【How to Enter Wiring Losses in PVSyst｜4 Steps for Beginners】

Table of Contents

• Understand what wiring losses are

• Information to check before entering wiring losses in PVSyst

• Step 1: Organize system conditions and wiring sections

• Step 2: Enter DC-side wiring losses

• Step 3: Enter AC-side wiring losses

• Step 4: Check wiring losses on the results screen

• Considerations when deciding input values for wiring losses

• Common wiring loss setting mistakes beginners make

• Design points to reduce wiring losses

• Verify PVSyst simulation results against site conditions

• Summary

Understanding What Wiring Loss Is

Wiring loss is the loss that occurs when electricity flows through cables in a photovoltaic power system. The power generated by the solar modules is sent to the grid via string cables, junction boxes, combiner boxes, power conditioners (inverters), AC cables, and power receiving/transforming equipment. Along this path, a portion of the energy is lost as heat due to the resistance of the cables. This is wiring loss.

Wiring losses vary depending on the size of the power plant, cable length, cable cross-sectional area, voltage, current, wiring route, and equipment layout. For example, even with the same installed capacity, losses tend to be larger when the distance from the modules to the power conditioner is long; conversely, placing equipment closer together makes it easier to reduce losses. In addition, configurations that carry large currents at low voltages tend to result in higher wiring losses.

In PVSyst, wiring losses are entered as part of the simulation conditions. The entered wiring losses are reflected in the annual energy production and in the loss diagram. Therefore, if the wiring loss value is set too low compared to the actual value, the energy production may be overestimated. Conversely, if a value larger than the actual is entered, the design proposal may appear unduly unfavorable.

What beginners should first grasp is that wiring losses are not "a loss at a single point" but losses that occur along the electrical paths throughout the entire power plant. In PVSyst, the basic approach is to treat the DC side and the AC side separately. The DC side covers the range from the photovoltaic modules to the input side of the power conditioner. The AC side covers the range from the output side of the power conditioner toward the substation/transformer equipment and the grid connection point.

Wiring losses should be entered as an approximate value in the early stages of design, and then reviewed based on cable length and conductor type as the design advances. Trying to enter perfect values from the outset tends to stall the work, but leaving provisional values until the end lowers the reliability of the simulation. When using PVSyst, it is important to consciously update the wiring losses at each stage—initial study, basic design, detailed design, and pre-submission check.

Information to confirm before entering wiring losses in PVSyst

Before entering wiring losses in PVSyst, first organize the necessary information. If you just open the input screen and cannot determine which values to enter, you will end up setting them based on intuition. Because wiring losses directly affect the simulation results, it is important to prepare values supported by evidence as much as possible.

First, what I want to confirm is the basic configuration of the power generation equipment. Identify the module capacity, the number of strings, the number of power conditioners, the presence or absence of junction boxes and combiner panels, and the distance to the substation/transformer equipment. In particular, clarifying which parts are treated as the DC side and which as the AC side makes it easier to avoid duplicate input values.

Next to check is the cable length. Wiring losses are greatly affected by cable length. Confirm the approximate distance for each segment, such as the distance from the module to the junction box, the distance from the junction box to the power conditioner, and the distance from the power conditioner to the power receiving and transformation equipment. In the early stages you may not know the exact actual lengths, but even then estimating distances from placement or layout drawings makes it easier to justify the validity of the input values.

Cable type and cross-sectional area are also important. In general, cables with a larger cross-sectional area have lower resistance, so for the same current the losses are smaller. However, it is not simply a matter of increasing the cross-sectional area; ease of installation, cost, voltage drop, allowable current, protection coordination and other factors must be considered together. PVSyst simulations do not make the final electrical design decision themselves, but to see how wiring conditions affect power generation it is necessary to make the cable conditions as close to reality as possible.

Also, it is important to vary the input granularity depending on the design stage. At the site selection and preliminary study stages, it is common to provisionally assume a general wiring loss rate. Conversely, at the stage of preparing generation forecasts for submission or conducting design comparisons, it is necessary to verify DC-side and AC-side losses separately based on the actual wiring plan. In practice, simulations are sometimes reused with the initial values left unchanged, so be clear about which design-stage conditions the inputs are based on.

