Six Perspectives for Learning How to Interpret PVSyst for Commercial Projects

Table of Contents

• Why interpreting PVSyst is important for commercial solar projects

• Viewpoint 1: Read from assumptions as well as energy yield

• Viewpoint 2: Read PR as an indicator of commercial viability

• Viewpoint 3: Assess the validity of irradiance and meteorological data

• Viewpoint 4: Interpret loss items as power plant risks

• Viewpoint 5: Read the impact of PCS capacity and oversizing

• Viewpoint 6: Identify causes of discrepancies through report comparison

• For commercial PVSyst, alignment with on-site conditions is important

• Summary

Why Interpreting PVSyst Is Important for Commercial Solar Power

In commercial solar power generation, it is not sufficient to view PVSyst results merely as an annual energy production figure. PVSyst is a widely used software for simulating the energy output of solar power plants, but the numbers it produces are calculated based on many assumptions such as input conditions, weather data, layout conditions, equipment configuration, loss settings, output limitations, and operating conditions.

Therefore, understanding how to read PVSyst means not only reading the numbers on the results screen but also verifying the conditions under which those numbers were generated. In commercial projects in particular, even slight differences in energy production can affect revenue from electricity sales, lending decisions, payback periods, O&M planning, performance guarantees, EPC contracts, and the evaluation of PR tests.

With residential solar, you can make a reasonably informed judgment based solely on approximate annual generation and estimated electricity bill savings. However, for commercial solar, the same 1% difference in generation can translate into a large monetary difference. At plants ranging from a few MW to several tens of MW, a 1% difference in annual generation has a significant impact on long‑term revenue, so interpreting PVSyst requires a higher degree of accuracy.

Also, PVSyst results serve as documentation to verify whether a power plant’s design is appropriate. Various factors that occur in an actual power plant—module orientation, tilt angle, row spacing, PCS capacity, DC wiring, AC wiring, transformers, shading, soiling, temperature rise, IAM, mismatch, degradation, and so on—are reflected in the simulation.

What’s important is not to view PVSyst’s results simply as “right or wrong,” but to interpret them in terms of “under which assumptions they are reasonable,” “which factors are pushing the energy production up,” and “which losses are being underestimated or overestimated.”

This article organizes how to read PVSyst for commercial solar from six perspectives. It explains a way of thinking you can use when you receive a PVSyst report, when comparing reports from other companies, when you feel the estimated power generation is too high or too low, or when you want to verify the basis for a business feasibility assessment.

Perspective 1 Read from the assumptions, not just the power output

When looking at a PVSyst report, what many people check first is the annual energy yield. For example, figures such as the annual output in MWh, the annual output per kW in kWh/kW, and the PR as a percentage. Of course, these are important results. However, for PVSyst used in commercial projects, simply looking at the results is not sufficient.

The first thing to check is what assumptions are used in the calculation. PVSyst's annual energy production varies depending on module capacity, PCS capacity, installation azimuth, tilt angle, meteorological data, albedo, soiling, shading, wiring losses, temperature losses, PCS losses, and other conditions. In other words, even for the same power plant, the results can change significantly if the assumptions change.

For commercial solar power, you need to carefully read the items in the first part of the PVSyst report, such as Project, Site and Meteo, Orientation, System, Near Shadings, and Detailed losses. In particular, the plant's location, the meteorological data used, the tilt angle, azimuth, module model, PCS model, string configuration, and oversizing ratio must always be checked as basic information.

For example, even a slight shift between the recorded location of a power plant and the actual planned construction site can change solar irradiance and air temperature conditions. In mountainous areas, snow-prone regions, coastal areas, basins, former golf course sites, and reclaimed land, topography and surrounding environment can more easily alter meteorological conditions. For commercial projects, it is necessary not just to use meteorological data from a nearby point, but to assess whether that point actually represents the conditions at the power plant.

