7 Steps, in Order, to Master Reading PVSyst

When evaluating the power generation of a solar power plant, the PVSyst report becomes an extremely important document. This is because it consolidates into a single report the information required for making generation business decisions—annual energy production, PR, Specific Yield, losses, monthly variations, PCS limitations, irradiance, temperature impacts, and so on. However, for someone looking at a PVSyst report for the first time, it can be hard to know where to begin.

PVSyst results are documents whose essence is hard to grasp by merely scanning them from top to bottom. Even if you look only at the final energy production, you cannot judge whether that figure is reasonable, conservative, or optimistic. Looking only at PR can also lead to incorrect comparisons with other projects unless you understand the influence of irradiance conditions, system capacity, loss settings, and output limitations. In other words, when reading PVSyst, it is important not only to memorize the meaning of each item but also to decide the order in which to check them.

This article organizes the order you should follow when reading PVSyst into seven steps. It is aimed at practitioners involved in the design, estimation, business feasibility assessment, energy production review, explanations to clients, and internal review of solar power plants. Even those not familiar with PVSyst terminology will find a clear, practical explanation of the order in which to check items to make it easier to explain the basis for the estimated energy production.

First, confirm the annual power generation that will serve as the conclusion

When reading PVSyst, the first thing you should check is the final annual energy production. In the report, the annual amount of electricity sent to the grid is shown with expressions such as Grid Injection, Energy injected into grid, or EGrid. The figure used directly for project planning and estimates of sales revenue is basically the value closest to this final output.

However, the important point here is not to judge performance solely by the annual power generation. Annual power generation will naturally be greater when the installed capacity is larger, and sites with higher solar irradiance have an advantage. Therefore, simply looking at the generation figures alone cannot tell you the quality of the design or the validity of the analysis conditions.

In the first step, the goal is to get a sense of the overall scale of the project. For example, check what the annual power generation is in MWh, which seasons have higher or lower generation on a monthly basis, and whether it deviates significantly from the expected generation. If extremely high or low values appear here, the cause will be investigated in subsequent steps.

When looking at annual energy production, you should simultaneously check the system capacity. If you look at energy production alone without knowing the DC capacity, AC capacity, PCS capacity, and the DC/AC ratio, you can draw the wrong conclusions. The same annual energy production means different things for a 1 MW system and an 800 kW system. When interpreting PVSyst, it is important to always check the system capacity that produces that energy together with the absolute value of the energy production before examining the latter.

Also, when explaining to financial institutions or the project owner, the final generation figure is the most closely watched number. Therefore, when presenting this figure, you should not simply say “the annual generation is this value,” but be able to explain that “this generation is the result of the specified solar irradiance, system capacity, loss conditions, and PCS limits.” In practice, it is effective to state the conclusion first and then trace the supporting rationale.

Next, check the efficiency relative to equipment scale using Specific Yield

After checking the annual generation, next look at the Specific Yield. Specific Yield is an indicator that shows the annual generation per 1 kWp of installed capacity. It is commonly expressed in kWh/kWp. While annual generation depends on system size, Specific Yield is an indicator that makes it easier to compare generation efficiency per unit of capacity.

For example, if you look only at annual electricity generation, larger power plants will show larger figures. However, by looking at Specific Yield, it becomes easier to compare generation efficiency between projects of different system sizes. Between a project with 1,200 kWh/kWp and one with 1,350 kWh/kWp, you can conclude that the latter produces more electricity per unit of capacity.

The reason to check Specific Yield early when interpreting PVSyst is that it allows you to intuitively assess the plausibility of the annual energy production. Even if the annual generation appears large, a low Specific Yield may indicate poor generation efficiency relative to the installed capacity. Conversely, if the annual generation appears small but the installed capacity is also small, the Specific Yield may be reasonable.

When looking at Specific Yield, it must be considered together with local solar irradiation conditions. Expected Specific Yield will differ between Hokkaido, Tohoku, Kanto, Chubu, and Kyushu even with the same equipment specifications. In addition, it varies with snowfall, azimuth, tilt angle, shading, PCS capacity, overloading ratio, soiling, and temperature conditions. Therefore, when judging whether Specific Yield is high or low, it is important to consider region and design conditions separately.