Wiring losses entered into PVSyst are not merely an operational input but a parameter that reflects the on‑site layout and electrical design. Therefore, rather than having the simulation engineer decide them alone, determining the values while coordinating information with the design engineer, the construction team, and the electrical engineer enhances the explanatory power of the results.

Step 1: Organize the system conditions and wiring sections

The first step in entering wiring losses in PVSyst is to organize the system conditions and the wiring sections. Rather than immediately entering numbers on the input screen, first briefly break down the flow of electricity through the generation equipment. This will make it easier to see whether each loss should be assigned to the DC side or the AC side.

In photovoltaic power generation systems, PV modules generate direct current power, which is collected at the string level and input to the power conditioner. After being converted to alternating current by the power conditioner, it is sent via AC cables to the substation/transformer facilities and the point of interconnection. When considering wiring losses in PVSyst, this flow is divided into the DC side and the AC side.

The range included on the DC side generally extends from the modules to the input side of the power conditioner. This covers string cables, extension cables, wiring up to the junction box, and wiring from the junction box to the power conditioner. Depending on the system configuration, a junction box may not be used, but even in that case there is DC wiring from the modules to the power conditioner, so DC-side losses should be considered.

The scope included on the AC side ranges from the power conditioner output to the substation equipment and onward to the point of interconnection. If power conditioners are distributed or the distance to the substation equipment is long, wiring losses on the AC side may not be negligible. In large-scale power plants, AC-side losses can vary depending on equipment layout and collector methods, so care is needed when comparing design proposals.

The important point here is not to enter the same loss twice. For example, if an external electrical calculation has already determined a wiring loss rate by summing the DC side and the AC side, entering that value redundantly in multiple locations in PVSyst will result in losses larger than actual. Conversely, if you enter only the DC side and forget to include the AC side, you will obtain a more favorable energy yield than in reality.

In practice, it is useful to create a simple memo to organize wiring losses. For example, write out the ranges such as the DC side from the module to the power conditioner, and the AC side from the power conditioner to the substation/transformer equipment. Then, for each section, organize the cable length, cable size, voltage, current, and the expected loss rate. Rather than relying solely on PVSyst’s input screens, keeping a separate record of the design conditions that form the basis will make reviews later less confusing.

Beginners tend to think of wiring losses as a single percentage, but in practice it is important to clarify which section the losses refer to. When explaining PVSyst simulation results to colleagues or clients, it is more credible to say, "I separated the DC-side and AC-side losses and entered them based on the current layout," than simply saying, "I entered the wiring loss."

Step 2: Enter the wiring losses on the DC side

The next step is to enter the DC-side wiring losses in PVSyst. DC-side wiring losses are the losses that occur from the DC power generated by the solar modules until it reaches the power conditioner. In PVSyst, you enter the loss conditions for the DC wiring from the system settings or the detailed losses configuration screen.

What you should first consider for the DC-side input is its relationship with the string configuration. The number of strings, the number of modules in series per string, and the input conditions of the power conditioner change the DC-side voltage and current. Wiring losses are strongly affected by current, so for the same cable length, configurations with higher current have greater losses. After entering the array configuration and string design into PVSyst, it is natural to then check the DC-side wiring losses.

There are two ways to input DC-side wiring losses: entering them as a loss rate, or entering them based on cable conditions. The easiest method for beginners to start with is to input the DC-side loss rate as a percentage. For example, if the DC-side wiring loss rate is specified in design documents or internal standards, enter that value. However, rather than simply using a generic value, you should verify whether it is reasonable for the size of the installation and the wiring distance.

For a more detailed assessment, losses are considered based on cable length and cross-sectional area. If the DC wiring distance is short and the cable size is appropriate, losses will be relatively small. On the other hand, in large sites where the distance between the module arrays and the power conditioner is long, DC-side losses tend to increase. In particular, care should be taken with layouts that centralize the power conditioner in one location, as DC wiring can become long.

