6 ways to check wiring losses in the PVSyst manual

Table of Contents

• Basics to understand before looking at wiring losses in the PVSyst manual

• Why are wiring losses important in power generation simulations?

• Method 1: Check the wiring losses on the DC side

• Method 2: Check the AC-side wiring losses

• Perspective 3: Consider the impact on annual power generation, not just the loss rate

• Perspective 4: Reassess the assumptions about cable length and cross-sectional area

• Perspective 5: Reading the connections between loss items in the report

• Perspective 6: Check the risks of underestimating and overestimating

• Points to note when using the PVSyst manual in professional practice

• Summary

Basics to Understand Before Looking at Wiring Losses in the PVSyst Manual

When simulating the energy production of a photovoltaic system, attention tends to focus on module output, solar irradiance, tilt angle, azimuth, and shading effects. However, in actual design and feasibility assessments, wiring losses that occur before the generated power reaches the power conditioner and the receiving equipment cannot be ignored. The purpose of reading the PVSyst manual is not simply to learn the on-screen operations, but to understand which input values correspond to which losses and how they ultimately affect the final energy yield.

Wiring losses are the power losses that occur due to the electrical resistance of cables. The DC power generated by solar modules is routed through junction boxes, combiner boxes, power conditioners, and then through AC-side wiring to the grid interconnection point or to load equipment. Because current flows during this process, the cables have resistance, and some of the power is dissipated as heat due to that resistance. In PVSyst, these electrical losses are entered as inputs and are reflected in the simulation results.

Users who search for "PVSyst manual wiring losses" have several concerns: they don't know where the wiring loss settings screen is, the difference between the DC side and the AC side is unclear, they can't judge whether the entered loss rates are reasonable, they don't understand the meaning of the loss items that appear in reports, and they are unsure how to reflect cable length and cross-sectional area from design drawings. This article organizes six key ways to view wiring losses when reading the PVSyst manual and clearly explains the practical points you should check.

The important point is not to treat wiring losses as mere percentage figures. For example, even if DC wiring loss is shown as 1.0%, whether that is reasonable in design, underestimated, or overly conservative varies by project. In large ground-mounted projects, cable lengths tend to be long, while in rooftop projects wiring conditions can change significantly depending on the arrangement of switchboards and power conditioners. In self-consumption projects, the AC-side wiring and the distance to the point of interconnection affect not only energy yield estimates but also voltage drop and equipment planning.

When consulting the PVSyst manual, it's important not only to check where on the screen to enter values, but also to confirm as a set which section of wiring those values represent, which current conditions are assumed, and which items in the final report they will be reflected in. If you proceed with simulations without a sufficient understanding of wiring losses, the estimated energy production may be overestimated compared to actual output, or conversely the results may become unnecessarily conservative.

Why Wiring Losses Are Important in Power Generation Simulations

Wiring losses are a relatively inconspicuous factor in the performance evaluation of solar power generation. They are not a visually obvious element like solar irradiance or shading analysis, nor are they an item that appears to be directly tied to product selection like modules or inverters. For that reason, simulation beginners often proceed with default values or simply reuse values from past projects. However, because wiring losses directly affect annual power generation, they must be treated carefully in business planning and design reviews.

One reason wiring losses are important is that they represent the difference between the power generated and the power that is actually available. Even if a solar module’s output is sufficient, if the wiring resistance is large, part of it is lost during transmission. Losses tend to increase especially under high-current conditions. When factors such as long cable lengths, small cross-sectional areas, improper division of circuits, and distant placement of power conditioners coincide, wiring losses can reach a level that cannot be ignored.

Also, wiring loss is a factor that readily reflects the quality of the design. Even if module capacity and installation angle are identical, losses will change if the layout of electrical equipment or the cable routing differs. When comparing multiple proposals in PVSyst, you must compare under the same assumptions for wiring conditions as well as the number of modules and azimuth; otherwise you cannot make a correct judgment. If only one proposal underestimates wiring loss, it may look favorable in simulation but fail to achieve the expected energy generation in the as‑built design.