Also, the relationship between module capacity and PCS capacity is important. In commercial solar installations, it is common to adopt an oversizing design in which DC capacity is larger than PCS capacity. A higher oversizing ratio tends to improve generation efficiency under low irradiance, but under high irradiance it makes clipping due to PCS output limits more likely. Therefore, it is necessary to check not only the annual generation but also the extent of the losses caused by PCS limitations.

Furthermore, installation azimuth and tilt angle directly affect energy production. If they are close to due south and set at an appropriate tilt angle, annual energy production tends to be higher, but land shape, site development conditions, racking layout, drainage planning, road layout, constraints from adjacent properties, and so on mean that ideal conditions cannot always be achieved. When reading PVSyst, it is important to verify that the azimuth and tilt settings match the actual design drawings and survey maps.

Thus, when reviewing PVSyst results, it is important not to start by looking at the annual energy production, but to read what assumptions that production was calculated from. If you compare only the production figures without checking the assumptions, you may overlook differences in the meteorological data, loss settings, PCS settings, and shading conditions.

For commercial projects, a PVSyst report is not merely a power generation document but a technical document that links design conditions with revenue assumptions. Rather than viewing it as good because generation is high or bad because it is low, the first reading should verify whether the underlying assumptions are realistic, explainable, and suitable for use in contract and financing decisions.

Perspective 2: Interpreting PR as an Indicator of Business Viability

One of the important indicators when interpreting PVSyst is PR. PR stands for Performance Ratio and is an indicator of how efficiently a solar power plant converts the solar irradiance it receives into electrical energy. In commercial solar power, PR is often used to compare the performance of plants and is frequently checked among EPC, O&M, investors, financial institutions, and technical consultants.

However, PR is not simply better when higher. When PR is high, it may indicate excellent design, but it can also mean that loss settings are too lenient, temperature conditions are favorable, soiling losses are underestimated, shading is not sufficiently accounted for, wiring losses are set too low, or PCS limits are not correctly reflected. Conversely, a low PR does not necessarily mean the design is poor; it may realistically incorporate conservative loss settings, severe weather conditions, snow, shading, or output curtailment.

In PVSyst for commercial projects, when reading PR you first confirm which stage the PR value refers to. In PVSyst reports the annual PR is shown as a simulation result, but behind it there is a Loss Diagram. By looking at the Loss Diagram you can see how much is lost at each stage—starting from solar irradiance, through array output and PCS output, up to the point of grid connection.

When interpreting PR as a business indicator, it is important to compare power plants under the same conditions. For example, comparing PR is meaningful if they are in the same region, have similar tilt angles, similar mounting systems, and similar overloading ratios. However, if you simply compare snowy regions with warm regions, fixed-tilt systems with tracking systems, low-voltage distributed systems with mega-solar, or mountainous sites with flat land, it becomes difficult to tell whether PR differences are due to design differences or environmental differences.

PR is important for commercial projects because it may be used for power plant performance evaluation and warranty conditions. In PR testing after plant completion, performance is evaluated based on measured solar irradiance, air temperature, and power generation. If the PR in PVSyst is close to contractual or warranty criteria, it is necessary to confirm in advance that its calculation assumptions are consistent with the measured evaluation.

For example, PVSyst calculates temperature losses, but in actual PR tests the methods for correcting module temperature and ambient temperature conditions may be specified separately. Also, if PCS power factor settings, output limits, auxiliary power, transformer losses, or the definition of the point of interconnection differ between the PVSyst report and the measured evaluation, the assumed PR and the actual evaluation results may diverge.

When looking at PR, it is useful to check monthly values as well as the annual value. Even if the annual PR appears reasonable, examining monthly data can reveal trends such as a large decline in summer, excessively high values in winter, unnatural treatment of snowy months, or fluctuations during the rainy season that do not match reality. For commercial projects, not only annual revenue but seasonal generation patterns and cash flow are important, so understanding how to read monthly PR is also essential.

PVSyst's PR is a convenient indicator that expresses a power plant's performance as a single number. However, isolating the figure and judging based solely on it can lead to misunderstandings. In commercial projects, it is important to treat PR as a metric to be interpreted across a plant's health, design quality, loss settings, profitability, and warranty conditions.

