5 Ways to Read PVSyst's System Output｜Check Final Energy Production

Which items does PVSyst's System Output display?

When reading a PVSyst report, the number you ultimately want to check is "how much electricity this power plant is expected to generate." The place that comes closest to that answer is the System Output. It is the section where you check the energy finally delivered to the grid after passing through various calculations such as irradiance, plane-of-array irradiance, array output, inverter output, AC losses, transformer losses, and so on.

In simulations of solar power generation, the intermediate numbers are also very important. For example, whether the solar irradiance incident on the module surface is sufficient, whether the impact of shading is too great, whether temperature losses are reasonable, how much inverter clipping occurs due to oversizing, and whether AC wiring losses or transformer losses are excessive — there are many points to review in the design. However, the first questions asked by financial institutions, project owners, EPCs, O&M teams, and internal management are often: "How much is the annual electricity sales?", "What is the expected generation?", and "Which final value will be used for contractual evaluation?"

System Output is the entry point for checking that final value. In PVSyst’s Loss Diagram, it corresponds to the position that starts from solar irradiation and, after array losses, inverter losses, and AC-side losses, shows the energy remaining at the end. Being able to read this correctly makes reading the entire PVSyst report immediately more practical for real-world use.

One important point to note is that you cannot fully judge a plant's quality by looking at the System Output alone. The System Output is an important result that indicates the final generated energy, but the reasons why that number is high or low are hidden in the preceding conditions. Whether the irradiance data is set high, the loss settings are lax, the module capacity is large, the PCS capacity is small causing clipping, or output curtailment and grid constraints are reflected — depending on these factors, the same System Output can mean different things.

Therefore, what is important in reading the System Output is not simply reading the power generation figures. You need to check at which point the energy is measured, after which losses it has passed, whether it can be used as the amount sold to the grid, whether it has the same definition as the comparison target, and whether there are any unnatural biases when viewed month-by-month or hour-by-hour.

In this article, I organize five key perspectives to keep in mind when reading PVSyst's System Output in professional practice. To prevent confusion when reviewing energy production, comparing other companies' reports, evaluating P50 and P90, preparing documents for bank submissions, or checking internal explanatory materials, I explain, in sequence, the approach to interpreting the final energy output.

1 System Output verifies the location of the final power output

The first point when reading PVSyst's System Output is to confirm where in the energy flow that figure sits. In photovoltaic simulations, the solar irradiance arriving from the sun does not directly become the amount of electricity sold. The horizontal-plane irradiance is converted to the tilted plane, and then, by sequentially subtracting nearby and distant shading, IAM, soiling, snow, module temperature, mismatch, wiring losses, inverter losses, AC wiring losses, transformer losses, and so on, it approaches the final output.

System Output is an item that appears toward the end of this flow. In PVsyst notation it is often displayed as E_Grid or EGrid, and it is generally treated as the energy injected into the grid, that is, the amount of electricity leaving the power plant. In reports it is shown as an annual value in kWh or MWh, and it may also be available as monthly values.

What's important here is not to confuse "System Output" with "the amount of energy generated by the module." The energy that modules or the array produce on the DC side exists before the System Output stage. Array output is the DC-side energy after being affected by temperature, DC wiring, mismatch, and so on. By contrast, System Output is the energy after inverter conversion and after reflecting additional AC-side losses. Therefore, System Output is used not to verify DC-side performance but to confirm the amount of energy the plant can supply externally.

For example, even if the array output looks large in a PVsyst report, if inverter overload losses and AC losses are significant, the System Output will not grow as much as expected. Conversely, even when the array output is modest, if the loss settings are small and the balance with PCS capacity is good, the System Output can appear relatively stable. Therefore, it is important not to look only at the final energy yield, but to trace the flow to see how much energy remains from upstream stages.

In practice, it is efficient to first check the annual value of the System Output and then review the monthly values. The annual value serves as the basis for project financials and revenue from electricity sales. Monthly values are used to check seasonal variations, the impact of snow accumulation, temperature-related losses in summer, reductions in solar irradiance during the rainy season, the angle of solar radiation in winter, and the effects of output control. Anomalies that are not visible from the annual figures alone become easier to detect when the data are broken down by month.