Furthermore, wiring losses also affect the credibility of reports. In simulation materials submitted to financial institutions, clients, asset owners, EPC contractors, and others, it is important to be able to explain the breakdown of losses. If, when asked about wiring losses, you can only respond with "it's the default value" or "it's the same as past projects," the validity of the design conditions may be called into question. By organizing the meaning of each loss item and the rationale for inputs while referring to the PVSyst manual, you can enhance the explanatory power of your report.

Understanding how to interpret wiring losses is not about making the energy production appear larger. Rather, it is to assess a more realistic estimate of energy production and to identify opportunities for design improvements. Using thicker cables can reduce losses, but it increases cost. Distributing power conditioners can, in some cases, shorten DC-side cable lengths, but you also need to consider maintainability, constructability, and the balance with AC-side wiring. PVSyst can be used as a tool to numerically evaluate such design decisions.

Method 1: Confirm DC-side wiring losses

When checking wiring losses, the first thing to confirm is the DC-side wiring loss. The DC side refers to the direct-current circuit from the photovoltaic modules to the power conditioner. The cables from the strings to the junction box, combiner box, and the power conditioner input are the main targets. When reading the PVSyst manual, you need to confirm on which screen the DC wiring loss is set and under what assumptions it is calculated or entered.

DC-side wiring losses are closely related to string configuration and cable routing. Even with the same installed capacity, differences in the number of strings, the positions of junction boxes, or the distance to the power conditioner will change cable lengths and current conditions. In particular, for ground-mounted solar power plants, array layouts often cover wide areas, so wiring from remote sections to the power conditioner tends to be long. Even for rooftop installations, if roof surfaces are divided into multiple sections or the installation location of the power conditioner is restricted, DC wiring can become longer than anticipated.

When checking DC wiring losses in PVSyst, you should not simply look at the loss rate alone but confirm which design conditions that loss rate reflects. For example, you need to check whether cable lengths were entered as rough estimates, whether they reflect the actual distances taken from drawings, whether round-trip lengths were considered, and whether the cable cross-sectional area matches the specifications actually planned for use. It is important to read the manual descriptions and avoid misunderstanding the meanings of the input fields.

A common mistake on the DC side is simply underestimating the representative distance from the modules to the power conditioner. In actual construction, the distance is not the straight-line distance but the length along racking and conduit routes, the locations of switchboards, underground burial, or along rack routes. Even if something appears close on the drawings, the construction route may require a detour. If the values entered into PVSyst are too close to the straight-line distance, the wiring losses may be shown smaller than they actually are.

Also, it’s important to note that DC-side wiring losses tend to have a greater effect during periods of high generation. The stronger the solar irradiance and the higher the current, the larger the losses due to cable resistance. Therefore, loss rates that may appear small on an annual average still require attention when evaluating peak output or considering voltage drop. Check how DC losses are handled in the PVSyst manual and, if necessary, cross-check with the electrical design calculations.

A practical approach to assessing DC wiring losses is to run simulations initially with approximate values and then update them to drawing-based figures as the design progresses. During the basic design phase, detailed cable routes are often not yet finalized, so it can be difficult to obtain exact values. However, if approximations are left unchanged after entering the detailed design or estimation phase, the discrepancy between the simulation and the actual design can become large. If you use PVSyst results for final decisions, always verify whether the DC-side wiring conditions have been updated.

Approach 2: Check AC-side wiring losses

The next important aspect when looking at wiring losses is the AC-side wiring loss. The AC side refers to the alternating-current circuits that run from the power conditioner to the AC combiner panel, power receiving and transformation equipment, the point of interconnection, or the load equipment. In the PVSyst manual, DC-side and AC-side losses are sometimes treated differently, so it is important not to confuse which losses you are looking at.

