10 Checklist Items to Avoid Mistakes When Interpreting PVSyst

The PVSyst report is a useful document that lets you review, in one place, a solar power plant’s energy production, losses, design conditions, meteorological conditions, and system configuration. However, because it contains many items and numerous abbreviations, if you follow only the numbers without knowing where to look, it can be easy to make incorrect judgments.

In practical work, there are many situations where people look at PVSyst results and judge whether “the annual energy production is high or low,” “the PR is reasonable,” “the losses are not too large,” and “the content can be used for bank submissions or internal approvals.” However, if you draw conclusions based solely on the final energy production, you may overlook differences in input conditions, differences in meteorological data, how system capacity is interpreted, and differences in loss settings.

When reading PVSyst, the important thing is not to look at the result numbers in isolation. Rather, you should check, in order, which assumptions produced those numbers, which losses are deducted at which stages, and whether the items being compared and the conditions are consistent.

In this article, to avoid mistakes when reading PVSyst, we organize 10 checklist items that should always be checked in practice. These items can be used as basic verification steps when reading PVSyst reports for power generation assessment, design review, internal sharing, customer explanations, and review of materials for financial institutions.

Table of Contents

• What to check first when reading PVSyst

• Check item 1: Verify project conditions and site information

• Check item 2: Verify the type of meteorological data and the annual average irradiation

• Check item 3: Verify the definitions of DC capacity and AC capacity

• Check item 4: Verify the Specific Yield

• Check item 5: Verify the Performance Ratio

• Check item 6: Verify the Array Loss

• Check item 7: Verify the System Loss

• Check item 8: Verify shading losses and IAM losses

• Check item 9: Verify temperature losses and module conditions

• Check item 10: When comparing, verify differences in assumptions

• Common mistakes when reading PVSyst

• How to share PVSyst check results within the company

• Improve PVSyst accuracy by combining it with on-site verification

• Summary

What to check first when reading PVSyst

When reading a PVSyst report, many people first look at the annual energy production and the Performance Ratio. Of course, the final energy production and PR are important. However, looking at those alone is not enough to judge whether the results are reasonable.

For example, even for the same power plant, the annual energy production will change if the meteorological data used is different. If the definition of DC capacity differs, the Specific Yield will also change. If the AC-side output limits, power factor, wiring losses, transformer losses, or auxiliary losses are set differently, the final Grid Injection value will also change.

In other words, the important thing when interpreting PVSyst is to start from the assumptions and read toward the results, rather than starting from the results.

First, we verify the site, meteorological data, modules, PCS, array configuration, tilt angle, azimuth, and installed capacity. Then we sequentially review solar irradiation, Reference Yield, Array Yield, Final Yield, Specific Yield, Performance Ratio, and the Loss Diagram.

Reading it in this order makes it easier to explain the reasons the power output is high, the reasons it is low, and the reasons it differed from other simulations. Conversely, if you look only at the final power output first, you may overlook differences in input conditions and make incorrect comparisons.

The PVSyst report is not merely a table of results but a document that shows the process by which the generated energy is calculated. Therefore, the basic way to read it is to track, at each stage, which energy is being reduced and by which losses.

Checklist Item 1: Confirm project conditions and site information

The first things to confirm are the project conditions and the site information. In PVSyst, the plant’s location, elevation, time zone, meteorological data, installation azimuth, tilt angle, and so on greatly affect the results.

Latitude and longitude in particular affect calculations of solar irradiance and solar altitude, so if an incorrect location is set the assumptions underlying the expected power generation can be invalidated. Even when using meteorological data from nearby locations, actual generation characteristics can differ depending on elevation, terrain conditions, snow conditions, and whether the site is coastal or inland.

A common pitfall when interpreting PVSyst is looking only at the energy production reported and skipping verification of the site conditions. For example, for a project in Hokkaido, if there is snow but the effects of snow are not taken into account, winter energy production can appear overstated. Likewise, for a mountain-area project, if the surrounding terrain and nearby obstacles are not sufficiently included, discrepancies with measured data tend to occur.

Also, always check the tilt angle and azimuth. South-facing, east-west-facing, low-tilt, and high-tilt orientations affect not only annual power generation but also seasonal and time-of-day generation patterns. In particular, for agrivoltaic, rooftop, self-consumption, and battery-coupled systems, simple annual generation figures alone may not be sufficient for evaluation.

