7 Ways to Read PVSyst Results | A Guide to Interpreting Energy Production

When you first look at PVSyst results, many similar numbers appear—energy production, ratios, losses, solar irradiation, array output, grid-side output, etc.—so it’s easy to get confused about where to start checking. In practice, rather than chasing all variables from the outset, it’s overwhelmingly easier to start with the numbers closest to the conclusions and then go back to the causes and background conditions. In PVSyst’s official tutorial, the main results, the loss diagram, and various graphs are likewise organized in sequence as the core of the report.

Also, a PVSyst report is not just a document that shows the total annual energy production. It is structured so you can track how much solar irradiance there was, where and how much energy was lost, how much difference exists before and after the inverter, and which months are underperforming. In other words, it is easier to understand PVSyst if you think of reading it in the order of "Conclusion," "Background," "Losses," and "Seasonal differences," rather than by rote memorization of abbreviations.

This article organizes and explains seven perspectives that practitioners should first grasp when reading PVSyst results. It not only covers how to read annual energy production, but also, in a single flow, summarizes the conditions on which those figures are based, which losses are affecting them, and where to look to identify design weaknesses. The content is useful not only for those who will be using PVSyst from now on, but also for those who are in the position of receiving and reviewing result reports.

Table of Contents

• Key points to grasp before reading PVSyst

• 1 Grasp the overall picture with Main results

• 2 Read Produced Energy・Specific production・PR separately

• 3 Understand the differences between GlobHor・GlobInc・GlobEff

• 4 Trace the reduction in generated energy using the Loss Diagram

• 5 Understand the differences between EArray・EOutInv・E_Grid

• 6 Organize performance using the Normalized Performance Index

• 7 Look for anomalies in monthly results and daily trends

• Summary

Prerequisites to Keep in Mind Before Reading PVSyst

Before reading a PVSyst report, the first thing to understand is that the numerical results include both “numbers close to the conclusion” and “numbers that are intermediate steps.” Produced Energy, Specific production, and PR are numbers close to the conclusions. On the other hand, variables such as GlobHor, GlobInc, GlobEff, EArray, and EOutInv are numbers that indicate the background conditions and intermediate stages leading to those conclusions. If a beginner tries to read everything with the same weight at first, it’s easy to lose sight of which values are important. In other words, PVSyst is a tool that’s easier to understand if you work backwards from the conclusions rather than trying to understand everything at once.

Another important point is that PVSyst’s energy production is not determined by a single number; it becomes the final value through multiple stages such as irradiance conditions, optical losses, array losses, inverter losses, and AC-side losses. In the official documentation, within the list of variables for the Grid system, items like EArrRef, EArrNom, GIncLoss, TempLoss, EOutInv, and E_Grid are organized by stage. In other words, PVSyst results only become clear when you read not just the “final energy production” but also where and by how much it was reduced.

To avoid getting lost in practical work, it's effective to adopt the following sequence: first grasp the conclusions in the Main results, then check the irradiation conditions, then follow the flow of losses in the Loss Diagram, and, if necessary, dig down into energy variables and monthly results. Simply having this sequence makes a PVSyst report easier to read—not a "collection of difficult tables" but a document that explains the reasons for the power generation step by step.

1 Grasp the overall picture in Main results

The first item to look at is Main results. In PVSyst’s official tutorial, it explains that the fourth page of the report shows “energy production, specific production and performance ratio.” Furthermore, in the Results dialog there is a box on the right that summarizes the main results at a glance, which is described as “for quickly spotting obvious mistakes or getting a first impression for comparison.” In other words, Main results is the entry point to the entire report, and the basic approach is to grasp the overall picture of the project here first.

What you should check here first is whether the annual energy production is of the expected scale. If the Produced Energy is extremely small or strangely large relative to the installed capacity, you should question the assumptions before proceeding to later pages. PVSyst also provides a summary of the simulation conditions at the top of the results page and says you should "quickly check" for any obvious input errors. In other words, the Main results page is not only the page for looking at the numbers but also the page for spotting any oddities in the input conditions.

Also, the reason to look here first is that all other pages of the report exist to explain this result. If you later look at the loss diagram or the monthly results, it's easy to get lost if you don't know what is ultimately being explained. In other words, in PVSyst the most natural way to read is to start from the Main results and trace “why this annual energy production occurred.”