When reading the System Output, first clarify which point the energy quantity refers to. Whether it is inverter output, the receiving point, the equivalent of the export (sales) meter, or after transformer losses will change what you compare it to. In particular, when comparing with other companies' reports or past projects, differences in this point can be the cause of discrepancies.

2 Confirm the difference between E_Grid and EOutInv

A common source of confusion when reading the System Output is the difference between EOutInv and E_Grid. EOutInv generally refers to the energy on the inverter output side. In other words, it is the energy that enters the inverter from the DC side and, after being affected by inverter efficiency, overload, operating range, and so on, comes out on the AC side. On the other hand, E_Grid is treated as the energy injected into the grid, reflecting subsequent AC wiring losses, transformer losses, and, in some cases, grid-side constraints or unused energy.

If you don't understand the difference between these two, misunderstandings can arise when comparing generation figures. For example, if you treat EOutInv as the final generation, you may mistake the figure before AC wiring losses and transformer losses are deducted for the amount exported to the grid. Conversely, if you intend to look at E_Grid but instead refer to EOutInv, you may end up reporting a slightly higher generation than actually occurred.

For projects with small AC losses, the difference between EOutInv and E_Grid may not be very large. However, when the plant scale is large, the distance from the PCS to the point of interconnection is long, a step-up transformer is included, or on-site consumption and auxiliary equipment losses are taken into account, the difference becomes non-negligible. In particular, for high-voltage and extra-high-voltage projects, it is important to be clear about how much of the system is included in the simulation.

When reviewing PVsyst results, check in the Loss Diagram which losses are applied after EOutInv. If items such as AC wiring loss, transformer loss, auxiliary loss, grid limitation, or unavailability are set, they will be deducted before reaching the System Output. If these losses are large, they should be explained as factors causing the reduction in the final energy output.

This difference is particularly important when comparing reports from other companies. If one report is based on inverter output while another is based on energy at the grid connection point, it is natural that the figures will differ for the same power plant. When creating a comparison table, you should check which variable is being used rather than simply placing annual generation figures side by side.

Also, when output curtailment or grid limits are configured, care must be taken in how EOutInv is handled. In PVsyst, the way grid limits are treated as losses at which stage can change how they appear in the reports. Because the energy equivalent to the inverter output and the energy actually injected into the grid are not necessarily the same, it is important to check the Loss Diagram together with the explanations of the result variables.

When reading System Output, you can obtain a lot of information simply by looking at the difference between EOutInv and E_Grid. If that difference is small, AC-side losses can be considered relatively low. If the difference is large, you need to check settings such as AC wiring, transformers, auxiliary equipment, constraints, and uncommissioned or non-operational equipment. When reviewing final generation, it is practical to check not only E_Grid but also the immediately preceding EOutInv.

View the monthly System Output, not just the 3-year values

System Output is often checked as an annual value, but in practice verifying monthly values is extremely important. Even if the annual generation looks reasonable overall, breaking it down by month can reveal anomalous fluctuations. Characteristics such as a large drop in generation in a particular month, a winter decline larger than expected, summer generation not increasing in line with solar irradiance, or weak peaks in spring or autumn are difficult to identify without looking at monthly System Output.

When examining the monthly System Output, start by checking seasonal trends. Generally, generation tends to be higher in spring and autumn, when solar irradiance is high and temperature-related losses are relatively small. In summer, although irradiance is abundant, losses from rising module temperature increase, so it does not necessarily produce the maximum generation. In winter the sun angle is low and, depending on the region, snow or shading can have a large impact. Comparing this seasonality with the shape of the System Output makes it easier to notice any inconsistencies in the settings.

For example, in projects located in snowy regions such as Hokkaido and Tohoku, the winter System Output can drop significantly. This is influenced not only by reduced solar irradiance but also by a decrease in effective insolation due to snow and by assumptions about how much snow will accumulate on module surfaces. Reports that do not account for snow may show higher winter power generation. Conversely, if snow losses are overestimated, the winter System Output can be unnecessarily low.