When looking at location information, it is important not to judge solely by the project name or site name, but to verify the latitude and longitude, elevation, the meteorological data used, tilt angle, and azimuth. If these are incorrect, then even if you later check detailed losses, the fundamental assessment will be off.

Checklist Item 2: Confirm the Types of Weather Data and the Annual Average Solar Radiation

The next thing to check is the meteorological data. PVSyst’s results can vary significantly depending on which meteorological dataset is used. Meteonorm, SolarGIS, satellite data, measured on-site data, nearby observation station data, and so on—different data sources show different trends in irradiance, temperature, diffuse irradiance, and wind speed.

In PVSyst reports, you check solar irradiation–related items such as Global Horizontal Irradiation, Diffuse Horizontal Irradiation, and Global Incident in Collector Plane. Particularly important is the relationship between horizontal plane irradiation and tilted plane irradiation. Even if the horizontal plane irradiation is the same, the irradiation incident on the array plane varies depending on the tilt angle and azimuth, and on how reflected and scattered components are treated.

If the annual average solar irradiance differs substantially from other sources, the difference in energy production may arise from differences in the meteorological data rather than from design or losses. For example, when the same power plant is evaluated by multiple companies, the PR may be similar while the annual energy production differs significantly. In such cases, the first thing to suspect is the assumed solar irradiance.

Temperature data is also important. Solar modules produce less output as temperature rises. Therefore, even with the same solar irradiance, temperature losses are larger in regions with higher temperatures or under low-wind conditions. When reading PVSyst, you need to check not only the irradiance but also how the temperature is set.

When using measured data, also check for missing data, anomalous values, time units, time zone, the pyranometer's installation angle, and the cleanliness of the pyranometer. If the meteorological data are inappropriate and you run simulations, even if the calculations in PVSyst are correct, the real-world assessment will be incorrect.

Checklist item 3: Verify the definitions of DC capacity and AC capacity

When reading PVSyst, the items you should always check are the DC capacity and the AC capacity. In a solar power plant, there is DC capacity based on module capacity and AC capacity based on the PCS and grid-connection equipment. Which one you use as the reference changes the evaluation of Specific Yield, capacity factor, overloading ratio, and output limitation.

In PVSyst, the sum of the modules' nominal outputs is treated as the Nominal PV Power. Meanwhile, the rated output of the PCS and the grid injection limit are constraints on the AC side. When the DC capacity is large and the AC capacity is small, clipping due to the PCS limit occurs during periods of high solar irradiance.

If you look only at annual energy production, the plant's performance can sometimes appear low. However, in designs that significantly oversize the DC side and constrain the AC side, a certain amount of clipping is sometimes tolerated to increase generation during mornings, evenings, and periods of low irradiance. Therefore, the presence of clipping itself is not necessarily a bad thing.

It is important to verify that the DC/AC ratio aligns with the design intent. By reviewing items such as the Pnom ratio, Inverter sizing, and Overload loss, you can determine how much module capacity has been stacked relative to the PCS capacity.

Also, when comparing with other companies’ reports or past studies, always confirm that the definition of DC capacity is the same. If they differ — for example, whether they are looking at module STC capacity, PCS rated capacity, or interconnection capacity — comparisons of kWh/kWp and plant utilization rates will be skewed.

To avoid mistakes when interpreting PVSyst, it is essential to confirm what kW installation it is being evaluated as before looking at the energy output.

Check Item 4: Verify Specific Yield

Specific Yield is a metric that indicates the annual electricity generation per 1 kWp of installed capacity. It is generally expressed in kWh/kWp and is used as an indicator that makes it easy to compare plants even when their scales differ.

When interpreting PVSyst, you can assess the appropriateness of the energy production relative to system size not only by the annual energy production itself but also by looking at the Specific Yield. For example, although the absolute values of annual energy production differ greatly between a 10 MW plant and a 50 MW plant, looking at the Specific Yield makes it easier to compare generation performance in the same location and under the same design conditions.

However, care is required when comparing Specific Yield. First, confirm whether the capacity used as the denominator is DC capacity or AC capacity. Generally, DC capacity is often used as the reference, but some documents use generation relative to AC capacity or grid-connected capacity. If you compare without checking this difference, the assessment can vary greatly even for the same amount of generation.