Perspective 3: Assessing the Validity of Solar Radiation and Meteorological Data

When interpreting PVSyst, solar irradiance and meteorological data are particularly important for commercial projects. In photovoltaic simulations, the irradiance you input forms the basis of the energy production. No matter how finely you adjust equipment settings and loss settings, if the meteorological data itself does not match reality, the reliability of the annual energy yield will decline.

In PVSyst, various meteorological data sources may be used, such as Meteonorm, NASA-derived datasets, SolarGIS, processed Japan Meteorological Agency data, and on-site observational data. For utility-scale solar PV projects, it is necessary to verify which data are being used, the representative period they cover, how the global horizontal irradiance and the diffuse irradiance on the horizontal plane are provided, and whether the temperature data are appropriate.

When assessing solar irradiance, first check the annual Global Horizontal Irradiation, that is, the global horizontal irradiation on a horizontal plane. Next, check the value after it has been converted to tilted-plane irradiance. Because solar modules are installed tilted rather than flat, irradiance on the horizontal plane is converted to irradiance on the tilted plane. This conversion involves azimuth angle, tilt angle, direct (beam) irradiance, diffuse irradiance, reflected components, and so on.

In commercial projects, annual energy production can vary by several percent even within the same region depending on the meteorological data used. A difference of several percent can have a significant impact on project finances. Therefore, when reading a PVSyst report, it is important not to try to explain differences in energy production solely by loss settings, but first to check for differences in solar irradiation.

Ambient temperature data is also important. Solar modules lose output as temperature rises. In regions with high summer temperatures, temperature-related losses are larger. PVSyst uses the ambient air temperature from meteorological data and a thermal model to estimate module temperature and calculate temperature losses. Therefore, if the temperature data is set lower, temperature losses will be smaller and the estimated energy production may be higher.

In regions with snowfall, extra caution is required. In areas with snow, winter power generation can decrease when modules are covered by snow, while albedo can increase due to reflection from the snow surface. How PVSyst handles snow and albedo can significantly change predicted winter generation. If snow losses are not adequately accounted for while only the snow-surface reflection is favorably represented, winter generation may appear overestimated.

When reading solar irradiance and meteorological data, it is also useful to compare them with performance records from nearby power plants or public meteorological data. If there are records from nearby plants, you can compare annual generation per 1 kW and monthly generation trends. However, since actual values include output curtailment, shutdowns, failures, soiling, snow accumulation, and measurement errors, you should clarify the conditions and examine them rather than making a simple comparison.

PVSyst’s energy production starts from solar irradiance. If the irradiance is 1% higher, then, all other conditions being equal, the energy production will generally be higher as well. Therefore, in commercial assessments it is essential to verify the validity of the meteorological data, not just the Loss Diagram and PR. Being able to explain the basis for the meteorological data is extremely important for investment decisions and technical explanations.

Perspective 4: Interpreting Loss Items as Risks to the Power Plant

The part of a PVSyst report that requires the most careful review is the losses section. In PVSyst, the generated energy is reduced step by step by various losses as it progresses from solar irradiation to the final grid output. This flow is shown in the Loss Diagram.

In commercial solar power, it is important to interpret loss items not merely as calculation deductions but as the power plant’s design risks, construction risks, and operational risks. This is because many of the losses correspond to actual site conditions and design quality.

Typical losses include near shading, far shading, IAM losses, soiling losses, temperature losses, low-irradiance losses, mismatch losses, module quality losses, LID, wiring losses, PCS losses, PCS output limitation, transformer losses, auxiliary power consumption, transmission losses, and so on. The PVSyst results can vary greatly depending on the extent to which these items are expected.

Shadow losses are particularly important for commercial-scale projects. Shadows cast by mountains, trees, constructed slopes, surrounding buildings, utility poles, fences, adjacent arrays, and between racking rows affect energy production. Even when performing 3D modeling in PVSyst, it is necessary to verify that the actual terrain and obstacles are correctly represented. At power plants with complex terrain, how shadows are interpreted determines the accuracy of energy yield assessments.