When entering data into PVSyst, understand what percentage loss is being applied relative to the reference output. Wiring losses are not a simple fixed value but vary according to generation and current conditions. Since their treatment in the simulation differs depending on the input method, set values while checking the meaning of each input field. In particular, avoid confusing whether the loss is relative to standard operating conditions or a loss calculated within the annual simulation.

After entering the DC-side wiring loss, verify its relationship with the other loss items. It should be treated as a separate item from mismatch loss, soiling loss, temperature loss, and power conditioner loss. Including other losses in the wiring loss makes the breakdown of losses unclear. To ensure that the loss diagram shows how much is lost due to each factor, it is important to organize and enter the wiring loss specifically as a wiring-related loss.

When beginners enter DC-side wiring losses, rather than aiming for overly detailed settings from the start, it is better to input them at a reasonable level of granularity appropriate to the design stage and make them updatable later. By increasing accuracy in stages—using rough estimates for initial studies, values closer to the actual wiring plan for detailed design, and values based on the final design before submission—you make PVSyst easier to use in practice.

Step 3: Enter the AC-side wiring losses

The third step is to enter the AC-side wiring losses. AC-side wiring losses are the losses that occur as the power converted to AC by the power conditioner travels to the substation/transformer equipment and the grid connection point. They can be overlooked compared with the DC side, but they are an important factor affecting power generation when the system is large or when the distance from the power conditioner to the substation/transformer equipment is long.

When considering AC-side losses, first confirm the arrangement of the power conditioners. If the power conditioners are distributed within the generation area versus concentrated in a single location, the AC wiring distances and power collection methods differ. Also, whether power is collected at low voltage or stepped up and transmitted on the high-voltage side changes how wiring losses are evaluated.

In PVSyst, AC-side losses are also set as items in the detailed losses. The input value is treated as representing the cable losses after the power conditioner output. Because its scope differs from DC-side wiring losses, take care not to duplicate losses entered on the DC side. If an external calculation summarizes the overall wiring losses, check whether the AC side is included in that calculation before entering it.

AC-side wiring losses are strongly influenced by the plant layout. For example, if the substation/transformer equipment is located at the edge of the site, the AC wiring distance to it becomes longer and losses may increase. Conversely, if electrical equipment can be placed closer to the center of the generation area, the wiring distance can sometimes be shortened. When comparing design proposals in PVSyst, it is important to consider not only module layout and tilt angle but also the differences in wiring losses caused by the placement of electrical equipment.

One point to note when entering values on the AC side is to avoid confusing the power conditioner’s conversion losses with wiring losses. The DC-to-AC conversion loss caused by the power conditioner is a separate factor from wiring loss. AC-side wiring loss refers to the losses that occur as the current passes through cables after conversion. Mixing these when inputting data will make the loss breakdown inaccurate and make it difficult to explain the simulation results.

Also, AC-side losses vary depending on how far downstream of the receiving and transforming substation you include in the simulation scope. Whether the assessment is made at the generator terminal, at the point of interconnection, or at the power sales meter location can change the wiring extent that needs to be considered. In practice, since the evaluation point may differ depending on the submission recipient or internal company standards, it is necessary to clarify which losses are included in the PVSyst results.

After entering the AC-side wiring losses, confirm that the overall wiring losses, combined with the DC side, fall within a reasonable range. By checking not only the DC and AC sides individually but also how the combined total affects power generation, you can more easily detect excessive entries or missing inputs.

Step 4: Check wiring loss on the results screen

The fourth step is to check the wiring losses on the results screen after running the simulation. In PVSyst, the conditions you entered are reflected in the simulation results and displayed as individual loss items in the loss diagram and the report. Do not be satisfied with merely entering the wiring losses; always verify how much they actually affect the results.

