5 Ways to Interpret PVSyst Annual Results | Check Annual Energy Production

When handling solar power system design and energy yield forecasting in practice, the first thing you notice is the annual results. Because annual energy production, specific yield, PR, loss rates, and so on are summarized, at first glance it may seem sufficient to look only at these. However, the annual results in PVSyst are not merely a list of answers. They are the annualized organization of the results obtained by stacking the site's solar irradiance, incident illumination conditions, array losses, and system losses. PVSyst's official documentation also explains that the results include many variables and that the Loss Diagram is particularly useful for identifying design weaknesses.

Especially for practitioners searching for "how to read PVSyst", the first hurdle is often deciding where to look in the annual results. If it is unclear whether a project should be evaluated solely by annual energy production, whether PR should be emphasized, or how to interpret loss rates, comparisons and explanations become unstable. In PVSyst, Specific Production corresponds to the annual Yf, and PR is defined as a summary value obtained by dividing the energy effectively produced by the incident energy and the system capacity. In other words, the annual results contain numbers with different roles mixed together, so it is important to decide on an order for reading them.

Annual results are easy to read but can also lead to misunderstandings. For example, it is risky to take a simulation’s annual results at face value as if they were definite future values. PVSyst’s explanation of the P50–P90 assessment also states that simulation results strongly depend on the nature of the meteorological data used: multi-year average data are generally easier to treat as averages, whereas single-year data cannot be regarded as representative values as-is. Annual results are convenient output figures, but it is important to read them as aggregated results of the underlying assumptions.

This article organizes how to read PVSyst’s annual results in practical work into five steps. First, look at annual energy production to understand the scale; next, check the specific yield to confirm the performance per installation; then examine solar irradiation and incident-light conditions to assess the foundation; follow the Loss Diagram to trace where reductions occur; and finally position PR as the summary value of the annual results. It also clarifies commonly misunderstood points and summarizes how the accuracy of on-site conditions relates to confidence in the annual results.

Table of Contents

• Key assumptions to grasp before reading power generation forecasts

• How to Read 1｜Confirm annual generation as the "scale"

• How to Read 2｜Assess per-installation performance using specific yield

• How to Read 3｜Verify the fundamentals using solar irradiance and incident light conditions

• How to Read 4｜Track where losses occur using the Loss Diagram

• How to Read 5｜Do not evaluate PR in isolation; use it as a summary metric of annual results

• Common misunderstandings

• The accuracy of on-site conditions affects confidence in the annual results

• Summary

Key assumptions to keep in mind before reading power generation forecasts

Before reading the annual results, the first thing to understand is that PVSyst’s results are organized as a flow. Solar irradiation occurs, it is received at the installation surface, converted to power by the array, and—after losses—becomes the final grid injection and usable energy. In PVSyst’s normalized indicators, Yr is the reference ideal yield, Ya is the array output, and Yf is the final usable energy; the differences are represented as Collection Losses (Lc) and System Losses (Ls). To read the annual results correctly, it is important to keep this structure in mind first.

Also, the annual results are a compressed result of all assumptions. If conditions such as site weather data, azimuth, tilt, shading, temperature, module quality, mismatch, wiring, and inverter efficiency change, the annual results will change as well. The official documentation also explains that detailed simulations can analyze thermal behaviour, wiring, module quality, mismatch, incidence angle losses, far shading, near shading, and that those results appear in the report and the Loss Diagram. In other words, annual results should be read not as mere numbers but as an aggregation of the assumptions.

In practice, simply understanding this premise makes your perspective much more stable. Instead of judging the annual results the moment you see them, you begin to consider under what conditions those numbers were produced. If you look in this order — annual energy production as the input number, specific yield as the sense of efficiency, irradiance as the foundation, the Loss Diagram as the causes, and PR as the summary value — the resolution of the results increases. Not judging by a single metric is the basic principle for using PVSyst in practice.