Also, Specific Yield is strongly affected by solar irradiance conditions. In regions with high solar irradiance, Specific Yield tends to be higher, while in regions with low solar irradiance, it tends to be lower. Therefore, judging "good performance" or "poor performance" based solely on Specific Yield is risky.

When interpreting Specific Yield, also check Reference Yield and Performance Ratio. Reference Yield is an indicator that approximates the theoretical input, representing the plane-of-array irradiance normalized by the system capacity. If Specific Yield is low, to distinguish whether this is due to low irradiance or large losses you need to examine Reference Yield and PR together.

In other words, Specific Yield is a useful comparative metric, but it should not be used as the sole basis for judgment. It only becomes a meaningful assessment when read together with location, solar irradiance, DC capacity, PR, and the loss breakdown.

Checklist Item 5: Verify the Performance Ratio

Performance Ratioは、PVSystの結果を見るうえで最も重要な指標の一つです。PRは、日射量に対してシステムがどれだけ効率よく発電できたかを示す指標です。日射条件の違いをある程度ならして、発電所の性能を比較しやすくするために使われます。

Performance Ratio is one of the most important indicators when viewing PVSyst results. PR is an indicator that shows how efficiently the system was able to generate power relative to the solar irradiation. It is used to smooth out differences in solar irradiation conditions to some extent, making it easier to compare the performance of power plants.

However, it is not as simple as a high PR always being good and a low PR always being bad. A common mistake when interpreting PVSyst is judging a power plant’s merits solely by looking at PR.

PR reflects many losses such as temperature losses, wiring losses, PCS losses, transformer losses, shading losses, IAM losses, mismatch losses, soiling losses, and auxiliary losses. Therefore, if PR is low, you need to check which loss is causing it using the Loss Diagram.

On the other hand, caution is also needed when PR is too high. It may be caused by loss settings not being sufficiently applied, soiling losses being underestimated, auxiliary equipment losses not being set, wiring losses being underestimated, shading not being taken into account, or snow effects not being included. In practice, attention is paid not only to PRs that are too low but also to those that are too high.

When comparing PRs, confirm that the targets are based on the same assumptions. Even for the same power plant, PR will change if solar irradiation data, temperature conditions, loss settings, output limits, power factor conditions, or the handling of auxiliary loads differ. Especially when comparing with materials prepared for financial institutions or with third-party evaluations, it is important not to treat PR differences simply as better or worse, but to clarify the differences in assumptions.

PR is PVSyst's overall performance indicator. However, PR is a conclusion, not a cause. After looking at the PR, always return to the loss breakdown to confirm why that value was obtained.

Checklist Item 6: Confirm Array Loss

Array Loss is an important item for checking losses that occur on the solar module side, that is, on the DC side. In PVSyst, various losses occur from the moment solar irradiance strikes the modules until it is extracted as the array output.

Typical array losses include temperature loss, IAM loss, mismatch loss, wiring loss, soiling loss, losses related to LID and degradation, and low-irradiance loss. Because these significantly affect energy production, it is necessary not only to look at the total value but also to check which loss categories are large.

Temperature losses in particular account for a large proportion at many solar power plants. Because module temperature rises cause output to drop, temperature losses are greater in summer and in high-temperature regions. They also vary depending on the racking ventilation conditions, whether the installation is rooftop-mounted or ground-mounted, and whether there is air circulation behind the modules.

IAM loss is the effect of sunlight being reflected at the glass surface when it is incident at an oblique angle. It becomes significant under conditions with a high frequency of low-angle incidence, such as in the morning and evening or during winter. It also varies with tilt angle, azimuth, and the module’s glass characteristics.

Mismatch losses and wiring losses also need to be checked. DC-side losses vary depending on string design, module variability, shading patterns, cable length, and cable size. If wiring losses are too small, verify whether the assumptions about cable length and cross-sectional area are realistic.

When reviewing Array Loss, check not only the magnitude of the loss rate but also whether it conforms to the design conditions and the actual site conditions. Even if the numbers look typical, the simulation is insufficient if they do not match the actual racking layout, terrain, cable routing, snow conditions, and soiling conditions.