The first thing to check is the loss diagram. The loss diagram allows you to visually confirm at which stages and in what way losses occur in the flow from solar radiation to the final output. Wiring losses are shown as losses on the DC side or the AC side. Here, confirm whether the wiring losses are larger than expected or, conversely, smaller than expected.

If wiring losses are shown as large, the input values may be excessive. It is also possible that the design actually has long wiring distances. It is important to distinguish whether this is an input error or a design-related issue. For example, mistakes to consider include accidentally treating the cable length as round-trip rather than one-way (double-counting), entering the same loss for both the DC and AC sides, or re-entering losses that were already included in external calculations.

Conversely, caution is also required when wiring loss is extremely small. There may be cases where the default value has been left unchanged, the AC-side input has been forgotten, or the actual cable distance has not been reflected. In particular, if wiring loss is too small for power plants on large sites or projects where the distance to the receiving and transformer facilities is long, the input conditions should be reviewed.

Next, what we want to check is the impact on annual energy production. Wiring losses are a factor that directly reduces energy output. Even a small change in the loss rate can cause a large difference in annual production when the installed capacity is large. When comparing design proposals, you cannot make a correct comparison if the input conditions for wiring losses are not consistent across proposals. For example, if one proposal inputs detailed wiring losses while another leaves them at their default values, you will not be able to tell whether the difference in generation is due to the quality of the design or differences in input conditions.

When reviewing a report, check what proportion of the losses is attributable to wiring losses compared with other losses. They may appear small compared with temperature losses or shading losses, but wiring losses can be adjusted through design improvements. If you want to improve energy output even slightly, revisiting wiring routes and equipment layout can be effective.

When submitting simulation results, be prepared to explain the basis for the wiring loss inputs. Rather than presenting only the PVSyst results, clarify which wiring range was targeted, how the DC side and the AC side were treated, and whether the input values are estimates or based on detailed design—organizing this information will make it easier to respond to questions.

Considerations when determining input values for wiring loss

When deciding the input value for wiring loss, beginners' biggest concern is "what percentage should I use?".

To conclude, there is no single correct answer that applies to every project. Wiring loss varies depending on equipment capacity, voltage, current, cable length, cable size, equipment layout, and installation conditions, so it must be assessed on a case-by-case basis.

However, in practice there is a stage-based approach. At the preliminary assessment stage, detailed wiring routes and cable sizes are often not yet determined, so provisional settings are made using internal standards or commonly assumed values. At this stage, it is important to make clear that these are only rough estimates. When the wiring design is finalized at a later stage, treat the inputs on the assumption that they will be updated.

During the basic design stage, the placement of power conditioners, the locations of junction boxes and collector panels, and the positions of power receiving and transformer equipment become largely determined. At this stage, losses are re-evaluated separately for the DC and AC sides while estimating cable distances. Because wiring losses change depending on equipment layout, when comparing multiple options it is desirable to reflect the wiring conditions for each option.

During the detailed design phase, cable sizes and wiring routes become concrete. At this stage, it is ideal to reflect loss values derived from electrical calculations in PVSyst. PVSyst simulations are intended for energy yield prediction, but if the input conditions deviate from the detailed design, their ability to explain the actual installation is reduced. Before submission and during final checks, verify the consistency between PVSyst input values and the design documentation.

When deciding on input values, be careful not to enter overly optimistic figures. If you input a small wiring loss, the estimated power generation will look higher, but if the prediction becomes excessively large compared to reality, it will be difficult to explain later. Conversely, if you are overly safety-conscious and enter values that are too large, the design proposal may appear unnecessarily unfavorable. The important thing is to set conservative values that nevertheless do not deviate significantly from actual conditions.

It is also important to recognize wiring losses not as a fixed value but as an item that changes with design modifications. Changing the module layout, the location of the power conditioner, the cable route, or the voltage configuration will alter wiring losses. When using PVSyst, rather than entering wiring losses once and leaving them, it is practical to update them in line with comparisons and revisions of design proposals.