In practice, Specific Yield is a metric that is easy to use for internal comparisons. By comparing with past projects, you can check whether the Specific Yield for the current project is higher or lower. In particular, comparing with projects in the same region, with similar slope angles, and with similar PCS configurations makes it easier to see differences in analysis conditions and design.

However, it is risky to judge performance based solely on Specific Yield. For example, in snowy regions the module surface can be covered with snow in winter even when there is solar irradiance. In mountainous areas, the effects of terrain shading may appear. In projects where PCS output limits are stringent, peak clipping occurs during periods of high irradiance. Therefore, Specific Yield is an entry point for assessing efficiency, and you must then check the PR and loss items to understand why that value was obtained.

Check overall system performance with Performance Ratio

After Specific Yield, the next metric to check is the Performance Ratio, or PR. PR is a key indicator that shows how efficiently a photovoltaic power generation system converted the solar irradiance it received into electrical energy. Because it is often used when describing a plant’s performance, it is a very important parameter when interpreting PVSyst.

PR is not determined solely by the amount of solar irradiance. It is determined by the combined effect of various losses such as module temperature, IAM losses, soiling, shading, mismatch, wiring losses, PCS losses, transformer losses, and power curtailment. Therefore, it is easier to understand PR if you consider it an indicator of how effectively the system is generating power with minimal losses.

One point to be careful about when looking at PR is that a high PR does not necessarily mean a good project. For example, even in regions with harsh solar irradiance conditions, PR can appear high if the loss assumptions are small. Conversely, if you conservatively account for snow, shading, temperature effects, or PCS limits, PR will be lower. In short, PR is not merely a figure of merit but an entry point for understanding which losses and to what extent they were assumed.

When reading PVSyst PR, you will often compare it with other companies' reports and past projects. What you need to watch out for is whether the baseline assumptions for the items being compared are the same. If the irradiation data differs, the treatment of snow differs, the Soiling Loss settings differ, the DC wiring losses or AC wiring losses differ, or the PCS output limits differ, simply lining up PR values side by side will not produce a valid comparison.

Especially for commercial solar, differences in PR affect energy production and profitability, so accountability is important. When the project owner asks, “Why this PR?” you must be able to break down and explain temperature losses, wiring losses, PCS losses, shading, soiling, snow, output limitations, and so on. PR is both the ultimate performance metric and a guide for checking the loss composition.

Also, checking PR on a monthly basis deepens understanding. In summer, temperature-related losses tend to be greater, while in winter performance is more susceptible to the solar incidence angle and to snowfall. By reviewing monthly PR and monthly generation together, you can capture seasonal characteristics that are not visible from annual PR alone and more easily identify trends specific to a project.

Confirm assumptions for solar radiation and meteorological data

After checking PR, next verify the solar irradiance and meteorological data. PVSyst's results are highly dependent on the meteorological data you input. No matter how finely you set the loss parameters, if the underlying irradiance data are not appropriate, the reliability of the final energy production will be low.

In PVSyst you will encounter several solar-irradiation related items such as Global horizontal irradiation, Diffuse irradiation, and Global incident in collector plane. What is particularly important in practice is how the horizontal-plane irradiation is converted to irradiation on a tilted plane and, as a result, how much irradiation actually reaches the module surface. Because a photovoltaic system generates electricity from the irradiation incident on the module surface, simply looking at regional solar irradiation is not sufficient.

When reviewing meteorological data, also check the data source used. Results can vary depending on whether Meteonorm, satellite data, nearby observation data, SolarGIS-based data, etc., were used. Even for the same location, the annual average irradiance and monthly distribution may differ depending on the data source. Therefore, when reading PVSyst, it is essential to confirm not only the power generation results but also which meteorological data they are based on.