AC-side wiring losses are strongly dependent on the layout of electrical equipment across the whole power plant. Losses vary depending on where the power conditioners are placed, the distance to cubicles and receiving equipment, and how the sections transmitted at low voltage and high voltage are divided. In particular, in large-scale projects the AC wiring from the power conditioners to the collection equipment can become long, and AC-side losses impact power generation assessments.

Even for self-consumption solar power systems, checking the AC side is important. When installing solar panels on the roofs of factories, warehouses, or commercial facilities, the wiring route from the power conditioner to the distribution board and load equipment can become complex. Because on-roof, indoor, outdoor racks, existing conduits, and panel modifications may be involved, it cannot be judged by simple distance alone. When evaluating energy yield in PVSyst, if the AC-side wiring losses are not set appropriately, there may be a discrepancy with the actual usable amount of electricity.

When assessing AC-side wiring losses, clarify how far downstream from the power conditioner output the simulation covers. Whether the reported output in the generation report represents the value at the power conditioner outlet or a value closer to the grid connection point, and which location is designated as the evaluation point within the project, will change how losses are handled. Confirm terminology in the PVSyst manual and make sure to share the evaluation point with the report readers.

A common oversight on the AC side is assuming losses are smaller than on the DC side and proceeding with initial or ad hoc values. Indeed, in some projects AC-side losses can be small. However, depending on the distributed placement of power conditioners, the location of the collection panel, the distance to the substation/transformer equipment, the voltage class, and cable specifications, non-negligible effects may occur. Especially when comparing designs, it is necessary to confirm that the assumptions about AC-side wiring have not changed between proposals.

Another point to note is not to treat AC-side losses as something that can be resolved solely by energy-yield simulations. In AC wiring, not only losses but also voltage drop, allowable current, protection coordination, constructability, and whether switchboards can be expanded should be considered. PVSyst’s loss settings are useful for energy-yield assessment, but they are not a substitute for actual electrical design. While referring to the manual to understand the loss items, detailed electrical design should be verified separately based on calculation reports and design standards.

Perspective 3: Consider the impact on annual power generation, not just the loss rate

When checking wiring losses in PVSyst, the results are often shown as a loss rate or a loss amount. What you should be careful about here is not to judge based only on the percentage display. Rather than simply deciding that a wiring loss below 1% is not a problem or that 2% is too large, you need to consider how it affects annual energy production, electricity sales, self-consumption, and the project's financial performance.

For example, in projects with small system capacity, a 1% loss may appear relatively small in terms of annual energy generation. On the other hand, in mega-solar or large-scale self-consumption projects, a 1% difference can translate into a large difference in annual energy generation. The larger the generation, the more even a slight difference in loss rate will affect revenue and savings. Therefore, when checking how wiring losses are presented in the PVSyst manual, it is important to make a habit of reading both the loss percentage and the annual energy generation.

If you look only at loss rates, you can misprioritize design improvements. Making cables significantly thicker to improve wiring losses by 0.2% may raise material and installation costs. Whether the resulting increase in annual energy production justifies the higher costs must be judged on a case-by-case basis. PVSyst results serve as input for that judgment. The goal is not to minimize wiring losses for their own sake, but to balance energy production, cost, and constructability.

Also, by examining the impact on annual power generation, you can compare wiring losses with other loss items. When compared with shading losses, temperature losses, mismatch losses, inverter losses, and so on, you can confirm what proportion wiring losses account for in the overall total. If wiring losses are clearly larger than in other projects, there may be room to reconsider cable routing and panel layout. Conversely, even if wiring losses are very small, you should question whether the input values are realistic.

In PVSyst reports, by checking the loss diagram and the breakdown of annual energy production, you can see how wiring losses are reflected in the final output. When doing so, make sure that the wiring conditions entered at the simulation-assumption stage match the loss items shown in the report. If values you thought you entered are reflected under different items, or if the intervals you expected are not included, you may misinterpret the report.

In practice, comparing wiring losses across multiple scenarios is also effective. Create a standard case, a conservative case with longer cable runs, and an improved case with revised cable sizes, and check the differences in annual energy production. This lets you understand how much changes in wiring design affect energy production. By referring to the PVSyst manual and comparing only the wiring losses under the same conditions, it becomes easier to explain the rationale for design decisions.

