6 Ways to Interpret PVSyst Output Results｜Clarifying Common Points of Confusion

PVSyst's output results can be hard to know where to look when you're not familiar with them. Because similar-looking numbers — annual energy production, specific yield, PR, solar irradiation, losses, array output, inverter output, grid-side energy, etc. — appear together, trying to understand everything from the start can actually obscure the overall picture. The official PVSyst documentation also organizes the results on the assumption that you read the "main results summarized in the report" in combination with "auxiliary results such as loss diagrams and monthly and daily breakdowns."

In practice, what's important is not memorizing abbreviations. First, grasp the figures that are closest to the conclusion, then work back through the solar irradiance conditions, losses, energy flows, and monthly variations. Simply adopting this order will turn a PVSyst report from "a document full of numbers that's hard to understand" into "a document that reveals the reasons for the power generation." In this article, I narrow the focus to six points that beginners are especially likely to get confused about, and organize them with an emphasis on how to interpret power generation.

Table of Contents

• 1 Look at the Main results first

• 2 Do not confuse Produced Energy, Specific production, and PR

• 3 Understand the differences between GlobHor, GlobInc, and GlobEff

• 4 Use the Loss Diagram to see where losses occurred

• 5 Understand the differences between EArray, EOutInv, and E_Grid

• 6 Check monthly results and Daily Input/Output for anomalies

• Summary

1 View the Main results first

When reading PVSyst output results, the first thing to look at is the Main results. In the official tutorial, the fourth page of the report is described as the “main results of the simulation,” and it explains that energy production, specific production, and performance ratio are summarized there. Furthermore, the summary values in the Results dialog should be used to get a first impression for comparisons and changes in conditions. In other words, this is the entry point when reading PVSyst.

First, the three points to check here are roughly how large the project's annual power generation is, how much generation is expected relative to the installed capacity, and how coherent the system as a whole appears to be. Even without advancing to root-cause analysis, just by looking at this page you can grasp "where the conclusion of this project lies." In other words, it's easier to understand Main results if you think of it as a page for viewing the map of the entire report rather than for detailed analysis.

One mistake beginners often make is to jump straight to the loss diagrams and graphs at the end. If you do that, you lose sight of why you are looking at those losses. If you first grasp the Main results, the solar irradiance and losses you examine afterward are easier to understand as "information that explains these results." In other words, in PVSyst it is less confusing to read from the conclusions back to the causes.

2 Do not confuse Produced Energy, Specific production, and PR

The most confusing aspect of PVSyst results is that people tend to think Produced Energy, Specific production, and PR are similar numbers. In the official tutorial, these three are presented as the "three relevant quantities", and their meanings are explained separately. Produced Energy is the "basic result", Specific production is the "Produced energy divided by the nominal power of the array", and PR indicates the "quality of the system itself, independently of the incoming irradiance". In other words, the three are not different expressions of the same energy production but indicators with different roles.

Produced Energy is a total metric for seeing how much electrical energy was ultimately obtained. It is the most intuitive and easy to understand, but it tends to make projects with larger system capacity look more favorable. When comparing a 10 kWp system and a 5 kWp system, it is natural that the former’s Produced Energy will be larger, and that alone does not mean it is a "good design." In other words, while useful for grasping the total amount, it is insufficient for comparing the quality of projects themselves.

Specific production is the value obtained by dividing Produced Energy by installed capacity. In the official Normalised performance index, Yf is the final system yield and is organized as a concept corresponding to annual specific yield. In other words, Specific production is a figure for seeing how much is generated per 1kWp after normalizing for system size. It is particularly useful for comparing projects, different orientations, and different capacities. It is suited to situations where you want to see the ease with which that equipment configuration generates power, which can be difficult to discern from total generation alone.

PR has another meaning. In the official help, PR is defined as Yf / Yr, or E_Grid / (GlobInc × PnomPV), and is presented as a way of viewing overall performance that includes optical losses, array losses, and system losses. In other words, PR is an indicator of how efficiently the system is operating under those conditions, rather than how much electricity was generated. Even if PR is high, total energy production can be small if the installed capacity is small, and conversely, a project with a slightly lower PR can produce sufficient energy if it has a large installation. Therefore, using PR as a substitute for total energy production is risky.

In practice, it's easier to understand if you read these three as "total amount," "per unit of capacity," and "system quality." First grasp the total amount with Produced Energy, look at Specific production to see how easily it generates electricity per unit of capacity, and use PR to check the final performance including losses. Simply viewing them in this order makes PVSyst's results much easier to organize.

