane irradiance to inclined-plane irradiance, and then takes into account factors such as nearby shading, IAM, soiling, and so on. The final PR is an indicator that reflects this sequence of conversions and losses. Therefore, by following the progression of GlobHor, GlobInc, GlobEff, and so on—not just PR—you can see what changes are occurring at the irradiance stage.

Annual PR and Monthly PR Mean Different Things

For PVSyst's Performance Ratio, it is important to look not only at the annual PR but also at the monthly PR. The annual PR is a convenient indicator that summarizes the overall performance of the plant into a single number, but it can be insufficient for root-cause analysis. By looking at the monthly PR, seasonal loss trends and design weaknesses become easier to identify.

For example, if PR decreases in summer, the main cause to consider is temperature loss due to increased module temperature. Because solar modules’ output falls as temperature rises, PR can drop even in summer when irradiance is high. This is especially true for rooftop-mounted or poorly ventilated ground-mounted installations, where module temperatures tend to be higher and temperature losses larger.

On the other hand, when PR is low in winter, factors such as snow cover, low irradiance, shading, decreased solar altitude, Soiling Loss, morning and evening shadows, and the PCS’s low efficiency under light load may be affecting it. In cold regions, temperature losses are smaller, so some months would normally show higher PR. However, when snow is taken into account or in terrain where winter shadows are longer, the monthly PR decreases significantly.

Spring and autumn PRs are useful for assessing a design’s characteristics because temperature conditions are relatively favorable and solar irradiance tends to be stable. Whether performance is extremely poor only in summer, only in winter, or generally low throughout the year will change which loss items you should examine.

When looking at monthly PR, it is important to check it together with monthly energy generation. Even a month with a high PR can contribute little to energy generation if solar irradiance is low. Conversely, a month with a slightly lower PR can have a large impact on annual revenue if irradiance is high and energy generation is large. Therefore, monthly PR should not be viewed in isolation but read together with monthly E_Grid, monthly GlobInc, and monthly losses.

In PVSyst's graphs and tables, monthly energy production, irradiance, and PR are displayed. The important thing is that when you find a month with abnormally low values, do not jump to conclusions immediately; check the cause in the Loss Diagram and Detailed Losses. For example, a low PR in June does not necessarily mean it was caused by the rainy season—temperature, diffuse irradiance, shading, PCS limitations, soiling settings, and other factors may be involved.

Identify the Causes of Low PR from the Loss Diagram

When the Performance Ratio is low, the first thing to check is the Loss Diagram. The Loss Diagram is a chart that shows, as solar irradiation energy progresses through the module, array, PCS, and grid output, at which stages and by how much energy is lost. When reading PVSyst's PR, the Loss Diagram is central to root-cause analysis.

If PR is low, first check the irradiance-side losses. If near-shading loss, far-shading loss, IAM loss, soiling loss, or snow loss are large, the effective irradiance incident on the modules is reduced. When these losses at this stage are large, improving downstream electrical losses will only yield limited gains in energy output.

Next, we look at module-side losses. Temperature losses, low-irradiance losses, module quality losses, mismatch losses, degradation, LID, etc. are involved here. If temperature losses are large, check whether the racking height, ventilation, installation method, temperature model, and the settings for Uc and Uv are appropriate. If mismatch losses are large, check the string configuration, module-to-module variation, the way shadows fall, and the MPPT configuration.

Next, check DC wiring losses, PCS losses, AC wiring losses, transformer losses, auxiliary power, and so on. If DC wiring losses are large, examine whether cable length, cross-sectional area, voltage, current, connection box and PCS layout are appropriate. If PCS losses are large, check the PCS efficiency curve, overloading ratio, low-load operation, clipping, power factor settings, output limits, and so on.

When reading the causes of a decline in PR, it is important not to view loss items individually and in isolation, but to consider them in connection with the overall plant design. For example, increasing the DC/AC ratio makes module capacity larger relative to PCS capacity, and annual energy generation may increase, but clipping losses may increase and PR may fall. In this case, a lower PR does not necessarily indicate a poor design. It should be evaluated in balance with profitability and equipment costs.

Also, in PVSyst's Loss Diagram losses are shown as percentages, so while it is easy to see which losses are largest, their impact in absolute terms can be easily overlooked. When assessing the effect on annual generation, checking how large the difference is in kWh or MWh makes it easier to set priorities for improvements.

