5 Ways to Interpret PVSyst Specific Yield | Basics of kWh/kWp

When reading a PVSyst report for a solar power plant, Specific Yield is as important as the energy output itself. Specific Yield is sometimes referred to in Japanese as 比発電量、設備容量あたり発電量、単位容量あたり発電量, and its unit is expressed as kWh/kWp. Simply put, it is an indicator that shows how many kWh of electricity are generated annually per 1 kWp of installed solar panels.

When you look at the results from PVSyst, the plant's total annual energy generation is displayed prominently, so your attention tends to go to the MWh or GWh figures first. However, if plant sizes differ, you cannot judge performance simply by comparing annual generation. A 10 MW plant and a 50 MW plant will, of course, have a larger generation for the 50 MW plant. For this reason, Specific Yield is used to compare plants on an equal-size basis.

By reading the Specific Yield, you can grasp how efficiently that power plant is generating relative to its capacity. When comparing multiple scenarios in PVSyst, differences in irradiance conditions, panel azimuth and tilt, shading impacts, loss settings, overloading ratio, PCS capacity, curtailment, and effects such as snow and soiling all ultimately appear in the form of kWh/kWp. In other words, Specific Yield is a practical metric for side-by-side comparison of design proposals.

In this article, we organize five important perspectives for reading the Specific Yield in PVSyst. We explain, in order, the meaning of the unit, its relationship to annual energy production, the difference from PR, how to view it month by month, and how to use it in design reviews.

Table of Contents

• Read Specific Yield as the energy generation per 1 kWp

• Verify consistency between annual generation and DC capacity

• Use it as a metric separate from PR

• Assess seasonal factors using monthly Specific Yield

• When comparing designs, evaluate it together with loss factors

• Points to note when viewing Specific Yield in PVSyst

• How to organize kWh/kWp for internal sharing

• Approach to on-site verification and utilization of LRTK

• Summary

Specific Yield should be read as the amount of electricity generated per 1 kWp.

The basis of Specific Yield is to look at how much electrical energy can be generated per 1 kWp of a solar panel’s DC capacity. The unit is kWh/kWp. For example, if the annual Specific Yield is 1,200 kWh/kWp, it means that installing 1 kWp of solar panels will yield approximately 1,200 kWh of electrical energy per year.

Here the important point is that Specific Yield is an indicator that removes the effect of plant size. If a plant’s total annual generation is 12,000 MWh and its DC capacity is 10,000 kWp, the Specific Yield is 1,200 kWh/kWp. If another plant has an annual 6,000 MWh and a DC capacity of 5,000 kWp, its Specific Yield is also 1,200 kWh/kWp. Even though the total generation differs, the generation performance per unit of capacity can be read as the same.

In PVSyst, result screens and reports may display values labeled Specific production, Specific yield, or expressions close to the unit kWh/kWp/year. The notation varies somewhat depending on the report format and version, but in practice this value is checked as the annual kWh/kWp. When compiling Japanese materials, it is clearer to express it as specific generation, annual specific generation, or annual generation per unit capacity.

When looking at Specific Yield, it is important to first check whether the value is based on DC capacity or AC capacity. The kWh/kWp commonly used in PVSyst is based on the nominal capacity of the PV array, i.e., kWp. kWp refers to the nominal output of a solar panel under standard test conditions. Therefore, it is fundamental to consider that the denominator is the installed capacity of the solar panels, not the PCS capacity or the grid-interconnection capacity.

If you read this without keeping that premise in mind, projects with high DC/AC ratios can easily be misunderstood. For example, in a design with large DC capacity and small AC capacity, the plant’s total amount of electricity sold may appear large, while the Specific Yield per unit of DC capacity may be somewhat lower. This does not necessarily mean it is a bad design; it can be the result of a design philosophy of increasing DC-side capacity to use the PCS efficiently.

When reading Specific Yield, rather than judging it as simply high or low, you need to check which capacity is being used as the reference and which energy-point’s generation is being used as the numerator. In PVSyst, values differ depending on the energy point, such as Array output, Inverter output, and Grid injection. For final evaluations of power sales, it is common practice to look at the value obtained by dividing the amount of energy injected into the grid by the DC capacity.

Check consistency between annual power generation and DC capacity

The second point when reading Specific Yield is to check consistency by calculating it from the annual energy production and the DC capacity. Because a PVSyst report displays many figures, rather than accepting the shown Specific Yield at face value, verifying it by back-calculating from the key numbers can prevent misreading the report or transcription errors in comparison tables.