Perspective 4: Re-examine the assumptions about cable length and cross-sectional area

To correctly assess wiring losses, it is essential to verify the assumptions about cable length and cross-sectional area. Wiring losses are greatly influenced by the cable's resistance and the current. In general, resistance increases as the cable gets longer, and resistance decreases as the cross-sectional area increases. When setting wiring losses in PVSyst, understanding this basic relationship also makes it easier to judge the validity of the input values.

When checking cable lengths, it is important to be clear about which section is being considered. A photovoltaic system has multiple wiring sections, such as from the module to the junction box, from the junction box to the power conditioner, from the power conditioner to the combiner box, and from the combiner box to the receiving and transforming equipment. If you proceed without clarifying which section the values entered into PVSyst represent, you may end up double-counting or omitting losses.

You also need to verify that the cross-sectional areas are consistent with the design drawings and the specifications in the estimate. In initial studies provisional cable sizes may be used, but if sizes change during detailed design, the conditions in PVSyst must also be updated. Increasing cable size tends to reduce wiring losses, but it also affects cost and constructability. Conversely, reducing cable sizes to save costs can increase losses and voltage drop.

When reading the PVSyst manual, check whether the input field uses the 'direct specification of loss rate' method or the 'calculation from cable conditions' method. If you enter the loss rate directly, you need to have independent justification for that value. When calculating from cable length and cross-sectional area, take care not to make mistakes in the input units or assumptions. Confusing unit conventions, mixing up one-way and round-trip distances, or misidentifying the number of representative circuits can lead to errors in wiring loss settings.

In practice, the accuracy of cable lengths changes with each design stage. At the initial proposal stage, rough distances are often estimated from a schematic layout. In the basic design stage, panel arrangement and the main routes become apparent. In the detailed design stage, actual conduit routes, rack routes, vertical risers, and routing inside panels can be taken into account. Depending on which stage’s documentation the PVSyst simulation is used for, the required accuracy of the wiring conditions also changes.

When reviewing assumptions about cable length and cross-sectional area, it is also important to align the understanding among the design, construction, and simulation personnel. If the simulation person is entering the straight-line distance from the drawings while the construction person is expecting nearly double the length for the actual route, discrepancies will arise in the final energy yield assessment. By referring to the PVSyst manual and sharing the meaning of the input conditions among stakeholders, such misunderstandings can be more easily prevented.

Perspective 5: Interpreting the relationships among loss items in the report

When checking wiring losses in PVSyst, it is important to read not only the settings screen but also the loss items in the final report. In simulations, many losses are accumulated step by step, and the final effective energy production is calculated. Wiring losses are displayed as part of those, so you need to understand their relationship with the other loss items.

In the report, results are organized starting from the irradiance incident on the solar modules, with losses such as temperature, IAM, mismatch, DC wiring losses, inverter losses, and AC wiring losses applied in sequence. The exact presentation varies depending on the project and settings, but the basic point is to understand which loss is being subtracted at each stage. When consulting the PVSyst manual, be sure to review the input screens together with the report display rather than viewing them separately.

The wiring loss item may appear in different locations and have different meanings on the DC side and the AC side. DC-side wiring loss affects the direct-current power before it enters the inverter. AC-side wiring loss affects the power after it has been converted to alternating current by the inverter. If you do not understand this order, you may end up evaluating wiring losses twice or, conversely, overlooking some of them.

When reading a report, it's important to look not only at the magnitude of loss rates but also at how they balance with other factors. For example, in projects where thermal losses or shading losses are large, finely tuning only the wiring loss may have a limited impact on overall power generation. Conversely, in projects with little shading and well-optimized design conditions, improvements in wiring loss may be relatively more apparent. Rather than evaluating wiring loss in isolation, check its position within the total losses.