Checklist item 7: Verify System Loss

System Loss refers to losses, mainly on the AC side, that occur as the DC power output from the array passes through the PCS, transformer, wiring, and the point of interconnection to the grid. When reading PVSyst's final energy production, checking this System Loss is indispensable.

Representative System Losses include PCS losses, AC wiring losses, transformer losses, auxiliary equipment losses, output limitations, PCS clipping, grid injection limits, etc. These directly affect the final Grid Injection and Energy injected into grid.

PCS losses occur based on the conversion efficiency of the PCS. Losses vary depending on the PCS model, efficiency curve, input voltage range, and load factor. Rather than simply looking at the rated efficiency, the efficiency in the actual operating range is what matters.

AC wiring losses are influenced by the cable conditions from the PCS to the transformer and from the transformer to the interconnection point. Losses vary with cable length, cross-sectional area, voltage, current, and system configuration. In large-scale power plants, wiring losses on the AC side or the medium-voltage side may be non-negligible.

Transformer losses include load losses and no-load losses. There are losses that occur while the system is generating power as well as losses that occur simply from the transformer being connected. If transformer losses are set in PVSyst, verify that those values match the equipment specifications.

Auxiliary losses are another item that is easy to overlook. If monitoring devices, communications equipment, PCS standby power, air conditioning, battery-related equipment, and so on are present, auxiliary loads will affect annual power generation. Reports that do not include auxiliary losses and reports that include auxiliary losses closer to actual conditions will show differences in PR and final generation.

When looking at System Loss, it is important to confirm what is being counted as generated energy. Depending on whether it is the PCS output, after the transformer, at the point of interconnection, or equivalent to the sales meter, the meaning of generated energy changes.

Checklist item 8: Verify shading losses and IAM losses

Shading losses and IAM losses are items that are easy to overlook when reading PVSyst. However, they are very important for explaining actual differences in energy production and month-by-month differences in energy production.

Shading losses are classified into near shading and far shading. Near shading is the shadow caused by surrounding buildings, trees, the spaces between mounting rows, inverter equipment, fences, terrain, and so on. Far shading is the effect of distant terrain such as mountains and the horizon.

At ground-mounted solar power plants, shadows between racking rows are more likely to occur during winter and in the morning and evening. If the tilt angle is large, the row spacing is narrow, or there is a terrain gradient in the east–west direction, shading losses may increase.

In PVSyst, you can create a 3D scene to perform shading calculations, but if the 3D model is not accurate, the shading loss assessment will also be inaccurate. You need to verify that the actual mounting structure height, terrain, row spacing, module layout, and the positions of obstacles are reflected.

IAM loss is the reflection loss that occurs when sunlight strikes the module surface at an oblique angle. Unlike shading, it is not caused by being blocked by a shadow; rather, as the angle of incidence increases, the amount of light captured by the module decreases.

Both shading loss and IAM loss affect power generation in the mornings, evenings, and during winter. Therefore, checking monthly power generation and how losses occur, not just annual values, provides a reading that is closer to actual conditions.

When you see reports showing shading losses close to zero, check whether there really are no shadows or whether 3D shading simply hasn't been configured. On rooftops, in mountainous areas, on developed sites, and at sites surrounded by trees, ignoring shading effects can lead to an overestimation of energy production.

Checklist Item 9: Verify Temperature Loss and Module Conditions

Temperature losses are particularly important among the loss items in PVSyst. Solar modules experience reduced output as temperature increases. Therefore, even under the same irradiance, energy production is lower when module temperatures are higher.

In PVSyst, the calculation of module temperature is influenced by the Thermal Loss factor and the installation conditions. The way a module heats up differs between ground-mounted installations with good rear ventilation and installations placed close to a roof. Under poorly ventilated installation conditions, temperature losses can be larger.

When assessing temperature losses, check the meteorological data: ambient temperature, wind speed, mounting configuration, and the module temperature coefficient. In particular, the module temperature coefficient varies by model. Modules with smaller output reductions at high temperatures and those with larger reductions will have different annual energy production.

Also, as module conditions, verify the nominal output, tolerance, degradation rate, LID, and the validity of the PAN file. PVSyst performs calculations based on the characteristics of the module selected from its module database, but you need to confirm that these match the modules you will actually use.