The basic idea is simple. Divide the annual energy production by the DC capacity of the solar panels. If the annual energy production is 12,000,000 kWh and the DC capacity is 10,000 kWp, the Specific Yield is 1,200 kWh/kWp. If the annual energy production is shown in MWh, convert it to kWh before dividing. 12,000 MWh is 12,000,000 kWh, so dividing by 10,000 kWp gives 1,200 kWh/kWp.

This check is particularly important when comparing multiple PVSyst cases. For example, when comparing cases for the same power plant with tilt angles of 10 degrees, 15 degrees, and 20 degrees, looking only at annual energy production can lead to incorrect conclusions if the capacity settings differ slightly between cases. By examining the Specific Yield, you can normalize capacity differences and more easily determine which tilt angle is more advantageous.

Also, when organizing PVSyst reports for customer submissions or internal approvals, it's best to always check the three items—annual energy production, DC capacity, and Specific Yield—as a set. Annual energy production alone is influenced by the system size, and Specific Yield alone does not convey the total magnitude. By listing the DC capacity alongside, it becomes clear what scale of power plant the values correspond to.

For example, if option A's annual generation is 10,000 MWh and option B's annual generation is 10,200 MWh, option B would appear better. However, if option A's DC capacity is 8,000 kWp and option B's DC capacity is 8,300 kWp, the Specific Yield would be 1,250 kWh/kWp for A and approximately 1,229 kWh/kWp for B. In this case, the generation performance per unit capacity can be read as higher for option A.

In this way, Specific Yield is not merely an outcome metric but a check metric to confirm the efficiency of a design when comparing cases in PVSyst. You can tell whether an increase in annual energy production is due to design improvements or simply from increasing DC capacity.

However, a high Specific Yield does not necessarily guarantee better economics. Reducing DC capacity to balance with PCS capacity and thereby reducing clipping can increase Specific Yield. However, when equipment costs, land use, interconnection capacity, electricity selling price, output control, and maintainability are taken into account, a design with a slightly lower Specific Yield but higher total generation and project revenue may be more advantageous. Therefore, Specific Yield is an indicator of design efficiency, and the final decision should be made together with a project viability assessment.

Use as a metric separate from PR

In PVSyst, PR—i.e., Performance Ratio—is an important metric alongside Specific Yield. Because both describe a plant’s performance, they are easily confused, but their meanings are different. Specific Yield indicates the energy produced per unit of capacity, while PR indicates the proportion of the incident solar energy that was converted into useful electrical energy.

Specific Yield tends to be higher in regions with greater solar irradiation. For example, even with the same design quality, regions with higher annual solar irradiation have higher kWh/kWp. Conversely, regions with lower solar irradiation have lower Specific Yield. In other words, Specific Yield strongly reflects a region's solar irradiation potential.

By contrast, PR is a metric that, by removing differences in solar irradiance conditions to some extent, shows the magnitude of losses as a system. It indicates how much remains compared to the theoretically obtainable energy after accumulated losses such as temperature loss, shading loss, IAM loss, mismatch loss, wiring loss, inverter loss, transformer loss, auxiliary equipment loss, and grid constraints.

Therefore, a high Specific Yield does not necessarily mean a high PR. In regions with very good solar irradiance, Specific Yield can be high even if losses are somewhat large. Conversely, in regions with low solar irradiance, Specific Yield can be low even if PR is high. This is an important point when reading PVSyst results.

In practice, Specific Yield is a useful metric for forecasting power generation and comparing project viability. The unit kWh/kWp allows comparison on a like-for-like plant size basis, making it suitable for investment decisions and project comparisons. On the other hand, PR is suited to assessing the validity of design and loss assumptions. If PR is unnaturally low, check shading, wiring, temperature, PCS capacity, output curtailment, auxiliary losses, etc. If PR is unnaturally high, verify whether loss assumptions are too optimistic and whether solar irradiance data, albedo, soiling, IAM, temperature coefficient, and wiring losses are appropriate.

Specific Yield and PR should not be evaluated in isolation. Specific Yield is closer to the perceived amount of energy generation, while PR is an indicator closer to the health of system performance. In PVSyst reviews, it is effective to first use Specific Yield to understand the generation per unit of capacity, and then use PR to check what composition of losses produced that value.

For example, if you compare a case with a Specific Yield of 1,250 kWh/kWp and a PR of 82% to a case with a Specific Yield of 1,180 kWh/kWp and a PR of 84%, the former may reflect better solar resource but somewhat larger losses, while the latter may indicate more modest solar resource but better system efficiency. Looking only at Specific Yield without understanding this difference can lead to a mistaken judgment about design quality.