2 Read Produced Energy・Specific production・PR separately

Among the Main results, the particularly important ones are the three: Produced Energy, Specific production, and PR. In the official tutorial these three are listed as "three relevant quantities", with Produced Energy described as the "basic result", Specific production as "Produced energy divided by the nominal power of the array", and PR explained as an indicator that "shows the quality of the system itself, independently of incoming irradiance". In other words, these three may look similar, but their roles are completely different.

Produced Energy is the result showing how much energy was ultimately obtained for that project. It is the most straightforward figure, but by itself it is heavily influenced by system size. For example, when comparing a 10 kWp system and a 5 kWp system, it is natural that the Produced Energy of the former will be larger. In other words, it is suitable for comparing total amounts, but it makes it difficult to tell whether performance per unit of system size is good.

What you should look at there is Specific production. PVSyst defines this as Produced Energy ÷ Pnom, and the official Normalised performance index also explains that yield-type indicators are easier to compare when normalized by Pnom. In other words, Specific production is an indicator that evens out system size to show “how much generation per 1 kWp.” It is extremely useful when comparing projects or laying out options with different orientations or different capacities.

PR plays a different role. On the official PR page, PR is defined for grid-connected systems based on IEC EN 61724 as PR = E_Grid / (GlobInc × PnomPV), and it is described as a view close to the "global system efficiency" that includes optical losses, array losses, and system losses. Also, unlike Specific energy production, it is positioned as a comparative metric that does not directly depend on meteorological inputs or surface orientation. In other words, PR is a number for assessing how well the system functions as a whole, not the amount of energy produced.

A common misreading here is to assume that a high PR necessarily means a high energy output. PR is an indicator of the system’s health and low losses, not a number that replaces installed capacity or the solar irradiation conditions themselves. In other words, Produced Energy, Specific production, and PR should be read as serving different roles: total amount, per capacity, and system quality, respectively. Simply being able to separate these three makes PVSyst reports much easier to understand.

3 Understand the differences between GlobHor・GlobInc・GlobEff

The third thing to look at is the difference between GlobHor, GlobInc, and GlobEff. PVSyst’s energy production does not begin directly with the module output; rather, it is organized in the order of how much solar irradiance was input, how that irradiance was converted on the installation surface, and then how much light actually reached the cells. In other words, to understand the energy production you need to grasp the "input" numbers.

GlobHor is the horizontal global irradiation and is a basic value read from the weather data file. On PVSyst's weather data and irradiations page, GlobHor is defined as “Horizontal global irradiation as read on the weather data file”. In other words, it is the starting point for seeing how much solar radiation is coming from the sky in that area. If this value is low, annual power generation will be hard to increase no matter how good the system conditions are.

GlobInc is the solar irradiation converted to represent how much reaches the receiving surface after adjusting for the azimuth and tilt of the roof or mounting. It is officially described as “Incident global irradiation in the collector plane,” and the tutorial also states “after transposition, but without any optical corrections.” In other words, GlobInc is the incident irradiation after accounting for azimuth and tilt, and it will differ for south-facing, east- and west-facing, and north-facing surfaces even in the same area.

Furthermore, GlobEff is the “Effective global” obtained from GlobInc after subtracting optical losses such as horizon, near shadings, IAM, and soiling. The official documentation explicitly states “Effective = irradiation effectively reaching the PV-cell surface.” In other words, GlobEff is the “effective irradiation actually reaching the cell,” and it is a more realistic input for power generation. If this is significantly lower than GlobInc, you should strongly suspect optical losses such as shading, IAM, or soiling.

In practice, keeping the sequence GlobHor → GlobInc → GlobEff in mind makes it much easier to distinguish whether it’s a regional-conditions issue, an azimuth/tilt issue, or an optical-loss issue. When you find that the energy production in PVSyst is lower than expected, it’s important not to immediately suspect equipment performance, but to check the numbers on the input side in order.

4 Tracking Decreases in Power Generation Using a Loss Diagram

The fourth item to look at is the Loss Diagram. In PVSyst’s official documentation, the Loss Diagram is described as providing a “quick and insightful look into the quality of a PV system design” and as being used to identify the main sources of loss. It is also stated that it is always shown in the annual report and can be viewed on a monthly basis. In other words, the Loss Diagram is the page in PVSyst that most intuitively shows the "reasons for the results."