If the temperature losses reported are very small, check whether the installation conditions are more favorable than in reality. Conversely, if the temperature losses are large, check whether the ambient temperature conditions, racking conditions, ventilation conditions, and module temperature coefficient are appropriate.

Temperature loss is not just a number; it is a factor that reflects the on-site installation environment. Checking temperature loss is especially important for rooftop installations, self-consumption systems, confined sites, and locations with poor wind flow.

Checklist item 10: Verify differences in assumptions when comparing

The most common pitfall when interpreting PVSyst is when comparing multiple reports. When comparing in-house analyses, analyses from other companies, third-party evaluations, documents submitted to banks, EPC documents, manufacturer estimates, and so on, simply lining up only the final energy production and PR can lead to incorrect conclusions.

The first thing to check when making comparisons is the meteorological data. Even at the same location, using different meteorological data can change the annual solar radiation and temperature. Differences in power output are often due not to design but to differences in the meteorological data.

Next, check the definition of system capacity. Look at whether the DC capacity is the same, the AC capacity is the same, the PCS capacity is the same, or the interconnection limit is the same. If the basis for capacity is different, comparisons of Specific Yield and plant utilization rate will not be valid.

Loss settings are also important. Check whether the treatment of soiling losses, wiring losses, mismatch losses, transformer losses, auxiliary losses, shading losses, IAM losses, and output limits is the same. If one report includes auxiliary losses and another does not, the PR and final energy production will differ.

Also, when making comparisons, pay attention to how output control and clipping are handled. If PCS capacity, power factor, grid injection limits, battery charge/discharge conditions, or self-consumption rate differ, the Grid Injection value can vary significantly.

In PVSyst comparisons, it is more important to break down the reasons for differences than merely to find the differences in results. If you organize the differences in energy production into differences in irradiance, capacity, losses, output curtailment, and evaluation points, it becomes easier to explain them to the other party.

When creating a comparison table, list annual energy production, Specific Yield, PR, solar irradiation, DC capacity, AC capacity, major losses, auxiliary losses, shading losses, temperature losses, and the final output point to make it easier to identify the causes of differences.

Common Mistakes When Reading PVSyst

A common mistake when reviewing PVSyst is judging it solely by the final energy production. Annual energy production is important, but that value is the result of the meteorological data, system capacity, loss settings, and grid constraints. You cannot judge the validity by looking only at the results.

Another common mistake is judging performance solely by PR. PR is a useful metric, but if loss settings are low it can look high. Conversely, if realistic losses are carefully included, PR can be lower. In other words, PR alone does not determine the quality of the analysis.

Care must also be taken when comparing Specific Yield. The unit kWh/kWp is easy to understand, but unless you confirm what the denominator kWp refers to, the comparison will not be accurate. Mixing DC-capacity-based and AC-capacity-based references can greatly skew the evaluation.

Also, the Loss Diagram can be misinterpreted. Many of the losses in PVSyst are calculated with respect to the energy at the previous stage, so simply adding the loss percentages does not yield the final loss. You need to understand at which stage each loss occurs.

Ignoring shading losses and soiling losses is also a mistake. In particular, at power plants where site conditions are complex, standard values from desk studies alone may not reflect the actual situation. It is necessary to take into account site-specific conditions such as trees, terrain, snowfall, dust, farmland, coastal areas, and industrial zones.

PVSyst reports, with their neatly arranged numbers, tend to appear accurate. However, if the input conditions are inappropriate, the outputs will be inappropriate as well. When reading PVSyst, what matters is not the appearance of the numbers but the practice of verifying the assumptions and the calculation process.

How to share PVSyst check results within the company

It is important to organize PVSyst verification results in a format that is easy to share internally. In particular, when design staff, sales staff, construction staff, management decision-makers, and customer-facing staff are all looking at the same materials, explaining things using only technical terms can make it difficult to convey the intended meaning.

For internal sharing, first make the conclusion clear. Organize whether the power generation is reasonable, what is causing it to appear low, where differences with other companies' reports originate, and what points require further verification.

Next, rather than simply pasting the PVSyst numbers as-is, narrow down the items to review. Organize the information around annual energy production, Specific Yield, PR, annual irradiation, temperature losses, shading losses, wiring losses, PCS losses, transformer losses, and auxiliary losses to make assessment easier.