Interpreting seasonal factors using monthly Specific Yield

It is important to view Specific Yield not only as an annual value but also on a monthly basis. In PVSyst you can check monthly generation, solar irradiation, PR, losses, and so on. By using these, you can identify which seasons have higher Specific Yield and which seasons have lower Specific Yield.

When looking at monthly Specific Yield, it generally tends to be higher from spring to summer and lower in winter. However, patterns can vary greatly depending on region and design conditions. In snowy regions, Specific Yield can fall sharply in winter due to reduced solar radiation and snow losses. In hot regions, even if summer solar radiation is high, temperature-related losses from elevated module temperatures can reduce PR, so Specific Yield may not increase as much as expected.

When reading monthly values in PVSyst, you should not look only at the energy generation but also check GlobInc, GlobEff, EArray, EGrid, PR, temperature loss, shading loss, Soiling loss, etc. GlobInc is the irradiance on the tilted surface, and GlobEff is often used as a concept close to the effective irradiance that takes into account shading and IAM; these form the basis for monthly generation. It is natural if months with high irradiance show higher generation, but if irradiance is high and generation does not increase, suspect the influence of temperature loss, PCS clipping, curtailment, shading, or soiling.

Monthly Specific Yield is also useful for comparison with measured data. After operation begins, convert the monthly generation to kWh/kWp and compare it with PVSyst's monthly Specific Yield to reveal seasonal deviations. Using only annual values can mask trends—such as being higher than expected in spring and lower in summer—that cancel each other out. By examining the data month by month, it becomes easier to pinpoint when problems are occurring.

For example, if the measured Specific Yield falls far below PVSyst only in winter, candidates include snow cover, shading due to low solar altitude, string-level shading, snow removal operations, PCS shutdowns, and grid curtailment. If it falls below only in summer, check module temperature, PCS temperature derating, fans and ventilation, output control, shading from vegetation, and accumulation of soiling. If the discrepancy occurs only during the rainy season, it is necessary to check the representativeness of the meteorological data and the difference from the actual solar irradiance.

When reading monthly Specific Yield, be mindful of the differences in the number of days in each month. Between January and February, and between months with 30 days and 31 days, a simple comparison of monthly generation is affected by the number of days. For a more detailed comparison, organize not only monthly kWh/kWp but also average daily kWh/kWp/day, which makes it easier to identify seasonal differences and anomalous months.

When explaining PVSyst results to internal teams or clients, presenting only the annual Specific Yield can make it difficult to convey why that value was obtained. Organizing monthly graphs and tables together and explaining in which seasons generation is high and in which seasons losses are large will increase the persuasiveness of the report.

When comparing designs, evaluate them together with loss factors

Specific Yield is useful for comparing designs, but it is risky to decide superiority by looking at the value alone. In PVSyst, many factors influence Specific Yield, such as tilt angle, azimuth, array spacing, shading, overloading ratio, PCS capacity, wiring losses, transformer losses, auxiliary losses, soiling, albedo, and temperature conditions. Therefore, when comparing designs it is important to read Specific Yield together with the Loss Diagram.

For example, suppose there is a 20 kWh/kWp difference in Specific Yield between option A and option B. The design implications are completely different depending on whether that difference is due to variations in solar irradiation capture, differences in shading losses, or differences in PCS clipping. If changing the tilt angle increased winter solar irradiation capture and thus raised the Specific Yield, that can be considered a design improvement. On the other hand, if the Specific Yield rose simply because the assumed wiring loss was reduced, it is not a design improvement but a difference in assumptions.

A common point to watch for when comparing PVSyst results is that the input conditions across multiple cases are not completely aligned. If the meteorological data, albedo, Soiling loss, IAM, temperature model, wiring losses, PCS model, transformer losses, auxiliary losses, or output limitation conditions differ even slightly, they will affect the difference in Specific Yield. To compare design proposals fairly, you need to fix as many conditions as possible other than the items you want to compare.

In particular, the settings for DC wiring losses, AC wiring losses, transformer losses, and auxiliary losses directly affect Specific Yield. If you set the loss rates low, EGrid increases and Specific Yield also rises. However, if those settings do not match the actual design conditions, the result will be an overestimate. When using PVSyst values for clients or financial institutions, it is important to be able to explain the rationale for the loss settings.

In designs with a high DC/AC ratio, PCS clipping losses may increase. This can reduce the Specific Yield per unit of DC capacity. However, by effectively utilizing PCS capacity and increasing generation in the morning, evening, and during periods of low irradiance, the project as a whole can still be advantageous. In such cases, rather than dismissing a design solely because of a decline in Specific Yield, it is necessary to evaluate it by including generation per unit of AC capacity, equipment costs, power sales revenue, output control, and grid interconnection conditions.

