10 PVSyst Terms That Are Confusing to Read｜Beginner's Dictionary

When you first look at a PVSyst report, you often find yourself confused more by the meanings of the terms listed there than by the generation figures themselves. Even commonly used words in solar power design and energy-yield analysis—GlobInc, GlobEff, EArray, EGrid, PR, IAM, Soiling, Mismatch, Ohmic loss, Auxiliary loss, and so on—can make it difficult to judge which numbers to prioritize if you’re not familiar with them.

In particular, when reading PVSyst results, a literal translation of terms alone is not sufficient. For example, unless you distinguish whether a term refers to irradiance, the output of the PV array, or the AC energy after passing through the PCS, you cannot correctly explain why generation is high or low.

In this article, we narrow the list to ten terms that beginners commonly get confused by when reading PVSyst, and organize them as a dictionary for reading power generation analysis reports. This is not merely a glossary; we explain where to look in the report, what to watch for when making comparisons, and how to interpret the findings in practical work.

Table of Contents

• Interpret PVSyst terms in terms of energy flow

• Term 1 GlobHor

• Term 2 GlobInc

• Term 3 GlobEff

• Term 4 EArray

• Term 5 EGrid

• Term 6 PR

• Term 7 IAM loss

• Term 8 Soiling loss

• Term 9 Mismatch loss

• Term 10 Ohmic loss

• Points beginners should note when reading PVSyst terms

• In comparison reports, check the assumptions rather than the meanings of the terms

• Summary

Read PVSyst terminology by following the energy flow

The first thing to grasp when understanding PVSyst terminology is to be aware of which stage the energy is at within the power plant. The analysis of a photovoltaic system starts with the solar irradiance falling from the sky, then the irradiance incident on the photovoltaic module surface, the effective irradiance after subtracting shading and reflections, the energy converted to DC by the modules, the energy converted to AC by the PCS, and finally the amount of electricity delivered to the grid.

Many terms appear in PVSyst reports along this flow. What often confuses beginners is that they treat all the terms as the same type of value. However, in reality terms representing solar irradiance, terms representing losses, terms representing DC-side generation, terms representing AC-side generation, and terms representing performance indicators are mixed together.

For example, GlobInc is a term that describes the solar irradiance incident on the module surface and is not the generated power itself. On the other hand, EGrid represents the alternating-current energy ultimately delivered to the grid, so it is a metric closer to the amount of electricity sold or to power-generation assessment. PR is not an absolute value of generated energy but a ratio used to evaluate a plant’s performance taking installed capacity and irradiance conditions into account.

In this way, PVSyst terminology is easier to understand if you learn it within the flow from solar irradiation to grid interconnection, rather than memorizing terms in isolation. Below, we will go through 10 terms that beginners are particularly likely to get confused by, in order.

Term 1 GlobHor

GlobHor means global horizontal irradiance. Simply put, it is the solar irradiance that reaches a horizontal plane parallel to the ground. It is one of the basic solar radiation quantities provided as meteorological data, and in PVSyst analysis it is positioned close to the initial input conditions.

Solar panels are typically installed with a tilt rather than horizontally. Therefore, the irradiance on the horizontal plane is not directly the same as the irradiance on the module surface. In PVSyst, the horizontal-plane irradiance is converted into the irradiance incident on the module surface by accounting for installation tilt and azimuth, diffuse irradiance, direct irradiance, and other factors.

One point beginners should be aware of is that a high GlobHor does not necessarily mean that power generation will be correspondingly high. Power output is affected by many factors, such as tilt angle, azimuth, temperature, shading, reflection, wiring losses, and PCS losses. GlobHor is only a starting point for meteorological conditions and is not a figure that represents the performance of the power plant itself.

When comparing reports from multiple locations, first check GlobHor to see whether there are differences in the solar irradiation conditions in the first place. If a site's generation is high, it's important to determine whether that is due to good design or simply because the area receives more global horizontal irradiance.

Term 2 GlobInc

GlobInc denotes the irradiance incident on an inclined surface. Because it represents the solar irradiance reaching the plane of a photovoltaic module, it is a metric more directly related to energy production than GlobHor. When reading PVSyst results, it is a particularly important term for assessing irradiance conditions.

Even at the same location, GlobInc changes when the module tilt and azimuth change. For example, a south-facing layout with an appropriate tilt angle can sometimes increase annual solar irradiation compared with a horizontal plane. Conversely, east–west orientations or low-tilt designs change the hourly distribution of power generation, and GlobInc values differ.