3 Grasp the differences between GlobHor, GlobInc, and GlobEff

When looking at power generation, the variables related to irradiance are the next area that tends to cause confusion. In PVSyst’s official documentation, meteorological and irradiance-related variables such as GlobHor, GlobInc, and GlobEff are defined progressively. GlobHor is the global irradiance on the horizontal plane, GlobInc is the incident irradiance converted to the receiving surface, and GlobEff is the effective irradiance after optical losses. In other words, even though they are all called "irradiance," their meaning differs depending on which stage the value represents.

GlobHor is the starting point for the meteorological conditions at a location. In the documentation it is described as the horizontal solar irradiance read from the weather data file. This represents the potential of the region, and roof conditions such as whether it is south-facing or oriented east-west are not yet taken into account. In other words, looking at GlobHor alone does not tell you how much electricity a specific installation will generate.

GlobInc is the value converted — through azimuth and tilt — into the solar irradiance reaching the receiving surface. In the official tutorial, GlobInc is described as "after transposition, but without any optical corrections," and is explained as corresponding to POA. In other words, south-facing surfaces and east/west-facing surfaces will have different GlobInc even if they have the same GlobHor. When understanding differences between projects in PVSyst, looking at GlobInc makes the "differences in roof conditions" much more apparent.

GlobEff is a value that goes one step further. Officially, it is described as the effective irradiance that actually reaches the cell surface after optical losses such as far and near shadings, IAM, soiling, and so on. In other words, even if GlobInc is sufficient, if GlobEff is significantly reduced, it is likely that shading, angle of incidence, soiling, or similar factors are at work. When you feel that power generation is lower than expected, instead of immediately suspecting equipment performance, looking at the difference between GlobInc and GlobEff can go a long way toward clarifying whether a problem is occurring on the light-entry side.

In practice, it becomes clearer if you look at GlobHor → GlobInc → GlobEff in that order, consciously separating “site conditions,” “roof conditions,” and “optical losses.” When you’re unsure about PVSyst results, rather than continuing to look only at the energy production, it’s easier to pinpoint the cause if you first check this flow of light.

4 Using the Loss Diagram to See How Power Generation Decreases

One of the clearest pages in PVSyst's output is the Loss Diagram. The official help explains that this figure is meant to quickly grasp the quality of the system design and identify the main sources of loss. The tutorial further states that the Loss Diagram on the fifth page shows the "energy balance and all losses along the system" and is a powerful indicator that will immediately reveal any sizing errors. In other words, the Loss Diagram is the central page for reading "why this energy production occurred."

The good thing about this diagram is that it shows, in sequence, where solar irradiance is reduced, where it is reduced in the array, and where it is reduced in the inverter and on the AC side. Because you can trace the energy flow on a single page from GlobHor to GlobInc, and then from GlobEff to EArrNom, EArrMPP, EOutInv, and E_Grid, you can intuitively grasp "where the largest drop occurred." In other words, this diagram lets you understand many of PVSyst's variables as a flow.

What beginners should focus on here is not memorizing the names of every loss item. First, it’s important to grasp whether optical losses are large, temperature losses are large, or whether mismatch, DC wiring, or inverter losses are particularly noticeable. The official Grid system variables also organize GIncLoss, TempLoss, ShdElec, MisLoss, OhmLoss, InvLoss, and so on, so you can return from the Loss Diagram to the variable definitions later for any area that caught your attention. In other words, the Loss Diagram is also an entry point to the detailed variables.

The Loss Diagram is also very well suited for comparing design options. The official tutorial explains that by comparing loss diagrams between variants, you can immediately grasp what changed. When comparing south-facing and east–west layouts, or designs with and without shading, you can see not only differences in annual energy production but also which losses caused those differences. In other words, the Loss Diagram is both a diagram that explains the reasons for the results and a diagram that helps identify design improvements.

5 Understand the differences between EArray・EOutInv・E_Grid

Among PVSyst's energy quantities, the differences between EArray, EOutInv, and E_Grid are especially confusing. According to the official Grid system variables, EArray is "the active energy at the array output, which includes deviations of the inverter operating point, but is not displayed in the Loss Diagram." EOutInv is "the energy available at the inverter output," and E_Grid is "the energy injected into the grid." In other words, it's easier to understand if you think of them as looking at energy at three separate points: the panel side, after the inverter, and the grid side.