Reading 1｜Confirm Annual Power Generation as the 'Scale'

The first metric to check in the annual results is, as expected, the annual energy production. This is the most straightforward figure for understanding the overall scale of the project and is easy to use in internal briefings and customer-facing summaries. In PVSyst result variables, for grid-connected systems the energy effectively produced, E_Grid, is central, and this value is also used in the numerator of the PR. By confirming the annual energy production first, you can readily grasp the level of annual output the project is assuming.

However, annual electricity generation is a "scale" figure, not a measure of "performance" itself. Projects with larger capacity tend to have higher annual generation, and if a site's solar irradiation conditions are favorable the figure will also tend to increase. For that reason, it is risky to judge a design as excellent based solely on annual generation. It is most useful to treat it as a first entry point—a figure to grasp how much annual output the project is expected to produce.

In practice, when looking at this metric, we first check whether it deviates significantly from past similar projects or projects in the same capacity range. If it is much larger than expected, one should suspect that the meteorological data or loss assumptions may be overly optimistic; if it is much smaller, there may be strong constraints somewhere — such as shading, irradiance/incident light conditions, or equipment configuration. Annual power generation also serves as an initial indicator to detect anomalies before a detailed assessment.

Also, when interpreting annual energy yield, it is important to note that simulation results do not necessarily represent a future representative year as-is. In PVSyst’s P50-P90 assessment, results using multi-year averages or TMY data are generally easier to treat as averages, whereas results based on a specific year cannot be regarded as equivalent to P50 as-is. Therefore, annual energy yield is a “number to check first,” but it is not a “number to finalize by itself.”

How to Read 2｜Evaluate per-installation performance using specific power generation

The metric you should check alongside annual energy production is specific production. In PVSyst’s normalized indicators, Yf represents the final system yield, and Specific Production is described as that Yf expressed on an annual scale. In other words, specific production is an indicator of how much useful energy is obtained per unit of installed capacity, and it is very useful when you want to compare projects by normalizing for their size.

In practice, this indicator is important because it reveals the "performance per unit of equipment" that annual generation alone cannot show. For example, a proposal with large capacity may appear to have high annual generation, but when judged by specific yield it may not be particularly superior. In such a case, although the scale is large, it may not be especially advantageous in terms of site conditions, irradiance conditions, or loss structure. Conversely, even if annual generation is modest, a high specific yield makes it easier to interpret the installation as having good performance per unit of equipment.

However, specific energy production is not万能 either. PVSyst's description of PR states that PR, unlike specific energy production, is not directly dependent on weather data or plane orientation, whereas specific energy production is affected by such site and installation differences. In other words, while specific energy production is useful for normalizing for scale, caution is needed when directly comparing projects with different site conditions.

Therefore, the most practical order for use in the field is: first look at scale by annual power generation, then assess each installation’s capability by specific power generation, and next check insolation and incident-light conditions to investigate the reasons for any differences. Taking this order prevents overreliance on specific power generation while still allowing it to be used as a highly effective comparative metric.

How to Read 3｜Confirm the foundation with solar radiation and lighting conditions

When reading annual results, the next things you must check are the underlying irradiance and the incident-light conditions. In PVSyst’s definition of PR, the energy that can potentially be produced is given by GlobInc × Pnom. Here, GlobInc is the incident energy on the array plane. In other words, the foundation of the annual results lies not only in the site’s natural conditions themselves but also in how the installation receives that irradiance.

The first thing to check is data that approximates the site’s horizontal-plane solar irradiance. This is the solar resource the location inherently possesses and is a precondition independent of the design. At sites with favorable insolation, annual energy production tends to be higher, whereas at harsher sites there is a physical upper limit no matter how much the design is refined. Therefore, after looking at annual generation and specific yield, it is important to confirm what level of site conditions forms their basis.

The next thing to check is the irradiance conditions. PVSyst’s results and the Loss Diagram stack many losses based on the energy that ultimately enters the collector plane. In the "Array losses" general considerations it is also explained that the starting point for considering the ideal yield is the incident irradiation in the collector plane after accounting for shading effects. In other words, the site’s solar irradiation does not directly translate into power generation; it only becomes "usable irradiance" after accounting for orientation, tilt, and shading conditions.