In PVSyst comparison reports, when GlobInc differs, simply comparing EGrid alone does not allow you to make a correct judgment. This is because if the solar irradiance input or installation conditions differ, the final energy production will change. When interpreting differences in energy production, you should first confirm whether GlobInc is based on the same assumptions or different ones.

For beginners, it's easier to understand GlobInc if you think of it as the "material for the power that reaches the panel." If the amount of power generated is the finished dish, GlobInc is the main ingredient. Having more material tends to increase power output, but if losses in the cooking process are large, the final EGrid may not grow as much as one might expect.

Term 3 GlobEff

GlobEff denotes effective irradiance. It can be understood as the portion of the solar irradiance incident on the module surface that is effectively available for power generation after accounting for effects such as near shading, far shading, IAM losses, soiling, and the like.

While GlobInc is the solar irradiance incident on the module surface, GlobEff is the portion of that irradiance that is closer to being actually usable for power generation. In other words, by looking at the difference between GlobInc and GlobEff, you can determine how much solar irradiance is being lost.

For example, if rack spacing is narrow and shading is likely in the morning and evening, GlobEff can decrease even if GlobInc is sufficient. Also, if the tilt angle is low and there are many hours when sunlight arrives at a shallow angle, GlobEff can decrease due to IAM losses. Furthermore, if dirt and snowfall are set as Soiling, they also affect GlobEff.

An important point when interpreting PVSyst is not to immediately blame the PCS or wiring for low energy production. By first checking whether there is a large drop between the GlobInc and GlobEff stages, you can distinguish whether the issue lies on the irradiance acquisition side or the electrical conversion side.

Term 4 EArray

EArray represents the DC-side energy obtained from the photovoltaic array. It is easiest to understand as the amount of electrical energy before it enters the PCS. It represents the result after the modules convert solar irradiance into DC power and after accounting for losses such as temperature loss, low-irradiance loss, mismatch loss, and DC wiring loss.

EArray is an easy-to-use metric for evaluating solar PV modules and DC-side design. For example, if GlobEff is the same but EArray is lower, possible causes include module temperature, module characteristics, DC wiring, mismatch, string design, and so on.

On the other hand, EArray is not the final amount of electricity sold. Because it is the value before conversion to AC by the PCS, PCS efficiency, AC wiring losses, transformer losses, output limits, and the like have not yet been fully reflected. Therefore, when assessing the overall performance of the plant, you need to check not only EArray but also EGrid.

Beginners often confuse EArray and EGrid. EArray refers to the DC side, while EGrid is a figure closer to the AC-side final output. If, in report comparisons, EArray is high but EGrid is low, losses after the PCS or output limitations may be affecting the result. Conversely, if EArray is already low, you should focus on the solar irradiance received at the module surface and DC-side losses.

Term 5 EGrid

EGrid represents the alternating current energy sent to the grid. Among PVSyst results, it is an important figure that directly relates to energy yield assessments and project feasibility evaluations. In many cases, this EGrid is the value cited as the annual energy production.

EGrid reflects losses downstream of the power plant, such as PCS conversion losses, AC wiring losses, transformer losses, auxiliary equipment losses, and output limitations. Therefore, it is generally smaller than EArray. By looking at the difference between EArray and EGrid, you can determine how much loss is occurring downstream of the PCS.

In interpreting PVSyst, it is important not only to view EGrid as the final result but also to check the process that leads to it. If EGrid is lower than expected, you need to break down and examine whether solar irradiance is low, whether there is significant shading, whether temperature losses are large, whether the DC capacity is too large relative to the PCS capacity causing clipping, or whether AC-side losses are large.

Also, when comparing multiple options, it is important not to judge them solely by the magnitude of the EGrid. If system capacities differ, the EGrid will naturally change. Looking at the energy generation per 1 kW of DC capacity and the PR together allows a fairer comparison of the efficiency and soundness of the design proposals.

Term 6 PR

PR is an abbreviation of Performance Ratio, and in Japanese it is called 性能比。It is an indicator that shows how efficiently a solar power plant is producing electricity relative to the given irradiance conditions and installed capacity.

PR is not the absolute value of energy production. It is a ratio used to assess a power plant’s performance by normalizing, to some extent, the effects of solar irradiance and installed capacity. Therefore, it is useful when comparing power plants in different regions or with different capacities.

However, a high PR does not necessarily mean a good design. For example, if overly conservative solar radiation data are used or the loss settings are set too low, the PR can appear artificially high. Likewise, in snowy regions or on heavily shaded sites, accounting for realistic losses can reduce the PR, but that is not necessarily a poor analysis.