Also, when using a PVSyst report as submission material, it may be necessary to explain wiring losses. From the perspective of clients or investment decision-makers, loss items are important information for judging the reliability of the energy production. If the wiring losses are too small, they may ask, "Does this really reflect the actual wiring?" If they are too large, they may ask, "Is there room for design improvement?" To answer such questions, it is necessary to be able to explain how the loss items in the report relate to the design conditions.

The ability to read the relationships in the report also helps when cross-checking simulation results. If the losses shown in the report are unusually small or large compared with the wiring loss conditions you entered, it provides an opportunity to review the input units and the scope of the items. By consulting the PVSyst manual and tracing where each setting is reflected, you can detect simple input errors or misinterpretations early.

Perspective 6: Check the risks of underestimating and overestimating

The final point to keep in mind when assessing wiring losses is the risk of both underestimation and overestimation. Wiring losses should not be understated. If they are set smaller than the actual values, generation will be overestimated and business plans and profitability/cash-flow calculations will become overly optimistic. Conversely, if they are set larger than reality, generation will be underestimated more than necessary, which can put project evaluations at a disadvantage.

The risk of underestimation is especially high during initial feasibility studies. When design details are not yet determined, using standard loss rates or values from past projects as-is can fail to reflect actual cable lengths or equipment layouts. Projects with large sites, complex roof geometries, distant points of interconnection, or constraints from existing equipment can impose more demanding wiring conditions. If generation is presented in PVSyst while wiring losses are underestimated, the projected output may be revised downward during subsequent detailed design.

One must not overlook the risk of overestimation. If wiring losses are made too large just to be on the safe side, the generated energy will be shown as lower than it actually is. In assessing project profitability, even slight differences in generated energy can influence decisions. Especially for self-consumption projects, because they affect the evaluation of electricity savings and the payback period, overly conservative loss settings can reduce the attractiveness of a proposal. Being conservative should be distinguished from being unduly strict without justification.

The proper approach is to clarify the basis for setting wiring losses. At the estimation stage, explicitly state that the values are approximate, and at the detailed design stage update them to values based on drawings and calculation sheets. Even when using values from past projects, confirm that equipment capacity, layout, cable routes, voltage class, and power conditioner configuration are similar. Simply using a value because “we always use this value” can lead to overlooking project-specific conditions.

When reading the PVSyst manual, pay attention to how initial values are handled. Initial values are convenient, but they are not the optimal values for every project. Even when using initial values, it is desirable to be able to explain why that value is acceptable. If you change an initial value, keeping a record of which design conditions were reflected in the change will be useful when reviewing the report later.

Sensitivity analysis is also effective for avoiding underestimation and overestimation. Simulate three wiring-loss scenarios—slightly low, standard, and slightly high—and check how much difference they make to annual energy production and project viability. If the differences are small, you can prioritize other factors; if they are large, you need to refine the wiring design in detail. It is important to use PVSyst not as a one‑off calculation tool but as a comparative tool for design decision‑making.

Points to Note When Using the PVSyst Manual in Practice

When using the PVSyst manual to check wiring losses, you need to separate operational procedures from design decisions. The manual is useful for understanding which settings to enter on which screens. However, the validity of the input values themselves must be judged based on the project conditions and design documentation. Entering values exactly as the manual instructs does not make them correct; what matters is whether the entered values match the actual conditions.

In practical work, the first thing to be aware of is clearly defining the simulation stage. During the initial proposal, basic design, detailed design, and pre-commissioning evaluation stages, the accuracy of the information available differs. In early stages you may use rough estimates, but if so you must assume they will be updated later. In the detailed design stage, it is desirable to align the simulation with electrical drawings, single-line diagrams, layout drawings, cable lists, and other related documents.

Next, it is important to keep a record of the input conditions. Looking only at the PVSyst project files or reports, you may not be able to tell why a particular wiring loss was chosen. Leaving notes on how cable lengths were measured, assumptions about cross-sectional area, the sections in question, the design stage, and the drawing numbers referenced will make it easier to explain later. Even if the person responsible for the simulation changes, handing over the conditions will be smoother.