Monthly solar radiation is also important. Even if the annual solar radiation is the same, whether it is concentrated in summer or remains stable in winter affects power generation, PCS clipping, and how temperature-related losses manifest. In regions with high summer solar radiation, losses due to temperature rise also tend to be greater. Even in regions with high winter solar radiation, if there is snowfall it may not be reflected in power generation.

Always check the ambient temperature data as well. Because solar modules lose output as temperature rises, temperature-related losses are larger in regions with high ambient temperatures. In PVSyst reports, temperature losses are reflected in the loss diagram, but you need to verify that the temperature conditions in the meteorological data are reasonable as a prerequisite.

Also, in snowy regions the concepts of albedo and snow losses are important. Snow surface reflection can increase incoming solar radiation, but if the module surface is covered by snow it cannot generate power. If you do not check how PVSyst treats snow and soiling, you may overestimate winter energy generation.

Solar irradiance and meteorological data are the foundation of energy yield simulations. If you discuss only losses and PR without checking these, your interpretation of the results will be superficial. To read PVSyst correctly, it is important to work backward from the final results and first confirm whether the irradiance conditions are reasonable.

Verify where generation losses are occurring in the Loss Diagram

The next thing to check is the Loss Diagram. The Loss Diagram is a chart that shows how much of the incident solar irradiance energy entering a photovoltaic (PV) system is lost at each stage. Within PVSyst, it is one of the pages most useful for explanations in practical work.

By viewing the Loss Diagram, you can see the flow from solar irradiance to module output, array output, PCS output, and energy injected into the grid. For example, IAM loss, shading loss, soiling loss, temperature loss, low-irradiance loss, mismatch loss, DC wiring loss, PCS loss, AC wiring loss, and transformer loss are displayed in order. This allows you to verify which losses were applied in calculating the final energy output.

The reason for emphasizing the Loss Diagram when reading PVSyst is that it makes it easier to explain the energy production. Annual energy yield or PR alone do not reveal where losses are occurring. However, by looking at the Loss Diagram you can determine whether temperature losses are large, wiring losses are large, PCS clipping is significant, or shading has a major impact.

Particular attention should be paid to losses that appear larger than usual. For example, if DC wiring losses are large, you should check cable length, cable size, voltage conditions, and the layout of junction boxes and PCS. If PCS losses or output limitations are large, check the DC/AC ratio, PCS capacity, power factor conditions, and output limit settings. If shading losses are large, check the terrain, surrounding obstacles, row spacing, and mounting arrangement.

In a Loss Diagram, attention must be paid to the sign of losses. Some items may appear positive and may be presented not as simple losses but as corrections or increase factors. For example, items related to albedo effects or conversion to sloped surfaces can, depending on conditions, look like additive elements. It is important not to judge by the item name alone, but to see from which energy stage to which stage the change occurs.

Also, the Loss Diagram is also very effective for explanations to clients. When explaining "why the generation ends up at this value," showing the step-by-step decrease in output is easier to understand than describing the losses in text alone. In internal reviews as well, focusing on the Loss Diagram makes it immediately clear which settings should be prioritized for checking.

Check PCS and grid-side constraints

A commonly overlooked aspect when reading PVSyst results is the limitations of the PCS and the grid side. In a solar power plant, the power generated by the modules does not all flow directly to the grid. The final amount of energy injected into the grid is determined after passing through PCS capacity, PCS efficiency, oversizing ratio, output limits, power factor conditions, transformer losses, AC wiring losses, and so on.

The first thing to check is the relationship between DC capacity and AC capacity. If the DC capacity is larger than the PCS capacity, the PCS may reach its limit during periods of strong sunlight and peak clipping may occur. This is not necessarily a bad thing. Oversizing can increase generation in the mornings, evenings, and during low-irradiance periods, potentially maximizing annual energy yield. However, if peak clipping is too large, the balance between module capacity and PCS capacity should be reconsidered.

Next, check the PCS efficiency. The PCS has a conversion efficiency, and losses occur in the process of converting DC to AC. In PVSyst these are reflected as PCS losses, but it is important to confirm that the PCS model/type being used and the efficiency curve are appropriate. If the equipment database selection differs from the actual equipment, it will also affect the power generation.