In projects with output curtailment, looking at monthly System Output can help estimate which seasons are most affected by the curtailment. If restrictions are more likely in seasons with many clear days and large generation peaks, there will be months in which System Output does not rise despite high solar irradiance. In such cases, it is possible that grid-side constraints or output caps — rather than mere equipment losses — are affecting generation.

Also, it is important to read the monthly System Output together with PVsyst's other monthly indicators. By looking at GlobInc, GlobEff, EArray, EOutInv, E_Grid, PR, Specific Yield, etc., side by side, you can see at which stage the generation is dropping. You can distinguish whether the System Output is low because solar irradiance is low, whether there is irradiance but array losses are large, or whether the array is producing but it drops on the AC side.

Annual values are necessary for assessing project viability, but monthly values are required to verify the validity of design and operation. In particular, for bank submissions, investor presentations, consultations with the EPC, O&M planning, and comparisons with actual generation, reviewing the monthly System Output enhances explanatory clarity. The final generation should be understood not only as an annual total but as the cumulative total across 12 months.

4 Viewing the relationship between System Output and PR

When reading PVSyst's System Output, you should also check its relationship with the Performance Ratio (PR). System Output indicates the final amount of energy. On the other hand, PR is an indicator of how efficiently the system generated electricity relative to solar irradiation and the system's installed capacity. In other words, System Output is the energy produced itself, while PR can be viewed as a comprehensive assessment of the generation system's efficiency and losses.

Projects with high power generation do not necessarily have better performance. If the installed capacity is large, the System Output will also be large, and if the solar irradiance conditions are good in a region, the power generation will be higher. Therefore, when comparing different projects, looking only at System Output can lead to mistakenly interpreting differences in capacity or irradiance as differences in performance. By looking at PR and Specific Yield together, a fairer comparison can be made.

For example, a project may show a high System Output but a low PR. In this case, although generation may appear large due to solar irradiance conditions and installed capacity, there may be significant losses within the system. It is necessary to check for shading, temperature effects, mismatch, wiring, PCS overload, AC losses, transformer losses, and non-operational units.

Conversely, there are cases where System Output is low but PR is high. In such cases, even for projects in regions with harsh insolation conditions, projects with small system capacity, or projects affected by snow or terrain conditions, the system may be operating relatively efficiently. Rather than judging solely by the absolute value of power generation, it is important to view PR as the conversion efficiency relative to insolation.

When reading the relationship between System Output and PR, Reference Yield and Specific Yield are also useful. Reference Yield is positioned as a theoretical solar irradiance potential based on plane-of-array irradiance, and Specific Yield indicates the energy produced per unit of installed capacity. Dividing System Output by the installed capacity yields a value in the form of kWh/kWp, making comparison easier. This allows the efficiency of energy production to be compared across projects with different capacities.

However, PR is not infallible. PR is affected by temperature conditions, irradiance conditions, oversizing design, azimuth and tilt angles, clipping, snow, and power curtailment. Also, in PVsyst, how losses are included in the PR and which energy is treated as the final output will change how it appears. Therefore, it is necessary to view System Output and PR together and then go back to check each loss item in the Loss Diagram.

A practical recommended reading order is to first check the annual generation using System Output, then look at generation per capacity using Specific Yield, and finally assess the overall plausibility of losses using PR. Viewing them in this order lets you check the three aspects—generation, capacity-normalized generation, and efficiency evaluation—in a balanced way. For internal or customer explanations, it also becomes easier to organize the message as, "The final annual generation is this value, on a per-capacity basis it is about this, and PR indicates that the level of losses is about this."

5 Back-calculating loss factors from differences in System Output

An important practical aspect of reading the System Output is to back-calculate the causes from the differences in the final energy output. In PVsyst, it is common to create and compare multiple cases and variants. For example, a case with a changed tilt angle, a case with a changed PCS capacity, a case with a changed DC/AC ratio, a case with modified wiring losses, a case with added snow losses, a case with changed albedo, a case reflecting output curtailment, and so on. In these comparisons, the differences in the System Output are the clearest result.