In practice, simply having this way of reading makes it considerably easier to organize the reasons for annual energy production. This is because it reveals whether the site conditions alone are favorable, or whether, including the installation conditions, the system is effectively capturing solar irradiance. Conversely, if you skip this step and only look at loss rates, you won't understand why a particular loss is significant. For annual results, it's overwhelmingly easier to understand if you first confirm the foundation and then proceed to the causes.

How to Read 4 | Trace where losses occur in the Loss Diagram

Once you've checked the foundation, the next thing to look at is the Loss Diagram. PVSyst explains that the Loss Diagram is particularly useful for identifying weaknesses in the system design, and that the effect of each loss can also be seen in hourly, daily, and monthly values. When reading annual results, this Loss Diagram becomes the central document for grasping where the losses are occurring.

The key point when reading a Loss Diagram is to look not only at the magnitudes of the rates but also at which stage the losses are occurring. In the general considerations of Array losses, array losses such as shading, IAM, temperature, quality, mismatch, and wiring reduce the ideal yield, and in the normalised indicators this is mapped to Collection Losses Lc. On the other hand, in the Normalised performance index, System Losses Ls are defined as the difference from Ya to Yf, that is, the losses from array output to the final useful energy. Knowing whether the reduction happens in upstream or downstream stages makes the direction for countermeasures much clearer.

For example, in projects where front-end losses such as shading or IAM are large, even improving inverter efficiency on the back end to some extent results in only a limited overall improvement. Conversely, in projects where the front end is well optimized, small differences in back-end System Loss tend to have a greater impact on annual results. The Loss Diagram should be read not only as a way to show at a glance what is wrong, but also as a diagram to help decide where to begin.

Also, PVSyst explains that each loss rate in the Loss Diagram is a percentage relative to the immediately preceding energy quantity and cannot be simply added together. What beginners most often stumble over is adding the loss rates without understanding this. The Loss Diagram is not a "table of rates" but a "flow diagram." With this recognition, interpreting the annual results becomes much more stable.

Interpretation 5 | Do not evaluate PR on its own; use it as a summary statistic of annual results

Finally, what I want to confirm is PR. In PVSyst, PR is defined as effectively produced energy divided by GlobInc × Pnom, and in the Grid system E_Grid is the basic numerator. Furthermore, PR is described as including optical losses, array losses, and system losses. In other words, PR is a very useful summary value in the annual results, but at the same time it is a composite result of multiple factors.

The key point here is not to judge PR on its own. PVSyst also notes that, unlike specific energy production, PR is less directly dependent on weather data and plane orientation, so it is considered suitable for system quality comparison between projects with different locations and orientations. However, PR still includes losses such as shading, IAM, temperature, mismatch, wiring, and inverter efficiency, so looking at PR alone won't reveal the causes.

In practical use, it is appropriate to consider PR as a summary metric for assessing how coherent the annual results are overall. Grasp the scale and sense of efficiency from annual generation and specific yield, confirm the baseline with solar irradiance and receiving conditions, and use the Loss Diagram to see where reductions occur; finally looking at PR increases confidence in the numbers. Conversely, if you look at PR alone first, you are more likely to be swayed by superficial appearance.

Another commonly misunderstood point is assuming that if a simulation's annual results are good, the PR will automatically be high. However, the magnitude of the annual results is also affected by site conditions, and PR is a value that represents the structure of losses. For that reason, treating PR not as a "conclusion" but as a "summary metric for organizing" is the most practical approach to interpreting annual results.

Common Misunderstandings

The most easily misunderstood aspect of PVSyst’s annual results is treating the annual energy production as if it were a definitive value. In PVSyst’s explanation of the P50–P90 assessment, it states that while results based on multi‑year averages or TMY data can generally be considered close to the mean, results from a specific single year cannot be regarded as a representative P50 value. In other words, annual results are a convenient output figure, but you must not ignore the nature of the meteorological data being used.