Power factor settings also require attention. If the PCS has power factor conditions, the way output limits appear can change depending on the relationship among active power, reactive power, and apparent power. Viewing results without understanding how PVSyst defines power factor can lead to misunderstandings about PCS limitations and their impact on PR. In particular, for projects with contractual power factor conditions or grid interconnection requirements, it is necessary to carefully check the configured settings.

AC-side losses must not be overlooked. Wiring losses and transformer losses from the PCS through the transformer, substation equipment, and the point of interconnection affect the final amount of energy injected into the grid. Attention tends to focus on DC-side losses, but what ultimately affects the entire plant's revenue is the final amount of energy delivered to the grid. Therefore, AC wiring losses and MV transformer losses should also be checked for consistency with estimates and design conditions.

When reading PVSyst, it's essential to check not only the module-side generation performance but also the power conversion and transmission flow downstream of the PCS. A solar power plant only has value when the electricity generated by the panels can ultimately be taken out as power that can be sold or used for self-consumption. Verifying the PCS and the grid side is an important step in linking generation to project financials.

Check seasonal variations and outliers in monthly results

Finally, review the monthly results. Annual generation and annual PR alone may overlook seasonal characteristics or anomalies. PVSyst allows you to check monthly generation, irradiation, PR, temperature, losses, and so on, so after looking at the annual values it is important to check the monthly behavior.

When you look at monthly results, you can see which months have higher and lower power generation. Generally, generation increases during periods with high solar irradiance and when temperature-related losses are not excessively large. Conversely, the rainy season, typhoons, snowfall, reduced winter solar irradiance, and high summer temperatures can create generation troughs that vary by region. By checking monthly generation, you can determine which seasons a project is strong in and which seasons it is weak in.

Monthly PR is also important. Even if the annual PR is reasonable, if the PR in a particular month is extremely low, it may indicate the influence of snow, shading, temperature, output limits, or soiling. Conversely, if the PR is unusually high in only certain months, it is also necessary to check solar irradiation and temperature conditions, as well as the influence of albedo.

Monthly results are a convenient metric for comparing against measured values. After operation begins, comparing the actual energy generation with PVSyst’s monthly generation lets you verify whether the system is producing as expected. However, when comparing with actual values you need to take into account actual weather, downtime, output curtailment, equipment failures, snowfall, soiling, the pyranometer’s accuracy, and so on. It is risky to conclude that a simulation was accurate or inaccurate based solely on a comparison of monthly generation.

Monthly results are also useful for explaining things to the client. Rather than showing only the annual generation, explaining, "This power plant generates more from spring through summer and decreases in winter due to reduced solar irradiance and snowfall," makes it easier for the client to understand the plant's characteristics. Monthly explanations are especially important for projects in snowy areas, mountainous regions, or those affected by shading.

By viewing the monthly results, you can understand the seasonal variations behind the annual values. When interpreting PVSyst, after confirming the annual results, breaking them down by month to check for any inconsistencies is an effective final check.

Reading the 7 steps in sequence reveals the overall picture of PVSyst

When reading PVSyst, the important thing is not to view each item in isolation but to read them in sequence and relate them to one another. First, grasp the conclusion from the annual energy production, check the efficiency per capacity with Specific Yield, and look at the overall system performance with PR. Then confirm the assumptions for solar irradiance and meteorological data, understand the breakdown of losses with the Loss Diagram, check PCS and grid-side constraints, and finally check seasonal variations with the monthly results. Reading in this order makes it easier to explain PVSyst results in practical work.

The reason many people get lost when reading PVSyst is that there are so many items and they don’t know where to start. However, if you decide on an order in which to check things, you can read even a complex report in an organized way. It is important not to stop at just looking at the annual energy production, but to trace how that production was calculated — under what conditions and through what losses.