Soiling loss is another item that is easy to overlook. In commercial-scale solar PV, the causes of soiling vary by region and include dust, pollen, yellow sand, bird droppings, volcanic ash, sea salt, dust from agricultural land, and dust from land development. It is important to check whether the soiling loss in PVSyst is treated as a constant for the year or set on a monthly basis, and how the cleaning effect of rainfall is being accounted for.

Temperature loss refers to the loss that occurs when module temperature rises. It varies with the racking's ventilation conditions, ground surface condition, installation height, rear-side airflow, and the module temperature coefficient. In PVSyst, this is related to the setting of the thermal loss coefficient. For ground-mounted commercial PV, ventilation conditions can be better than for roof-mounted systems, but heat can accumulate with low racks or dense layouts.

Wiring losses are an item that often requires technical explanation in commercial projects. You need to check which parts—such as DC-side wiring, AC-side wiring, MV wiring, and transformers—are included in PVSyst. For example, whether the calculation range includes the modules to the junction box, the junction box to the PCS, the PCS to the transformer, or the transformer to the point of interconnection will change how the losses appear.

Also verify that PCS losses and transformer losses match the actual equipment specifications. Even when using equipment characteristics from PVSyst’s database, the actual model to be adopted, operating range, efficiency curves, power factor settings, output limits, and night-time consumption may not coincide. Especially for large-scale projects, cross-checking with the specifications is important because PCS and transformer losses affect long-term revenue.

When reading PVSyst loss items, it becomes easier to understand if you separate not only the magnitude of the loss rates but also whether the loss can be managed on site, improved in the design, or varies with operation. Shading and wiring are items that are easy to improve at the design stage. Dirt and snow are affected by operational management. Temperature and solar irradiance are items that are strongly influenced by regional characteristics.

In commercial projects, loss items serve both as documentation explaining energy production and as a checklist for risk management. When reading PVSyst, it is important to review each loss item one by one to clarify what the primary factors reducing energy production are, which items can be improved, and which items are being treated conservatively.

Perspective 5: Interpreting the impact of PCS capacity and overloading

One thing you should always check in PVSyst for utility-scale solar is the relationship between PCS capacity and overloading. At solar power plants, it is common to design the modules' DC capacity to be larger than the PCS's AC capacity. This is generally called overloading.

The reason for oversizing is to use the PCS more efficiently. Solar modules do not always deliver their rated output. In the mornings and evenings, on cloudy days, in winter, and during periods of low irradiance, their output decreases. Therefore, by making the DC capacity larger than the PCS capacity, the PCS can operate near its rated output for longer periods, which can increase annual energy generation.

On the other hand, if the overloading ratio is too high, during high irradiance a DC output exceeding the PCS limit can occur, and the PCS will restrict the output. In PVSyst, this restriction is shown as an item such as "Inverter loss over nominal inverter power." For commercial projects, you should always check how large the PCS limiting losses are.

When evaluating PCS capacity and oversizing, you should not simply look at the ratio of DC capacity to AC capacity; you need to understand the design philosophy of the entire power plant. The optimal oversizing ratio varies depending on the feed‑in tariff, interconnection capacity, land area, module price, PCS price, the possibility of output curtailment, grid interconnection conditions, the contracted capacity with the utility, and whether battery storage is present.

For example, in projects where the interconnection capacity is limited, and therefore the PCS capacity cannot be increased, a design that increases DC capacity to secure generation during low solar irradiance is sometimes adopted. In that case, even if some clipping losses occur during high solar irradiance, it can be reasonable from an annual revenue perspective. Conversely, if losses due to PCS constraints are too large, the additional module capacity may not be contributing sufficiently to revenue.

Also, attention is required for the PCS power factor settings. In commercial solar power, grid interconnection conditions may require fixed power factor operation or reactive power control. You need to confirm how PCS capacity is treated in PVSyst, how the power factor setting affects the active power limit, and whether apparent power or active power is used as the reference.