Another common mistake is to treat specific yield and PR each, on their own, as universal evaluation metrics. Specific yield is useful for normalizing scale, but it includes differences in site and irradiance conditions. PR is suitable as a summary of system quality, but it is a single number that aggregates loss factors and does not indicate their causes. Precisely because both are useful, they need to be interpreted together with their background.

Also, trying to add up all the loss rates in the Loss Diagram to produce a "total loss" is a typical mistake. PVSyst treats each loss rate as a proportion relative to the energy at the previous stage and explains that they cannot be simply added together. Loss rates are meaningful not only in terms of their magnitude but also in terms of which stage they affect.

Moreover, it is risky to look only at annual results and not check the monthly results. PVSyst states that it can also disaggregate loss effects on an hourly, daily, and monthly basis, and seasonal differences or anomalies that are not apparent from the annual data alone are often found by checking monthly results. Annual results are the final summary, but to verify causes you need to return to a monthly perspective.

The accuracy of on-site conditions affects the credibility of annual results

To make the annual figures reliable for practical use, accurate assessment of site conditions is indispensable. PVSyst’s detailed simulations can reflect near shadings, wiring, equipment layout, orientation, tilt, and so on, but conversely that also means the accuracy of the input site information directly affects the results. The official documentation likewise explains that in project design fine effects such as near shadings, wiring, module quality, and mismatch are handled in detail.

For example, if nearby shading is not sufficiently reflected, the annual Shading Loss may appear too small. If the azimuth or tilt are slightly off, the appearance of GlobInc will change. If the wiring route, panel locations, or the distance to the point of interconnection differ from the actual site, the System Loss results will also be affected. In other words, the credibility of the annual results is determined not by the neatness of the report's appearance but by how accurately the site conditions have been entered.

In that sense, having a means to grasp on-site positional relationships with high precision is extremely important for improving the quality of PVSyst’s annual results. If distances to obstacles, equipment layout, azimuth, and route conditions can be accurately determined, it becomes easier to refine the assumptions for Shading Loss, incident light conditions, and System Loss. Whether one merely reads the resulting numbers or can personally be convinced by them depends strongly on the accuracy of these on-site conditions.

Seen from this perspective, it naturally points to LRTK—the iPhone-mounted GNSS high-precision positioning device—as a means of accurately grasping on-site positional relationships. By making it easier to confirm locations in the field, determine clearances to obstacles, and improve the reproducibility of equipment layouts, it becomes simpler to more accurately organize the assumptions to be entered into PVSyst. If you want to avoid leaving annual results as desk-bound numbers and move toward predictions that are convincing in practical work, the accuracy of such on-site assessment is a great help.

Summary

When reading PVSyst annual results, first check the annual energy production to understand the scale, next use the specific yield to assess performance per installed capacity, then establish the baseline with solar irradiation and incident-light conditions, trace where losses occur in the Loss Diagram, and finally position PR as a summary metric of the annual results. Just having these five steps lets you avoid ending with whether the annual generation is simply “big or small” and enables you to read it including the underlying context.

What’s important is not to treat the annual results as a single answer. PVSyst’s results are an aggregation of the meteorological data, installation conditions, shading, equipment configuration, and loss settings. That’s why reading them including the assumptions behind the numbers is the way to make them useful in practice. If you review in order not only the annual energy production but also the specific yield, the baseline irradiation, the Loss Diagram, and the PR, the way the results appear becomes much clearer.

To further ensure the reliability of such readings, it is indispensable to grasp the on-site spatial relationships with high accuracy. If you want to organize shadows and layout conditions more precisely, the perspective of utilizing LRTK, an iPhone-mounted GNSS high-precision positioning device, is also effective. By combining the ability to correctly read PVSyst’s annual results with the ability to accurately capture site conditions, it becomes easier to arrive at a more convincing verification of annual power generation and design decisions.