In particular, for estimates and feasibility assessments, being able to explain the generation figures is more important than the figures themselves. The points that stakeholders focus on differ: the project owner, financial institutions, designers, construction teams, and O&M staff. The project owner cares about annual generation and profitability. Designers care about wiring losses and PCS capacity. Construction teams care about layout and shading effects. O&M staff care about performance comparisons and monthly variations. If you can read PVSyst in sequence, it becomes easier to provide explanations tailored to each stakeholder.

Moreover, this seven-step process is also effective when comparing reports from other companies. Rather than simply comparing annual energy production or PR, checking, in order, the irradiance data, loss settings, PCS conditions, wiring losses, the handling of snow and soiling, and monthly trends makes it easier to identify the causes of differences. This enables you to organize and explain differences in assumptions, instead of making the simplistic judgment that a high PR is good and a low PR is bad.

PVSyst is a powerful tool for predicting the future energy production of a solar power plant, but unless you can correctly interpret the results, you cannot fully utilize it. The key is to treat the final energy output as the conclusion while sequentially checking the underlying irradiance, temperature, losses, equipment limitations, and month-by-month variations.

Connecting PVSyst interpretation to on-site verification and design improvements

PVSyst reports are not something to be left as desk-based energy yield calculations. Their practical value increases when the insights they provide are linked to on-site verification, design improvement, construction management, and post-completion energy yield verification.

For example, if the shading loss in the Loss Diagram is large, it is necessary to check the site’s topography, surrounding trees, buildings, and racking spacing. If wiring losses are large, there is room to review the placement of PCS and junction boxes, cable routes, and cable sizes. If PCS clipping is large, it is necessary to reconfirm the DC/AC ratio, PCS capacity, and output limiting conditions. If monthly results show a large drop in generation during winter, it is important to compare site conditions with the effects of snow, low solar irradiance, and shading.

Thus, interpreting PVSyst is not merely a technique for reading the numbers in a report, but also a verification procedure that connects design and the field. By confirming that the losses in the simulation are consistent with actual field conditions, the reliability of power generation forecasts is increased.

In recent years, the use of on-site 3D data and high-precision positioning information has increased in the design and construction management of solar power plants. For example, using a system like LRTK that combines an iPhone with RTK GNSS to acquire site positions with high precision makes it easier to verify racking positions, site grading status, boundaries, equipment layout, and inspection points in the field. By comparing the layouts and shading conditions assumed in PVSyst with the actual conditions on site, it is possible to reduce the gap between simulation and construction/operation.

Especially when there is a discrepancy in energy output, looking only at PVSyst results may not reveal the cause. Only by checking on-site factors such as tilt and orientation, nearby obstructions, site grading, equipment layout, cable routes, snow conditions, and the presence of weeds or soiling can you sometimes explain the difference from the simulation. Combining PVSyst interpretation with on-site verification is crucial for quality control of solar power plants.

Summary

The order to follow when reading PVSyst consists of seven steps: annual energy production, Specific Yield, Performance Ratio, irradiance and meteorological data, Loss Diagram, PCS and grid-side constraints, and monthly results. Checking in this order makes it easier to understand not only the final energy production figure but also the assumptions and losses through which that figure was calculated.

Because PVSyst reports contain a large amount of information, trying to read every detail from the start can easily be confusing. First, establish the annual energy production, which is the main conclusion, and then, in order, check the efficiency per unit of capacity, the overall system performance, meteorological conditions, the breakdown of losses, equipment constraints, and monthly variations.

In practice, it is important not only to use PVSyst’s numbers as-is but also to be able to explain why those numbers were produced. The ability to interpret PVSyst is required in a variety of situations, such as owner briefings, internal reviews, estimates, materials for financial institutions, comparisons with other companies’ reports, and verification of actual performance after commissioning.

If you have the ability to read PVSyst correctly, you can not only increase the reliability of energy generation forecasts but also link it to design improvements and on-site verification. By understanding final energy yield, PR, losses, irradiance, PCS, and monthly variations as a single flow, you will be able to evaluate the performance of a solar power plant more accurately.