An easy-to-overlook aspect when interpreting PCS capacity is the definition of the generation side and the transmission side. Depending on whether PVSyst’s results correspond to the PCS output, after the transformer, or the interconnection point, the amount of generation that should be used for project financials changes. The power sold by a plant is usually evaluated at the metering point. If you do not confirm whether the PVSyst output value corresponds to the metering point, you may misinterpret the generation figures.

By checking the monthly PCS curtailed losses, you can identify characteristics of an oversized design. If curtailed losses are concentrated from spring to summer, it can be interpreted that clipping is occurring during periods of good solar irradiance. If curtailed losses are also large in winter, snow reflection and increased module output due to low temperatures may be influencing the result.

In commercial PVSyst, PCS capacity and oversizing are design elements that directly affect profitability. When DC capacity is increased to boost energy generation, it is important to assess how much limitation loss occurs and whether the investment remains worthwhile even if those losses are accepted.

Perspective 6 Understanding the Causes of Differences Through Report Comparison

In commercial solar projects, it is common to compare multiple PVSyst reports for the same power plant. Multiple reports may be produced depending on the purpose — analyses by the EPC, by third-party organizations, by the project owner/operator, for financial institutions, and analyses before and after design changes.

What’s important here is not just to look at differences in annual energy production or PR, but to break down the causes of those differences. If PVSyst’s results are higher or lower than another company’s report, you need to determine which items are producing the discrepancy.

The first things to compare are DC capacity and AC capacity. If the number of modules, module model, number of PCS units, PCS model, or oversizing ratio differ, the annual energy production will naturally change. When comparing energy production, it's easier to see differences by comparing annual energy production per kW as well as total annual production.

Next, verify the differences in meteorological data. When the meteorological data used differ, variations in solar irradiation and temperature can be major factors causing differences in power generation. In particular for commercial projects, differences among SolarGIS, Meteonorm, on-site measurement data, and nearby station data can change generation by several percent. When comparing reports, it is effective to first place annual and monthly solar irradiation side by side for review.

Next, we will check the differences in installation conditions. When azimuth, tilt angle, spacing between racking rows, terrain shading, nearby shading, albedo, or snow conditions differ, the power generation will change even with the same equipment configuration. In particular, in mountainous or sloped areas, the method used to model shading conditions is likely to be the cause of the differences.

Comparing loss settings is also important. Compare soiling loss, wiring loss, mismatch loss, module quality loss, LID, temperature loss, PCS loss, transformer loss, auxiliary equipment loss, and so on, one by one. Rather than looking only at the total loss, check which losses are high and which are low.

For example, if a report shows a high PR, possible factors include low soiling loss, low wiring loss, low shading loss, low temperature loss, and low loss due to PCS limits. Sometimes one factor differs significantly, while in other cases several items each contribute a slight advantage.

When comparing reports, it is easier to understand if you read PVSyst's Loss Diagrams side by side. Starting from solar irradiance, then tilted-plane irradiance, irradiance after shading, array output, PCS output, and grid output, you look at how much difference there is at each stage. If differences appear in the early stages, the influence of meteorological data or shading conditions is large; if they appear in the later stages, the influence of PCS, wiring, transformers, auxiliaries, and so on is likely large.

In commercial projects, you may need to explain the comparison results of PVSyst reports to customers, financial institutions, technical reviewers, and internal approval committees. In such cases, simply saying "this one has higher energy production" or "this one has a lower PR" is not persuasive. You need to be able to explain which assumptions differ and why those assumptions are reasonable.

Investigating the causes of discrepancies can also lead to design improvements. If wiring losses are large, PCS placement and cable sizing would be candidates for review. If shading losses are large, racking layout and site development plans would be candidates for review. If PCS-limiting losses are large, the oversizing ratio and PCS capacity would be candidates for review.

In comparing PVSyst reports, it is important to examine the structure of the differences rather than which result is better. For commercial projects, being able to explain differences in energy production directly contributes to technical credibility.