Also, wiring losses are an item that requires close coordination with electrical designers. If only the person in charge of the energy yield simulation makes the decision, they may overlook actual installation routes and electrical constraints. Electrical designers take a comprehensive view of allowable current, voltage drop, protective devices, switchboard configuration, constructability, and so on. Cross-checking the loss settings in PVSyst with the electrical design calculations results in a more reliable energy yield assessment.

When reading the PVSyst manual, pay attention to the correspondence of terms. Even if the Japanese term "wiring loss" is lumped together, in PVSyst it may be divided into multiple labels or items such as the DC side, the AC side, ohmic losses, cable losses, and so on. Do not judge solely by the on‑screen item names; it is important to check which power stage the loss applies to. If your understanding of the terminology is vague, it can lead to misreading the report.

Finally, it is also important not to scrutinize wiring losses excessively. In energy yield simulations, many factors influence the results, including solar irradiance data, shading, temperature, module characteristics, inverter selection, degradation, and availability. Wiring losses are important, but they are just one component of the whole. Paying more attention to details can sometimes improve accuracy, but if other assumptions remain coarse the overall reliability will not increase. When using PVSyst, it is practical to vary the depth of checks according to their importance while keeping an eye on the balance among all loss items.

Summary

When checking wiring losses in the PVSyst manual, it is important not simply to look for the input fields on the screen but to comprehensively consider the meaning of the losses, the sections to which they apply, their impact on annual energy production, and their consistency with the design conditions. Wiring losses are the fundamental losses that occur in the process by which power generated by photovoltaic modules becomes usable power, and they affect the reliability of energy production simulations.

One approach is to check the wiring losses on the DC side. In the DC circuit from the modules to the power conditioner, string configuration, junction box location, cable length, and conductor cross-sectional area affect the losses. Be aware of the actual wiring route rather than the straight-line distance, and update the conditions according to the design stage.

Second, check the wiring losses on the AC side. For the AC wiring downstream of the power conditioner, the distance to the AC distribution board and substation/transformer equipment, the equipment layout, and the voltage class all matter. In self-consumption systems and large-scale projects, AC-side wiring conditions can affect power generation assessments, so don’t be reassured by looking only at the DC side.

The third point is to look not only at the loss rate but also at the impact on annual power generation. Instead of judging solely by a figure such as what percentage the wiring loss is, you need to check how much difference in annual energy output that represents and to what extent it affects business viability and the benefits of self-consumption. Comparing multiple options makes it easier to understand the effectiveness of improvements in wiring design.

The fourth point is to review the assumptions about cable length and cross-sectional area. The validity of the wiring loss depends heavily on the cable conditions you enter. Verify that the target segment, whether the measurement is one-way or round-trip, whether the length is an estimate on the drawings or the actual route, and the planned cable size all correspond. Misunderstandings about units or the scope of the segment can lead to discrepancies in the simulation results.

The fifth point is to read the connections between the loss items in the report. Wiring losses, together with temperature losses, shading losses, mismatch losses, inverter losses, and other losses, are reflected in the final energy output. By checking not only the input screen but also how they are presented in the final report, you can more easily find configuration errors or misinterpretations.

The sixth point is to check the risks of underestimation and overestimation. If wiring losses are underestimated, the power generation will be overestimated, and if they are overestimated, the project evaluation will be negatively affected. Even when using initial values or values from past projects, it is useful to confirm whether they match the project conditions and, if necessary, perform a sensitivity analysis.

The PVSyst manual is the starting point for learning how to work with wiring losses. However, what truly matters in practice is linking the settings you check in the manual to drawings, electrical design, construction conditions, and report explanations. If you can correctly assess wiring losses, the accuracy of power generation simulations improves, and the persuasiveness of design comparisons and business decisions increases. When using PVSyst, don’t treat wiring losses as a minor supplementary item; carefully interpret them as an important verification point that supports energy yield assessments.