For beginners, it's easier to understand PR if you think of it as a report card that roughly shows a power plant's efficiency. However, that report card changes depending on the assumptions. If settings such as weather data, module specifications, PCS specifications, wiring losses, soiling, shading, or output limits differ, the PR will also change. When comparing, it's necessary to check not only the PR figure but also the assumptions under which the PR was calculated.

Term 7 IAM loss

IAM loss stands for Incidence Angle Modifier loss and refers to reflection losses caused by the angle of incidence. The more obliquely sunlight strikes the module surface, the greater the proportion reflected at the glass surface, reducing the solar irradiance available for power generation. IAM loss represents this effect.

When the sun is high in the sky, light strikes the module surface relatively head-on, so IAM losses tend to be small. Conversely, during mornings, evenings, and in winter when the solar altitude is low, light enters at a shallow angle, increasing reflection and making IAM losses tend to be larger.

When looking at IAM loss in a PVSyst report, you need to consider it together with the installation tilt, orientation, and the local solar irradiance conditions. In low-slope rooftop installations, east–west oriented designs, or layouts that prioritize morning and evening generation, the impact of IAM can become more apparent.

Beginners should be careful not to overlook IAM loss as merely a small correction term. Although it may appear to be less than a few percent on an annual basis, when comparing projects this difference can lead to variations in energy yield and PR. In particular, when comparing reports from other companies, if the IAM settings differ the results may vary even for the same layout.

Term 8 Soiling loss

Soiling loss refers to losses caused by soiling. When sand and dust, pollen, bird droppings, volcanic ash, fallen leaves, snow, or other contaminants adhere to the module surface, sunlight has more difficulty reaching the cells and power output decreases. Soiling loss is the parameter you set in PVSyst to account for this effect.

Soiling loss varies greatly depending on the region and environment. In dry areas, places with a lot of sand and dust, near agricultural land or development sites, along roads with heavy traffic, coastal areas, and snow-prone regions, the effects of dirt and deposits need to be carefully considered. On the other hand, in regions where natural cleaning by rain can be expected, the annual average Soiling loss may be relatively small.

An important point in interpreting PVSyst is whether the Soiling loss corresponds to the actual conditions at the power plant. Simply inputting a general value may not reflect the local environment. In snowy regions, you need to consider not only "soiling" per se but also periods when snow prevents generation and the relationship with increased albedo due to reflection.

Beginners should understand Soiling loss as "loss of power generation caused by the module surface not being clean." When comparing reports, because annual energy production can change simply due to different Soiling loss settings, it is important to first confirm that the same assumptions are being used before looking at differences in PR or EGrid.

Term 9 Mismatch loss

Mismatch loss is the loss that occurs due to variations among modules and differences in conditions between strings. Even solar cell modules of the same model number exhibit slight differences in actual output characteristics. In addition, the way shadows fall, temperature, degradation, and wiring configuration can cause differences in output between strings. These differences prevent the overall system from achieving the ideal output, and this is called mismatch loss.

In photovoltaic power generation, multiple modules are connected in series and parallel to generate electricity. In series connections, if some modules have lower current, the output of the entire string is affected. Therefore, module selection, string design, and shadow distribution are related to Mismatch loss.

If the Mismatch loss is large in a PVSyst report, you should first check the string configuration and the effects of shading. In particular, mismatch can increase in locations with complex terrain, where racking orientations vary, or where partial shading occurs. Also, be careful when multiple tilts and azimuths are combined into a single MPPT.

For beginners, it is easier to understand Mismatch loss if you think of it as a loss where the whole is pulled down by the weakest part. Ideally all modules would generate power equally, but in reality there are differences in conditions, so the total output can be slightly lower than a simple sum.

Term 10 Ohmic loss

Ohmic loss refers to the loss caused by wiring resistance. In Japanese, it is sometimes called resistance loss or wiring loss. When electric current flows through a cable, the cable has resistance, so part of the energy is lost as heat. This loss is Ohmic loss.

In PVSyst you can set DC-side wiring losses, AC-side wiring losses, and, in some cases, medium-voltage-side wiring and losses around transformers. Wiring losses increase as cable length increases, as current increases, and as conductor cross-sectional area decreases. Therefore, the power plant layout, PCS placement, junction box placement, and cable sizing are directly linked to Ohmic loss.

What beginners should be aware of is that Ohmic loss, even if it appears as a small percentage, can lead to a non-negligible difference in annual energy production. For example, the difference between wiring losses of 1.0 percent and 1.5 percent may seem minor on its own. However, at large-scale power plants, when converted to annual energy output or revenue, it can amount to a substantial difference.