In commercial PVSyst, alignment with on-site conditions is important

PVSyst is a powerful simulation tool, but if the input conditions do not match the actual site conditions, the reliability of the results decreases. For utility-scale solar, it is important to review it while cross-checking with design drawings, survey maps, land development plans, racking layout drawings, single-line wiring diagrams, equipment specifications, cable routes, topographic data, obstacles that cause shading, and site photographs.

In particular, topography and site layout conditions have a major impact on power generation. On flat terrain the evaluation can be relatively straightforward, but on sloped sites, valley terrain, formerly forested land, former golf courses, and reclaimed/developed sites, orientation, slope, shading, drainage, racking height, and inter-row spacing become complex. It is necessary to check how well the model in PVSyst reflects the on-site conditions.

Also, to improve the accuracy of on-site verification, using GNSS surveying, point cloud data, drone surveying, and AR-based on-site checks is effective. For example, by leveraging GNSS positioning and on-site verification systems that can be used in combination with an iPhone, such as LRTK, it becomes easier to confirm design drawings, on-site location, racking layout, survey points, and construction status in the field. PVSyst's analysis results are desk-based simulations, but the terrain, location, layout, and construction status that underpin them need to be verified on site.

In commercial solar power projects, power output analysis, design, construction, and O&M are sometimes carried out by different personnel or companies. As a result, discrepancies between the conditions in PVSyst and the actual site can go unnoticed. Changes such as the azimuth being different from the drawings, PCS layout being altered, cable routes becoming longer, post-development terrain differing from the original plan, or increased shading from surrounding trees can all affect power generation.

When interpreting PVSyst, the important thing is not to treat the report as a standalone. Only by comparing PVSyst’s figures with site conditions, design drawings, construction conditions, and operational conditions can it be used as a decision-making resource for commercial projects.

Also, for commercial projects it is important to compare against actual performance data after the plant is completed. By comparing the monthly generation estimated by PVSyst with the actual monthly generation, it becomes easier to detect soiling, shading, outages, curtailment, equipment malfunctions, measurement errors, and the like. PVSyst can be used not only before construction but also for performance evaluation after the start of operation.

Summary

To understand how to interpret PVSyst for commercial use, it is important not just to look at the annual energy production or PR, but to read what assumptions those figures are based on.

First, confirm the site location, meteorological data, azimuth, tilt angle, module capacity, PCS capacity, and string configuration that form the basis for the power generation assumptions. Next, interpret PR as an indicator of plant performance and check not only the annual value but also monthly trends and the relationship with the Loss Diagram.

Furthermore, we verify the validity of solar irradiance and meteorological data and assess how differences in input data affect power generation. Regarding loss items, we treat shading, soiling, temperature, wiring, PCS, transformers, auxiliary equipment, and similar factors as risks to the power plant, and identify which losses are design or operational challenges.

Regarding PCS capacity and overloading, you need to check not only the ratio of DC capacity to AC capacity but also clipping losses, power factor, interconnection capacity, and the definition of the point of interconnection. When comparing multiple reports, it is important to break down and read not only the differences in generation and PR but also where differences arise in weather data, installation conditions, loss settings, and PCS conditions.

In commercial solar power, the results from PVSyst relate to investment decisions, financing, design studies, contracts, performance guarantees, and O&M planning. Therefore, reading PVSyst is not merely knowledge of software operation, but a technical interpretation for assessing a power plant’s commercial viability and risks.

When reading a PVSyst report, it is important not to take the numbers at face value but to check, in order, the assumptions, losses, comparisons, site conditions, and operating conditions. Doing so makes it easier to determine whether the estimated energy production is reasonable, whether it can be used for revenue planning, and whether there is room for design improvement.

In commercial solar projects, aligning the on-site conditions with the simulation conditions is the starting point for a reliable energy yield assessment. The ability to read PVSyst correctly is an indispensable perspective for protecting a plant’s profitability, improving design quality, and strengthening the ability to explain results to stakeholders.