The tutorial explains that the Loss Diagram is a powerful indicator showing the system’s energy balance and all losses, allowing assessment of design quality. The figure shows a flow starting from GlobHor, then GlobInc, IAM, GlobEff, EArrNom, Array losses, EArrMPP, Inverter losses, EOutInv, AC losses, and finally EGrid. In other words, by looking at the Loss Diagram you can immediately see at which stage—irradiance, array, inverter, or AC side—energy was lost.

Beginners shouldn't try to memorize all the losses in detail here. First, look at which losses are largest. It's often meaningful enough just to check whether optical losses are large, temperature losses are large, mismatches or wiring are having an effect, or losses are occurring on the inverter side. The official Grid system variables also list variables such as GIncLoss, TempLoss, MisLoss, and OhmLoss, making it easier to dig deeper later into any losses that concern you.

Also, the Loss Diagram is very well suited for comparing proposals. The official tutorial also explains that it should be used when comparing different variants. If you only look at differences in annual energy production, you may be able to tell which is better, but you cannot see why the difference occurred. However, when you place loss diagrams side by side, it becomes much clearer whether the difference is due to shading, orientation, or inverter settings. In other words, the Loss Diagram is also excellent as "materials for explaining results."

When working with PVSyst in a professional setting, it's most efficient to check the results in Main results and then use the Loss Diagram to identify where the weaknesses are. When you feel overwhelmed by the many numbers in PVSyst, returning to this diagram makes it easier to organize them.

5 Understand the differences between EArray, EOutInv, and E_Grid

The fifth item to look at is the differences between EArray, EOutInv, and E_Grid. In PVSyst several "energy" figures appear, and if you cannot organize them it is easy to be unsure which number to regard as the generated energy. On the Grid system variables page, variables for energy output and use are defined, such as EArray, EOutInv, E_Avail, E_Grid, E_Solar, and E_User. In other words, PVSyst has different numbers for each stage of the electrical flow.

First, EArray. In the tutorial it is described as “Energy produced by the PV array (input of the inverters)”. In other words, it is the energy produced by the array at a stage before inverter losses are applied. It is important when you want to look at the performance of the panels or the array.

Next is EOutInv. The tutorial describes it as “Available energy at the output of the inverter,” and the Grid system variables also show a flow in which, after EOutInv, wiring and night losses are subtracted to arrive at E_Avail. In other words, it’s easiest to understand EOutInv as a figure close to the AC-side immediately after the inverter. If this is substantially lower than EArray, inverter losses or operational constraints may be having a large impact.

And E_Grid is defined as “Energy injected into the grid.” In other words, it is the energy ultimately injected into the grid and is a concept close to the amount of electricity sold. In grid-connected systems, this E_Grid is an important figure that directly links to Produced Energy and PR in the Main results. However, in self-consumption projects it is dangerous to regard E_Grid alone as the total generated energy. In PR explanations, when there is self-consumption or internal use, E_Grid should be replaced by E_Grid + E_Solar. In other words, for self-consumption installations, unless E_Solar is also included, the generated energy is easily underestimated.

Understanding these three allows you to read PVSyst's figures in stages as "panel side", "after the inverter", and "grid side". In practice, when you feel the generated energy is low, checking whether it is low at EArray, drops at EOutInv, or drops further at E_Grid makes it much easier to narrow down the cause. In other words, grasping the differences between EArray, EOutInv, and E_Grid is an essential prerequisite for correctly comparing generation figures.

6 Organizing Performance Using the Normalized Performance Index

The sixth item to look at is the Normalized Performance Index. PVSyst’s official documentation explains that this normalized index was introduced by the JRC to make comparisons easier and is also positioned within IEC EN 61724. Because these indices are normalized by GlobInc and Pnom, they make it easier to compare performance while somewhat leveling out differences in system size and site conditions. In other words, you can think of them as a common language for further organizing and understanding the numbers in the Main results.

What is important here is the relationship among Yr, Ya, Yf, Lc, Ls, and PR. In the official description, Yr is the ideal Reference Yield; Ya is the Array Yield, defined as the actual array output divided by the installed capacity; Yf is the System Yield, defined as the system’s final useful energy divided by the installed capacity. And Lc = Yr − Ya is the array-side loss, while Ls = Ya − Yf is the system-side loss. In other words, by looking at the normalized metrics you can understand, separated into array-side and system-side, how much the value has been reduced from the ideal.