Also, in comparisons using PVSyst, the way wiring losses are set can differ from report to report. Whether the calculation is based on actual cable length and cross-sectional area or whether a percentage is entered as a standard value affects how persuasive the results are. When reading reports, it is important to check not only the Ohmic loss value but also the basis for it.

What Beginners Should Watch Out for When Reading PVSyst Terminology

When learning PVSyst terminology, rather than trying to memorize everything, it's important to distinguish and understand which stage the numbers refer to. By organizing whether a term relates to irradiance, DC-side generation, AC-side generation, loss items, or performance indicators, the overall structure of the report becomes clear.

What beginners should be especially careful about is not judging based solely on the final energy output. Whether EGrid is high or low is important, but if you cannot explain why, you cannot apply the analysis results in practice. You need to check, in order, whether GlobInc is low, whether losses up to GlobEff are large, whether the drop occurs at the EArray stage, or whether it drops after the PCS.

Even if you understand the meaning of the terms, overlooking the units can lead to misunderstandings. Solar irradiance is often shown as kWh/m2 (kWh/ft2), and generation is shown in kWh or MWh. Losses are often expressed as a percentage, but their meaning changes depending on what they are a percentage of.

When reading PVSyst, it's important to look not only at the magnitude of the numbers but also at their sequential relationships. By tracing the flow from GlobHor to GlobInc, from GlobInc to GlobEff, from GlobEff to EArray, and from EArray to EGrid, it becomes easier to identify where power generation is being reduced.

In comparison reports, check the assumptions rather than the meanings of terms

After understanding PVSyst terminology, it becomes important to verify the assumptions in the comparison report. When comparing multiple analysis results, even if the same terms are used, the numerical values will not completely match if the input conditions or settings differ.

For example, even when using the same term "PR", the basis for comparison changes if the meteorological data, module capacity, PCS capacity, loss settings, output limits, or the treatment of snow and soiling differ. The same applies to EGrid. While it is an easy-to-understand figure when viewed as generation, simply comparing proposals with different equipment capacities or solar irradiation can lead to incorrect judgments about the quality of the design.

In comparing PVSyst reports, it is important first to check the meteorological data, system capacity, tilt angle, azimuth, DC/AC ratio, loss settings, output limits, and the handling of shading. Then, by looking at GlobInc, GlobEff, EArray, EGrid, and PR in that order, it becomes easier to explain the causes of differences in generated energy.

On-site verification is also important. Drawings and desk-based analyses alone cannot fully capture actual rack/mounting positions, obstacles, terrain, construction tolerances, nearby structures, maintenance access routes, snow conditions, and so on. In recent years, on-site checks using iPhones with GNSS, AR-based overlaying of drawings, and the use of point cloud data to understand existing conditions have progressed. By leveraging systems that handle high-precision positioning on smartphones—such as LRTK—to assist with site positioning and drawing verification, it becomes easier to identify discrepancies between the conditions modeled in PVSyst and the actual site.

In particular, verification of shading, racking layout, terrain undulations, and azimuth affects the reliability of the analysis results. Understanding PVSyst terminology is important, but ultimately what matters is how well those figures reflect the actual conditions on site.

Summary

Terms in PVSyst that are easy to get confused are easier to understand if you separate them into irradiance, losses, DC generation, AC generation, and performance indicators. GlobHor is the irradiance on the horizontal plane, GlobInc is the irradiance incident on the module plane, and GlobEff is the effective irradiance reflecting losses. EArray is the energy on the DC side, EGrid is the AC energy delivered to the grid, and PR is an indicator for viewing the plant’s performance as a ratio.

IAM loss, Soiling loss, Mismatch loss, and Ohmic loss represent losses due to reflection, soiling, variations among modules or strings, and wiring resistance, respectively. Understanding these loss items allows you to explain more specifically why the energy output is high or low.

For beginners reading a PVSyst report, it is not necessary to memorize all the terms perfectly from the start. First, it is important to follow in order how solar irradiance is converted into generated energy and at which stages losses occur. If you check the flow from GlobInc to GlobEff, EArray, and EGrid, it becomes easier to grasp the overall picture of the report.

Also, in comparison reports, it is essential not only to understand the meanings of terms but also to align the underlying assumptions. Even for reports produced by the same PVSyst, results can vary greatly if the meteorological data, loss settings, shading treatment, wiring conditions, or PCS conditions differ. By understanding terminology like a dictionary while simultaneously checking the analysis conditions, you can turn PVSyst results into usable material for practical decision-making.