Ensure consistent preconditions in PR comparisons

PVSyst's Performance Ratio is a useful indicator for comparing multiple cases. However, when comparing PRs, you must align the assumptions; otherwise you will not make a correct evaluation. When comparing in-house analysis and third-party analysis, design option A and design option B, or an older report and a newer report, you need to confirm which conditions are the same and which are different.

The first thing to standardize is the meteorological data. If solar irradiance, ambient temperature, wind speed, or data sources differ, you won’t be able to tell whether differences in PR are due to design differences or weather differences. Next, check the module model, PCS model, capacity, string configuration, tilt angle, azimuth, racking type, shading settings, soiling losses, wiring losses, transformer losses, auxiliary losses, power factor setting, and how output limitation is handled.

Especially in commercial solar power, a few percent difference in PR can trigger significant debate. However, on closer inspection, one report may apply a relatively strict Soiling Loss while the other barely accounts for it, or one may consider snow while the other does not. In such cases, the PR difference reflects differences in assumptions rather than differences in analysis accuracy.

Also, the settings for DC wiring losses and AC wiring losses also affect the PR. In PVSyst, there are methods to calculate losses from wiring length and cross-sectional area, and there are methods to set a fixed percentage. In projects where PCS are distributed, DC wiring losses tend to be smaller, while in centralized PCS configurations long DC cables can increase losses. Comparing PR alone without taking these differences in design philosophy into account can lead to incorrect judgments.

Careful attention should also be paid to how power factor settings and output limits are handled. Depending on PCS capacity, apparent power, active power, power factor control, and grid interconnection conditions, the way losses are accounted for in PVSyst may change. In particular, in projects with PCS output limits or clipping, a reduced PR may be due not to a decline in plant performance but to deliberate capacity design or grid conditions.

What's important in PR comparisons is not to treat the difference in PR as a conclusion, but to be able to explain the breakdown of that difference. For example, if the PR is 1.5% lower than another company's report, being able to explain the difference as 0.4% temperature loss, 0.5% soiling loss, 0.3% DC wiring loss, and 0.3% PCS loss makes reviews and customer explanations easier.

Points to Note When Using PR in Practical Work

PVSyst's Performance Ratio (PR) is often used for design review, energy yield assessment, bank submissions, EPC estimates, and O&M comparisons. However, care is needed in how it is used. PR is a useful metric, but it is not a universal solution.

First, PR is not a metric that directly indicates profitability. To assess electricity sales revenue and investment recovery, you need to evaluate factors including annual power generation, electricity sales price, CAPEX, OPEX, output curtailment, degradation rate, maintenance costs, and tax conditions. Even if PR is high, the project may not be commercially viable if installed capacity is small, solar irradiance is low, or electricity sales conditions are poor.

Next, PR can decrease when the design is made overly conservative. For example, setting snow loss, soiling loss, wiring loss, and auxiliary loss to conservative values will lower the PR. However, this reflects a cautious incorporation of risk rather than poor plant performance. For financial institutions and in guarantee design, a well‑founded conservative PR can be more trusted than an optimistic PR.

On the other hand, it is dangerous to set losses too low in order to make the PR look high. Settings such as making Soiling Loss nearly zero, setting wiring losses lower than reality, not adequately accounting for shading, ignoring snow, or omitting auxiliary power will raise PR but can lead to large discrepancies with actual performance. Because PVSyst's PR can appear differently depending on the settings, you should not judge reliability based solely on the magnitude of the numbers.

Caution is also required when using PR for performance evaluation. The PVSyst simulation PR and the actual PR calculated from measured data can differ in measurement points and calculation methods. For measured PR, factors such as the pyranometer’s installation angle, soiling, calibration status, missing data, PCS shutdown, output control, grid outage, communication errors, etc., can have an effect. When comparing with simulation PR, it is necessary to make clear how far the measured data will be corrected.

PVSyst's PR is generally regarded as an annual representative value, but in O&M short-term PR is also examined. Short-term PR is susceptible to weather, temporary shutdowns, and sensor errors, so judging equipment anomalies based solely on PR for a single day or a single month is risky. For anomaly detection, it is necessary to combine methods such as comparison with days of similar conditions, comparison at the string level, comparison by PCS, and checking output by irradiance level.