Also, in difference comparisons, rather than simply writing "Plan A is higher" or "Plan B is lower", explain why the difference occurred. Distinguish whether the solar irradiance is different, the loss settings are different, the capacity definitions are different, or the treatment of output limits is different — separating these points will help organize the discussion.

When explaining to customers, it's also important not to overuse PVSyst's technical terminology. For example, explaining Performance Ratio as "an indicator of how efficiently a plant generates electricity relative to solar irradiance" and Specific Yield as "the annual energy produced per 1 kW of installed capacity" makes the concepts easier to understand.

If you want to standardize how PVSyst is read within the company, creating a checklist is effective. By establishing a routine to verify—each time—the site information, meteorological data, capacity, solar irradiation, PR, Specific Yield, major losses, shading, temperature, and comparison conditions, you can reduce the chance of oversight by the person in charge.

Improve the accuracy of PVSyst by combining it with on-site verification

PVSyst is a powerful simulation tool, but it is only meaningful when site conditions are accurately reflected. Things such as the site's topography, racking layout, obstructions, cable routes, PCS placement, surrounding trees, site development status, snowfall conditions, and susceptibility to soiling may not be apparent from the numbers in the report alone.

Therefore, to avoid errors in interpreting PVSyst, it is important to combine it with on-site verification. Use site photographs, drone images, point cloud data, survey data, drawings, single-line wiring diagrams, and cable route diagrams to confirm that the simulation conditions match reality.

Shading and terrain effects in particular are easier to verify with on-site data. Shadows between rows of racking, shadows from surrounding trees, mountain shade, ground slope, and differences in elevation after grading can be overlooked from design drawings alone.

Also, on-site data are useful for as-built verification after construction. If the actual racking positions or heights differ from the design, they can affect shading, cable lengths, and inspection/access routes. If PVSyst’s assumptions diverge from the site conditions, the discrepancy between simulation and measured results can become significant.

In such verifications, using smartphone RTK, GNSS positioning, AR displays, and point cloud viewing makes it easier to reconcile the site with the design conditions. A system like LRTK that combines an iPhone with GNSS to perform high-precision on-site position checks and compare them with drawings and point clouds can also help verify the assumptions for PVSyst.

For example, by verifying on site the racking positions, the extent of site grading, roads, drainage, PCS locations, fencing, and surrounding obstructions, and cross-checking them against PVSyst’s shading and layout conditions, you can detect discrepancies early between desktop studies and actual site conditions. Power generation simulations are not something that can be completed by software calculations alone; their accuracy is improved by combining them with on-site measurements and verification.

To read PVSyst correctly, it's important not only to check the figures in the report but also to assess whether the site conditions that underpin those figures are accurate.

Summary

To avoid mistakes when reading PVSyst, it is important not to look only at the final energy production or PR, but to check sequentially from the assumptions through the flow of losses.

First, we verify the project location, meteorological data, tilt angle, azimuth, DC capacity, and AC capacity. Next, we look at Specific Yield, Performance Ratio, Array Loss, and System Loss to trace how the energy generation is determined. After that, we check shading loss, IAM loss, temperature loss, wiring loss, PCS loss, and auxiliary equipment loss.

When comparing multiple PVSyst reports in particular, you should not just place the results side by side; you need to organize the differences in meteorological data, capacity definitions, loss settings, output limits, and evaluation points. Even for the same power plant, if the assumptions differ, the energy output and PR will change.

PVSyst reports are a powerful resource for explaining the performance of a solar power plant. However, using them without checking the input conditions and loss settings can lead to incorrect conclusions.

When reading PVSyst in practice, keeping the following sequence in mind makes it easier to assess: first check the assumptions, then check the solar irradiance, next check the capacity and energy production, and finally check the breakdown of losses. When comparing, organize the reasons for any differences into solar irradiance, capacity, losses, and constraints.

PVSyst is not simply software for producing power output. It is a tool for comprehensively checking design conditions, site conditions, equipment specifications, and loss settings, and for explaining the validity of a power plant. By using the 10 check items introduced in this article, you can reduce oversights when reading a PVSyst report and make evidence-based judgments more easily in internal or customer explanations.