EArray is a number close to the energy the array actually produced. If this is lower than expected, you should strongly suspect optical losses, array losses, shading, mismatch, etc. EOutInv is the energy that has passed through the inverter and appeared on the AC side. If there is a large difference compared to EArray, inverter efficiency, operational constraints, thresholds, or the like may be at work. In other words, by comparing these two you can fairly easily determine whether the problem lies on the panel side or the inverter side.

E_Grid is the energy ultimately exported to the grid, and it’s easiest to understand if you think of it as roughly the amount of electricity sold. However, this is the point that most easily confuses beginners. In self-consumption projects, if you treat only E_Grid as the “total generation” you will overlook the portion consumed within the building. Even the official explanation of the Performance Ratio usually uses E_Grid, but when there is self-consumption (E_Solar) it states that evaluation should be done using E_Grid + E_Solar. In other words, for projects that include self-consumption it is crucial not to draw conclusions based solely on E_Grid.

In practice, it's easiest to first look at EArray to check the magnitude of generation on the panel side, then look at EOutInv to assess the inverter's effect, and finally look at E_Grid to confirm the energy that ultimately went out. If the numbers in PVSyst feel overwhelming, simply dividing them into these three stages makes the energy flow much easier to understand.

Find anomalies in June monthly results and Daily Input/Output

The final items to check are the monthly results and the trends in the Daily Input/Output. Even if you understand the annual generation and loss flows, that alone can make it hard to see "which season is weak" or "which time of day the behavior is abnormal." The official PVSyst tutorial explains that page six shows the Daily Input/Output diagram and the distribution of power injected to the grid. Furthermore, because the Loss Diagram can also be reviewed by month, you can trace seasonal biases in the annual results. In other words, finally checking along the time axis is the finishing touch to interpreting PVSyst.

Looking at the monthly results, the seasonal imbalances between spring, summer, autumn, and winter become much clearer. If only winter shows a sharp drop, shading or slope effects may be responsible; if only summer drops, high-temperature conditions may be the cause; if only spring and autumn are higher than expected, the season’s weather conditions or interaction with the load may be factors. If you look only at the annual total, the discussion ends with “a little low” or “a little high,” but by examining monthly results it becomes much more specific. In other words, monthly results are very well suited for isolating causes.

Daily Input/Output diagram も非常に有効です。公式チュートリアルでは、よく設計された系統連系設備なら、collector plane の global incident irradiation に対して grid injected energy は概ね直線関係となり、高日射側で少し飽和するのは温度の影響だと説明されています。また、高日射条件で外れ点が目立つ場合は overload condition の兆候とされています。つまり、この図を見ると、温度による自然な頭打ちと、設計条件の問題による頭打ちを見分けやすくなります。

In practice, even if the annual figures look reasonable, if there are large monthly or daily variations, the project still has issues that require interpretation. Conversely, even if the annual figure is slightly low, looking at the monthly and daily data may show it falls within the range of meteorological variation. In other words, PVSyst’s output only becomes clear—allowing you to determine whether the system really behaves that way—when it is finally examined on a time axis.

Summary

Common points of confusion when reading PVSyst output results include: skipping the Main results and jumping into the details; mixing up Produced Energy, Specific production, and PR; not recognizing the stage differences among GlobHor, GlobInc, and GlobEff; treating the Loss Diagram as merely a figure; assuming EArray, EOutInv, and E_Grid represent the same generation value; and failing to check for monthly or daily biases. In other words, when interpreting PVSyst, the order in which you examine items is more important than the numbers themselves.

The important thing is not to dismiss PVSyst as a complicated software output. Look at the conclusion for annual energy production, check the solar irradiance conditions, identify where the largest losses are in the loss diagram, follow the flow of energy, and finally check for monthly and daily imbalances. Simply following this sequence will make the report much more practical to read. In other words, reading PVSyst is not about understanding everything at once, but about grasping items in order of their significance.

Also, if you really want to improve the accuracy of interpreting such reports, the precision of the input conditions is indispensable. If the roof edges, the positions of obstacles, elevation differences, or the way nearby shadows fall are ambiguous, the results of GlobEff and the Loss Diagram will also tend to vary. In particular, the conditions for shading and effective area directly affect how the power generation is perceived.

In that respect, as a means of accurately understanding spatial relationships on site, the iPhone-mounted GNSS high-precision positioning device LRTK is extremely effective. Because it makes it easier to precisely record the positions of roof edges and obstacles in the field, it becomes easier to improve the accuracy of the input assumptions for PVSyst. If you truly want to interpret PVSyst output in a form that is usable in practice, properly capturing local conditions with methods like LRTK is a major advantage that improves the accuracy of result interpretation.