Common Wiring Loss Configuration Mistakes Beginners Often Make

When entering wiring losses in PVSyst, there are several points that beginners tend to get wrong. Because wiring losses can look like a minor item, mistakes may go unnoticed as the simulation proceeds. However, since they affect power generation forecasts and design comparisons, it is essential to verify the inputs afterward.

One common mistake is confusing the DC side with the AC side. If you include the losses from the module to the power conditioner on the AC side, or include the losses after the power conditioner output on the DC side, the breakdown of losses becomes unclear. As a result, even when looking at a loss diagram, it becomes difficult to determine which section has issues.

The next most common issue is duplicate input. In some cases, overall wiring losses are calculated by an external electrical calculation and entered into the DC side of PVSyst, and then a similar value is also entered on the AC side. In that case, losses are calculated as larger than they actually are, and the annual energy production is displayed as lower. Verify the basis for the input value and be sure to understand the range that value covers.

Conversely, input omissions are also common. There are cases where a simulation is run with the default settings and the results are used with almost no consideration given to wiring losses. For small-scale rough assessments this may not be a major problem, but if input omissions occur in submitted power generation forecasts or feasibility evaluations, they will affect the reliability of the results.

Also, care must be taken when reusing the same wiring loss rate across projects. Even for installations with similar equipment capacity, wiring losses change depending on site shape, equipment layout, and wiring distances. Referring to values from past projects is useful, but you should not copy them verbatim; you need to confirm they are appropriate for the current project.

Errors in handling cable lengths can occur. If you enter only the straight-line distances shown on drawings, you may estimate them shorter than the actual wiring route. Cables are laid according to terrain, rack/pedestal placement, access routes, equipment locations, and construction conditions, so they may differ from a simple straight-line distance. While this may be unavoidable at the rough estimation stage, in the detailed design stage cable lengths are verified to approximate the actual route.

Furthermore, wiring losses and voltage drop are sometimes confused. Voltage drop is an electrical-design check that focuses on the decrease in voltage, while wiring loss focuses on the energy lost as power. They are related but not the same. It is important to understand what the values you enter into PVSyst mean.

Design Considerations to Reduce Wiring Loss

When checking wiring losses in PVSyst, if you find the losses are large there may be opportunities for improvement in the design. Wiring losses are not losses caused by natural conditions; they are items that can be reduced to some extent through equipment layout and electrical design measures. If you want to improve power generation, reviewing wiring losses is an effective point to consider.

First, consider shortening the wiring distance. Wiring losses increase as cable length grows. By reassessing the placement of power conditioners, junction boxes, combiner boxes, and power receiving/transforming equipment, you may be able to reduce wiring distances. Especially on large sites, where electrical equipment is located can change losses on the DC and AC sides.

Next, select the appropriate cable size. If the cable cross-sectional area is too small, resistance increases, resulting in greater losses and voltage drop. Conversely, choosing a cable larger than necessary can be disadvantageous in terms of ease of installation and cost. You should not decide cable size based solely on PVSyst results, but they can be used as a reference to check the impact on power generation.

String configuration and voltage design also play a role. In general, configurations that use higher voltage and keep current lower make it easier to reduce wiring losses. However, because equipment input ranges, safety standards, and design conditions must be met, simply raising the voltage is not necessarily the right solution. In PVSyst, it is important to assess wiring losses together with string design and equipment conditions.

The balance between distributing and centralizing equipment placement is also important. Distributing power conditioners can, in some cases, shorten DC wiring, but it affects AC-side collection methods, maintainability, and ease of construction. Conversely, centralizing power conditioners can make management easier, but may result in longer DC wiring. Which approach is more advantageous depends on project conditions, so it is effective to compare multiple options using PVSyst.

Local terrain and construction conditions also affect wiring loss. A route that looks short on drawings may in practice need to avoid level differences, drainage channels, roads, existing structures, slopes, and the like, which can lengthen the cable route. These factors are difficult to see in desk-based simulations, but if site conditions are not reflected, the entered wiring loss may deviate from the actual situation.