The advantage of this way of thinking is that it makes it easy to understand, in relative terms, where performance is being lost. Temperature, mismatch, wiring, shading, and so on mainly act as array-side losses, while inverter and grid-side conditions act as system losses. In other words, if Lc is large you should review array-side conditions, and if Ls is large you should review system-side conditions. This interpretation pairs very well with the Loss Diagram.

Also, we can restate the meaning of PR here. On the official page, PR is Yf / Yr, and it is said to correspond to E_Grid / (GlobInc × PnomPV). Furthermore, PR includes optical losses, array losses, and system losses, while being described as a comparative metric that, unlike Specific energy production, is less directly dependent on surface orientation and meteorological inputs. In other words, PR is useful for organizing "how efficiently that installation is operating," but it does not substitute for the total energy generation itself.

In practice, by looking at whether Specific production is high, PR is high, or which of Lc and Ls is larger, it becomes much easier to clarify the strengths and weaknesses of a system. When you want to understand PVSyst results in a more structured way, this Normalized Performance Index is very effective.

7 Search for anomalies in monthly results and daily trends

The seventh item to examine is the monthly results and daily trends. Even if you understand the annual conclusions and the flow of losses, it can still be difficult to see “which seasons are strong,” “which periods are weak,” or “where deviations from expectations tend to occur.” The official PVSyst tutorial explains that the Main results table displays monthly values and the yearly value side by side, and that a Daily Input/Output diagram of solar irradiation and grid injection is shown. In other words, PVSyst helps you grasp a system’s peculiarities not only from annual results but by looking at monthly and daily trends.

When you look at monthly results, patterns toward spring, summer, autumn, and winter become much easier to see. For example, if results are high in spring and autumn but drop only in summer, you are more likely to suspect temperature-related conditions. If they drop only in winter, it becomes easier to consider the effects of shading, solar altitude, and roof conditions. Because the Loss Diagram is officially described as viewable by month, looking at losses by month as well as annually makes seasonal weaknesses much clearer.

Examining daily trends is even more specific. In the official tutorial, the Daily Input/Output diagram explains that for a well-designed grid-connected system, the energy injected into the grid is roughly linearly related to solar irradiance, and that slight saturation on the high-irradiance side is due to temperature effects. It also notes that if outliers appear during high-irradiance periods, they are a sign of overload conditions. In other words, the daily graph is not merely a visual plot but a clue for detecting overloads and design inconsistencies.

In practice, even if the total annual generation is sufficient, if there are extreme biases on a monthly or daily basis, it is worth investigating the causes. Conversely, if the annual value looks somewhat low but the monthly figures are as expected, it may simply be within the range of weather or seasonal variation. In other words, by finally examining monthly and daily trends, you can read PVSyst’s results not as “a single number” but as “the operational character.”

Summary

When reading PVSyst results, the seven perspectives you should grasp first are: Main results; the differences among Produced Energy, Specific production, and PR; the relationship among GlobHor, GlobInc, and GlobEff; the Loss Diagram; the differences among EArray, EOutInv, and E_Grid; the Normalized Performance Index; and the monthly results and daily trends. Simply reading these in order turns a PVSyst report from "a document with many numbers that is difficult to understand" into "a document that reveals the reasons for the generated energy."

The important thing is not to try to understand PVSyst from all its variables. First grasp the conclusion in Main results, then look at the irradiance conditions and the flow of losses, and after that check the energy variables and monthly trends. Simply following this order makes it much clearer where to focus. In other words, reading PVSyst is more a matter of the order in which you read it than of how much knowledge you have.

Also, if you truly want to improve the accuracy of interpreting these reports, the precision of the input conditions is indispensable. If the roof edges, obstacle positions, elevation differences, or the way nearby shadows fall are ambiguous, the interpretation of the Loss Diagram and Main results will also tend to vary. In particular, the shading and effective-area conditions directly affect GlobEff and how losses appear.

In that respect, LRTK, an iPhone-mounted high-precision GNSS positioning device, is highly effective for accurately determining on-site positional relationships. Because it makes it easier to accurately record the positions of roof edges and obstructions on site, it helps improve the accuracy of the input assumptions entered into PVSyst. If you want to read PVSyst results in a form that is truly usable in practice, accurately capturing on-site conditions with methods like LRTK is a major advantage that enhances the accuracy of energy-yield interpretation.