However, concluding only that the System Output has increased or decreased is insufficient. If you cannot explain why the difference occurred, it cannot be used for design decisions. When examining differences in System Output, check the Loss Diagram to see which stage's losses have changed. By breaking down whether the difference is on the solar side, in array losses, in inverter losses, or on the AC side, you can explain the effects of design changes.

For example, if changing the tilt angle increases the System Output, you need to confirm whether that increase is due to increased irradiance on the tilted surface, whether it reflects easier snow shedding, or whether it’s caused by changes in shading effects. If you decide the tilt angle based solely on the final energy output, you may overlook the balance with constructability, wind loads, racking costs, and land-use efficiency.

If you change the PCS capacity, it is important to look at the relationship among EArray, EOutInv, and E_Grid. Increasing PCS capacity reduces clipping losses, but it affects equipment costs, transformer capacity, and grid interconnection conditions. Decreasing PCS capacity may lower costs, but can increase inverter overload losses at peak times and reduce System Output. Checking these differences on an annual and monthly basis reveals in which seasons losses occur.

When AC wiring losses or transformer losses are changed, the difference between EOutInv and E_Grid is the direct point of verification. Even if the DC-side design is the same, the final System Output can change depending on AC-side distance and cable size, transformer efficiency, and auxiliary equipment consumption settings. Especially in large-scale projects, a few percent difference on the AC side can have a significant impact on annual energy production and revenue from electricity sales.

Even when comparing with another company's report, it is efficient to use the differences in System Output as the starting point. First check the difference in annual energy generation, then sequentially examine the differences in irradiance, installed capacity, PR, and the Loss Diagram. If the irradiance data differ, the foundation for the comparison changes from the outset. If the installed capacity differs, the absolute value of generation also changes. If the loss settings differ, the System Output will change even with the same irradiance and the same capacity.

System Output is the result, but in difference analysis it can also serve as the starting point. By checking differences in the final values and tracing their causes back to earlier stages, you can improve the accuracy of PVsyst report reviews. This becomes a way of interpreting the report that can be used not only for simple power-generation checks but also for design decisions and presentation materials.

Workflow for verifying System Output in operational use

In practice, when checking PVsyst’s System Output you first look at the annual E_Grid. Confirm whether this can be used as a reference value for the amount of electricity sold and the final generated energy. Next, check the difference from EOutInv to see to what extent AC-side losses and transformer losses are reflected. After that, review the monthly System Output to see if there are seasonal fluctuations or any abnormal drops.

Next, cross-check against PR, Specific Yield, and Reference Yield. Even if System Output is high, a low PR may indicate significant losses. Conversely, if System Output is low, it may simply be due to poor solar irradiance conditions and still be reasonable from a system performance standpoint. Distinguishing between these cases makes it easier to explain the generation figures.

Furthermore, review the Loss Diagram to trace the flow leading to the final energy production. Check where the largest drops occur among solar irradiance, optical losses, array losses, inverter losses, and AC losses. In a PVsyst review, it is important not to stop at the System Output alone but to examine the path that leads to the System Output.

When preparing materials for internal sharing, it is clearer to organize System Output, PR, Specific Yield, and the main loss items as a set. By listing the absolute generation, generation per unit capacity, efficiency relative to solar irradiance, and the breakdown of losses, it becomes easier to explain to non-engineers. For clients and financial institutions, clarifying which values correspond to the amount of electricity sold and which losses are included will prevent later misunderstandings.

What you need to pay particular attention to is when comparing PVsyst results with actual generated energy. You must confirm which meter the actual energy was measured with. Whether it is the PCS output, the point of interconnection, the sales meter, or the value after on-site consumption is subtracted will change the comparison results. If PVsyst’s System Output and the measurement point of the observed data do not match, you cannot correctly assess the validity of the simulation.

System Output is the figure in the PVsyst report that is closest to business decision-making. However, to use that figure correctly, you need to align definitions, measurement points, ranges of losses, and comparison conditions. Carefully checking these items will make explanations of the energy yield more convincing.