Shading effects are also important when interpreting Specific Yield. Near shading, distant terrain shading, inter-array shading, and shading from surrounding obstacles affect monthly and time-of-day power generation. In particular, when shadows occur during winter or at sunrise and sunset with low solar elevation, the impact on annual Specific Yield can sometimes appear limited, but they affect generation outside peak times and seasonal energy balances. It is important to confirm that PVSyst's 3D shading and Near Shadings settings match the actual site conditions.

In design comparisons, rather than looking only at the difference in Specific Yield in kWh/kWp, converting that into how much difference it represents in annual electricity sales makes judgment easier. For example, for a 10 MWp power plant, if the Specific Yield differs by 10 kWh/kWp, the difference in annual generation is about 100,000 kWh. Multiplying by the feed-in tariff or the PPA price gives an estimate of the annual revenue difference. This makes the economic impact of design changes easier to see.

Notes when viewing Specific Yield in PVSyst

When reading PVSyst's Specific Yield, first check which generation figure is being used as the numerator. There are multiple stages of generation figures, such as array output, inverter output, and energy injected into the grid. For practical matters like power sales and project economics, values close to EGrid or Grid Injection—the energy ultimately sent to the grid—are most relevant. On the other hand, when evaluating module or array performance, values such as EArray can also be useful.

Next, confirm that the capacity used as the denominator is the DC capacity. PVSyst's kWh/kWp is generally based on the nominal DC-side capacity, but internal or third-party documents may instead use generation per AC capacity. Confusing DC-capacity-based kWh/kWp with AC-capacity-based kWh/kWac can lead to large errors in projects with high oversizing ratios.

Also, when transcribing values from a PVSyst report into Excel or similar, pay attention to unit conversions. If MWh, kWh, MWp, and kWp are mixed, Specific Yield can be off by a factor of 1,000. If you treat annual generation in MWh, make the DC capacity MWp as well before dividing, which yields MWh/MWp and is numerically the same as kWh/kWp. For example, dividing 12,000 MWh by 10 MWp gives 1,200 MWh/MWp, which has the same meaning as 1,200 kWh/kWp.

In PVSyst case comparisons, also check how output limitations and grid constraints are handled. If Grid limitation or Power limitation is set, the energy that could have been generated is curtailed, so EGrid and Specific Yield decrease. This is not a design issue but may be due to interconnection conditions, PCS settings, or contractual terms. When Specific Yield appears low in the report, check the Loss Diagram to see at which stage the energy is being lost.

In snowy regions, the treatment of Soiling loss and Albedo also has a major effect on Specific Yield. Whether generation stoppages or reductions caused by snow are treated as Soiling or set as monthly loss rates, and to what extent the reflection during snowfall is anticipated as Albedo, will change the Specific Yield in winter. For projects that account for snow, it is necessary to focus not only on annual values but also on winter monthly values.

In evaluating power generation in high-temperature regions and during summer, check for temperature losses. Solar panels tend to generate more power with higher irradiance, but output decreases as module temperature rises. Therefore, even in regions with high summer irradiance, large temperature losses can suppress the increase in Specific Yield. Checking whether PVSyst's Thermal Loss factor and module temperature conditions are reasonable makes it easier to interpret Specific Yield.

How to organize kWh/kWp for internal sharing

When sharing PVSyst results internally, Specific Yield is an extremely useful metric. Even when plant sizes differ, arranging values in kWh/kWp allows you to compare generation performance per unit of capacity. In particular, for initial screening of multiple projects, comparison of design options, comparison with other companies' simulations, and comparison with actual performance, using Specific Yield as a common metric helps to organize discussions.

In internal documents, arranging annual generation, DC capacity, AC capacity, DC/AC ratio, Specific Yield, PR, and major losses makes the information easier to understand. With Specific Yield alone, you cannot tell whether differences are due to solar irradiation conditions or to design. By including PR and the loss items, you can explain the background of the values.

For example, if Project A's Specific Yield is 1,250 kWh/kWp and Project B's is 1,150 kWh/kWp, Project A simply appears better. However, if Project A is in a region with high solar irradiance and Project B is in a region with snowfall or low solar irradiance, you cannot say this reflects a difference in design quality. If you look at PR and the loss breakdown together, Project B may actually be better in terms of system efficiency.

Specific Yield is useful when comparing with other companies' reports. However, before making comparisons, you need to check DC capacity, meteorological data, the target period, loss settings, output limits, grid injection point, PCS capacity, and the handling of auxiliary losses. Even if another company's annual generation is higher, if their DC capacity is simply larger, the difference in Specific Yield may be small. Conversely, if the Specific Yield differs significantly, there may be differences in solar irradiation data or loss settings.