How to Use PVSyst PR to Improve Design

PVSyst's Performance Ratio can be used not only to check results but also as a guide for design improvements. If the PR is low, thinking about which losses to reduce to increase energy production can bring you closer to a more rational design.

First, if shading losses are large, review the array layout, row spacing, tilt angle, azimuth, terrain, and surrounding obstacles. In mountainous areas and on reclaimed/developed land, terrain shading and long winter shadows can greatly affect PR. Widening row spacing will reduce shading losses but may also reduce installed capacity, so you need to evaluate not only PR but also total energy generation and land-use efficiency.

Next, if temperature losses are large, check the module installation conditions and the mounting structure. In well-ventilated installations the module temperature tends to be lower, resulting in smaller temperature losses. Roof-mounted or low-height mounting structures tend to trap heat, which can reduce PR. It is also important to ensure that PVSyst's temperature model settings match actual conditions.

If wiring losses are large, review the placement of the PCS and connection boxes, cable lengths, cable sizes, and voltage design. In particular, for large-scale projects, checking whether losses are concentrated on the DC side or the AC side will change the direction of improvements. Making the cables thicker reduces losses, but increases costs, so a balance with economic feasibility is necessary.

If PCS losses or clipping losses are large, reassess the DC/AC ratio and PCS capacity. However, clipping losses are not necessarily bad. Designing to reduce PCS capacity to lower equipment costs, while sacrificing only the limited high-output hours of the year, can be reasonable from a business perspective. In that case, PR may drop slightly, but investment efficiency could improve.

If soiling losses or snow losses are large, check the cleaning plan, snow removal policy, tilt angle, module lower-edge height, and the site’s dust and agricultural environment. By reviewing whether the settings in PVSyst match actual field conditions, you can avoid overestimation or underestimation. In particular, in snowy regions it is more important to appropriately incorporate the uncertainty of winter generation than to make the PR appear high.

When using PR for design improvements, it is important not to aim merely at increasing PR, but to optimize power generation, cost, risk, constructability, and maintainability together. PR is the entry point for improvements, and the final decision should be made based on the overall balance of the project.

Summary

PVSyst's Performance Ratio is a critically important indicator for understanding the performance of a solar power plant. However, PR is not simply a number showing a large amount of generated electricity; it is a performance ratio that indicates how much electrical energy is obtained from the incident solar irradiation after subtracting various losses.

When reading PR, first understand how it differs from the actual energy production, then check the assumptions for meteorological data and solar irradiance. After that, look at monthly PR as well as the annual PR to grasp seasonal loss trends. If PR is low, use a Loss Diagram to identify which losses—shading, temperature, soiling, wiring, PCS, transformers, auxiliary power, etc.—are affecting it.

When comparing other companies' reports or multiple cases, it is essential to align the assumptions such as meteorological data, equipment capacity, tilt angle, shading settings, loss settings, PCS conditions, power factor, and output limits. If you compare PRs with different assumptions as they are, you will be looking at differences in the configuration settings rather than the relative merits of the designs.

In practice, PR is a useful metric for design reviews, customer presentations, materials for financial institutions, and O&M evaluations. However, a high PR is not necessarily good, nor is a low PR necessarily bad. Conservative loss assumptions can make PR appear low, while underestimating losses can make PR appear high. What matters is not just the PR number itself but being able to explain the rationale behind it.

Learning to read PVSyst’s PR correctly greatly improves the validation of energy yield, the analysis of loss factors, the comparison of design proposals, and the accuracy of explanations to clients. In evaluating a solar power plant, it is important to use PR not as the final conclusion but as an entry point for interpreting the plant’s performance.

Also, to make the power generation simulations created in PVSyst more useful on site, it is important to manage and combine design drawings, survey results, as-built confirmations, site photos, and point cloud data. Using field tools like LRTK that leverage an iPhone and high-precision GNSS allows efficient on-site verification of a solar power plant’s racking positions, site preparation status, boundaries, inspection records, and post-construction as-built conditions. By combining the simulation results in PVSyst with high-precision positional information and 3D data acquired on site, it becomes easier to perform integrated quality control across design, construction, inspection, and operation and maintenance.