Notes on Reading PVSyst's System Output

When reading PVSyst's System Output, the thing you most want to avoid is looking only at the final energy yield and judging it in isolation. It is dangerous to make a simple judgment that a higher yield is good and a lower yield is bad. The energy production of a solar power system is determined by a combination of solar irradiance conditions, system capacity, design conditions, loss settings, grid conditions, and operational conditions. The System Output is the result of those factors, not the cause itself.

Also, even with the same PVsyst, results can vary depending on the person configuring it. If input conditions differ—selection of meteorological data, albedo, soiling, snow, shading, module degradation, mismatch, wiring losses, PCS settings, power factor, transformer, auxiliaries, output control, etc.—the System Output will also change. Therefore, when comparing reports, it is essential to check not only the differences in final energy production but also the differences in the configuration settings.

If you use the System Output value to estimate feed-in revenues, further caution is required. PVsyst results are simulations, and actual performance will vary depending on real weather, equipment availability, failures, curtailment, maintenance, grid outages, snow, and soiling. In project planning it is common to separately account for P50, P90, degradation over time, output control, availability, and maintenance outages. PVsyst's System Output is an important reference value, but it alone does not fully guarantee future generation.

Furthermore, in projects that include battery storage or self-consumption, the meaning of System Output can differ from that in conventional full-feed-in (all-power-sold) projects. How the generated energy is interpreted by E_Grid depends on whether it is used directly by loads, charged into batteries, or only the surplus is injected into the grid. In self-consumption systems, a small grid injection does not necessarily indicate low generation; the energy consumed on the demand side must also be included in the evaluation.

System Output is interpreted slightly differently for projects involving full-feed-in sales, surplus feed-in sales, self-consumption, battery storage, or output control. It is important to check the report’s assumptions and, after clarifying which form of energy should be treated as the final generated energy, make a determination.

Summary for Confirming Final Power Generation

PVSyst's System Output is an important metric for verifying the final electricity production of a solar power plant. It indicates the value at the stage where the energy that began as solar irradiation, after passing through various losses, is finally injected into the grid. For generation reviews, financial/energy balance calculations, documents submitted to banks, comparisons with other companies' reports, and reconciliation with actual results, it is one of the primary figures to check.

The basic way to read it is to first understand the location of the System Output. Check whether it is a value after passing through AC-side losses and transformer losses, rather than the array output or inverter output. Next, look at the difference between EOutInv and E_Grid to see how much loss there is from the inverter output to the point of interconnection. Also check monthly values as well as annual values to identify seasonal variations or any abnormal drops.

In addition, it is important to evaluate in combination with PR and Specific Yield. System Output represents the absolute amount of energy generated, while PR indicates the system’s efficiency relative to solar irradiation. When comparing projects with different capacities or different solar irradiation conditions, you need to look not only at System Output but also at the energy generation per unit of capacity and PR.

Also, when comparing cases or comparing with other companies' reports, start from the difference in the System Output and work backward to identify which loss items are causing the discrepancy. By checking, in order, differences in irradiance data, shading settings, temperature losses, wiring losses, PCS overload, AC losses, transformer losses, and output control, it becomes easier to explain the reasons for the generation differences.

Reading PVsyst’s System Output correctly is not simply a matter of checking the final energy output. It is the work of validating design conditions, organizing the basis for comparing reports, and strengthening the explanatory power needed to judge the financial viability of a power generation project. Rather than stopping at the final value, reading the flow of energy that leads to it is how to use PVsyst effectively in practice.

In designing and managing the construction of solar power plants, not only the energy yield on PVsyst but also the site’s topography, racking layout, surveying accuracy, and post-construction as-built verification are important. If the conditions assumed in desktop simulations differ from the actual site conditions, the evaluation of energy production will be affected. Combining a system that uses an iPhone and high-precision GNSS, such as LRTK, to verify on-site location information and as-built conditions makes it easier to consistently check design, construction, inspection, and maintenance. By reading the final energy yield in PVsyst and confirming construction status on-site with high-precision positioning, you can more concretely understand the gap between the plant’s planned values and reality.