For comparisons with actual results, divide the plant’s total measured generation by the DC capacity and present it as the measured Specific Yield. In doing so, it is important to annotate downtime periods, output control, communication outages, pyranometer anomalies, PCS stoppages, maintenance work, snow accumulation, and effects before and after mowing. PVSyst is a simulation based on certain assumptions, while actual results include operational factors. When examining the differences, rather than simply judging whether the simulation was right or wrong, identify and organize which factors caused the discrepancy.

When using kWh/kWp internally, it is effective to organize not only annual values but also monthly kWh/kWp. Viewing data by month makes it easier to see the effects of snowfall, the rainy season, high temperatures, output control, shading, soiling, and similar factors. In particular, for O&M and power generation diagnostics, comparing monthly Specific Yield with PVSyst monthly values becomes an entry point for anomaly detection and root-cause analysis.

Site verification and approaches to utilizing LRTK

PVSyst's Specific Yield is the result calculated based on design data and meteorological data. However, in actual power plants, small differences in site conditions can affect energy production. For example, the as-built topography, racking layout, panel orientation and tilt, surrounding trees, slopes, utility poles, fences, residual snow, drainage conditions, and vegetation growth may not fully match the assumptions used in the simulation.

Therefore, if the Specific Yield is lower than expected or if differences arise between PVSyst cases, on-site verification is also important. Rather than judging solely from the desk-based Loss Diagram, checking the actual site's topography, shading conditions, and whether the equipment layout matches the design drawings will make it easier to identify the cause.

In site checks like this, using a system such as LRTK that enables high-precision GNSS positioning with a smartphone makes it easier to verify the positional relationship between the drawings and the actual site. By combining an iPhone with high-precision GNSS to record equipment locations and reference points on site and cross-checking them against design drawings and survey data, it becomes easier to identify discrepancies between the assumptions in PVSyst and the actual site conditions.

For example, if you record with location information the positions of nearby structures and trees that cause shading, the layout of racking rows, elevation differences of the site surface, and any anomalies found during inspections, it will be easier later to cross-check them against PVSyst’s 3D model and the site drawings. The accuracy of field records is important for distinguishing whether a decline in Specific Yield is due to the design, construction conditions, or the site environment.

PVSyst is a powerful tool for power generation simulation, but the accuracy of the input conditions determines the reliability of the results. If site positioning, photographs, point clouds, drawings, and construction records are organized, the interpretation of PVSyst becomes more practical. Rather than treating Specific Yield as a mere number, confirming which on-site conditions influence that value leads to better design reviews and operational improvements.

Summary

PVSyst's Specific Yield is a basic indicator that shows the energy produced per unit of installed capacity of a photovoltaic power plant. The unit is kWh/kWp, and it indicates how much is generated per 1 kWp per year. Because it can be compared regardless of plant size, it is a convenient metric for comparing projects, design proposals, other companies' reports, and actual performance.

The basic way to interpret it is to first understand its meaning as the amount of energy generated per 1kWp. Next, back-calculate from the annual generation and DC capacity to check whether it is consistent with the value shown in PVSyst. It is important to use it as an indicator separate from PR. Specific Yield is strongly affected by solar irradiance conditions, while PR is suited to evaluating system losses.

Also, by looking at monthly Specific Yield as well as the annual Specific Yield, seasonal generation trends and abnormal causes become easier to understand. The impacts of snowfall, high temperatures, the rainy season, shading, output curtailment, and soiling are easier to grasp when viewed on a monthly basis. In design comparisons, the differences in Specific Yield should be checked together with the Loss Diagram and loss settings to identify which factors are causing those differences.

While a high Specific Yield is advantageous in terms of energy generation, that alone does not necessarily mean that the design or project viability is superior. It is important to make a comprehensive judgment including the overloading ratio, PCS capacity, equipment costs, terms for electricity sales, output control, maintainability, and site conditions. PVSyst's kWh/kWp should be regarded not as a standalone indicator for drawing conclusions, but as an entry point for interpreting design and operation.

When reading a PVSyst report, checking Specific Yield, annual energy production, DC capacity, PR, major losses, and monthly trends together is fundamental to avoid uncertainty in practice. Furthermore, by combining site positioning records, drawings, point clouds, and photographs, you can more accurately grasp the relationship between the simulation results and actual site conditions. Once you can correctly read Specific Yield, you can use PVSyst results not just as a simple generation table but as practical documentation for design decisions and improvement proposals.