To reduce wiring losses, it is necessary to balance not only energy generation but also constructability, maintainability, safety, and overall cost. PVSyst is an effective tool for checking how differences in wiring losses affect annual energy production. Because it enables numerical comparison of the effects of design changes, it is also easy to use for explaining them to stakeholders.

Verify PVSyst simulation results against on-site conditions

Even if you enter wiring losses into PVSyst and calculate energy yield, practical accuracy will not improve if the results do not match site conditions. Wiring losses vary not only according to values on drawings and calculations but also according to the actual layout, topography, construction routes, and equipment locations, so verification against site conditions is important.

The first thing to confirm is whether the layout assumed in PVSyst matches the actual on-site layout. If module placement, walkways, sloped terrain, obstacles, or equipment layout change, the wiring routes will also change. After entering the wiring losses based on the early-stage design layout, if the arrangement changes during site verification or detailed design, the wiring losses should also be reviewed.

Next, the accuracy of the on-site survey is also important. If the power plant site shape, elevation differences, and equipment placement positions are not accurately understood, estimates of wiring distances will be off. In particular, on sloped or graded/developed land, it is necessary to consider wiring routes that follow the actual terrain, not just distances on a plan view. Wiring losses themselves are electrical parameters, but their calculation depends on accurate on-site information.

Also, during the construction phase, changes may be made to the routes assumed at the design stage. Depending on site conditions, cable racks, buried routes, and equipment locations may be adjusted. Such changes do not necessarily have a major impact on power generation forecasts, but when preparing submission documents or conducting post-completion verification, it is important to understand the differences between the final construction conditions and the simulation conditions.

PVSyst's results are a powerful resource for predicting energy output, but they depend on the accuracy of the input conditions. Wiring losses are part of that, and the more they reflect actual site conditions, the more explanatory power the results have. Conversely, if site conditions remain ambiguous and only the input values are made more detailed, it will only increase apparent precision and may not match reality.

In designing solar power plants, it is important not to treat simulations and on-site checks as separate, but to have them complement each other. By checking energy yield and losses in PVSyst and conducting positioning, terrain verification, equipment-location confirmation, and construction-route checks on site, you can minimize the gap between desk studies and actual site conditions.

Summary

The method for entering wiring losses in PVSyst is not simply a matter of filling in numeric fields on the screen. Wiring losses are an important loss term that represents the power lost as the electricity generated by the solar modules is transmitted to the power conditioners and the substation/transformer equipment. If the input values are not appropriate, they will affect the annual energy production, the loss diagram, and the results of design comparisons.

The process beginners should grasp is to first understand the meaning of wiring losses, then clarify the scope of the DC side and the AC side, enter each loss into PVSyst, and finally check how they are reflected on the results screen. In particular, be careful not to confuse the DC and AC sides, make double entries, omit inputs, or reuse values from past projects.

It is important to review the input values for wiring losses according to the design stage. Use approximate values in preliminary studies, reflect equipment layout and cable distances in the basic design, and update them based on the actual wiring plan in the detailed design. When using PVSyst in practice, you should not enter the values once and be done; instead, you should update the simulation conditions to match design changes.

Wiring losses are also closely related to on-site conditions. If the power plant’s site shape, slope, equipment layout, or wiring routes change, the assumptions about losses will change as well. To perform accurate power generation forecasts in PVSyst, it is important to ascertain the site’s location and topographical information as accurately as possible.

Therefore, in the design and pre-construction checks of solar power plants, on-site high-precision position verification is essential in addition to simulations. LRTK, as a GNSS high-precision positioning device that can be attached to an iPhone, can streamline the verification of equipment locations, measurement points, topography, and planned construction positions on site. When examining wiring losses and power generation with PVSyst, if layouts and wiring routes can be confirmed based on accurate on-site position information, it becomes easier to reduce discrepancies between desk-based simulations and actual site conditions. By combining PVSyst’s power generation forecasts with LRTK’s on-site positioning, it becomes easier to make decisions that are more practical and field-oriented throughout the process from solar power equipment design studies to construction verification.