5 perspectives to compare with adjacent facilities to identify causes of low power generation

When you feel the power output of a solar power generation system is low, the first thing to check is not just the figures from your own equipment. By comparing with adjacent systems, other sections within the same site, or generation facilities operating under similar conditions, it becomes easier to determine whether the decline is a temporary effect of the weather, an equipment malfunction, or a result of management or measurement perspectives.

However, comparing neighboring installations is not simply a matter of looking at which one generates more electricity. If equipment capacity, azimuth, tilt, shading, commissioning date, measurement units, output control, shutdown history, and so on differ, the observed generation will also differ. Therefore, if you make judgments without aligning the premises of the comparison, you risk mistaking differences that are not actually problems for faults, or conversely, dismissing anomalies that should not be overlooked as "just the weather."

This article explains five perspectives for practitioners investigating "low power generation" when comparing low generation with adjacent installations to determine the cause. It organizes a way of thinking to narrow down causes without undue effort by combining on-site inspections, monitoring data, daily reports, electricity sales statements, and inspection records.

Table of Contents

• Initial prerequisites to verify when comparing with adjacent equipment

• Perspective 1: Compare by aligning the units of installed capacity and power generation

• Viewpoint 2: Compare solar radiation conditions and shadow patterns at the same time of day

• Perspective 3: Compare differences in orientation, tilt, and installation environment

• Viewpoint 4: Compare stoppage history, output control, and communication anomalies

• Perspective 5: Compare how dirt, deterioration, and partial malfunctions appear

• Approach to determining inspection priorities from comparison results

• Judgment mistakes to watch for when comparing adjacent equipment

• Summary: Increase the accuracy of comparisons to more quickly narrow down the causes of declines in power generation

Prerequisites to Check First When Comparing with Adjacent Equipment

When comparing the cause of low power output with adjacent facilities, it is important first to clarify exactly what is being compared. For example, even solar power installations located side by side on the same site may not have identical equipment capacity, number of panels, number of power conditioners, installation angles, shading from nearby trees or buildings, point of supply, or monitoring system settings. Even if installations look similar, you cannot determine the cause from a simple difference in generated power unless the comparison conditions are matched.

In practice, it is easier to organize comparisons by first dividing them into categories such as "different sections within the same site", "separate circuits within the same system", "adjacent separate installations", and "nearby similar installations". If it is a different section within the same site, weather and solar radiation conditions may be similar, but shading and terrain effects can differ by section. If it is a separate circuit within the same system, equipment configuration may be similar, but there may be differences in string-level faults or the condition around junction boxes. For nearby similar installations, even if weather trends are alike due to proximity, installation conditions and operating rules may differ, so extra caution is required when making comparisons.

Next, align the periods you are comparing. What can be read changes depending on whether you look at daily, monthly, or yearly units. Daily comparisons are suitable for detecting sudden stoppages, temporary shading, communication anomalies, and the presence or absence of output control. Monthly comparisons are suitable for detecting soiling, seasonal shading, and continuous performance degradation. Yearly comparisons help grasp long-term changes and differences in maintenance conditions. However, if you look only at yearly data, short-term stoppages or anomalies on specific days can be averaged out and become hard to see.

Also, confirm the types of data you are comparing. The generation shown on the monitoring screen, the sold electricity amount on the sales statement, the output for each power conditioner, current values per string, solar irradiance, ambient temperature, and inspection records each cover different scopes. Generated energy and sold energy are not necessarily the same thing. For systems with self-consumption, you need to distinguish whether generation is low or sold energy is low. If there are missing measurements in the monitoring device’s readings, the apparent drop may not be an actual generation decrease but could be due to communication or recording issues.

Comparison with adjacent facilities is not intended to instantly determine the cause, but to provide clues for prioritizing inspections. The more carefully you align the differences with the comparison targets, the easier it becomes to classify the causes of low power generation into weather, installation conditions, operation, equipment abnormalities, and measurement deficiencies.

Perspective 1: Align the units of installed capacity and power generation for comparison

A common mistake when comparing with adjacent installations is to compare only the generation figures without considering differences in installed capacity. Even if generation appears low, a smaller installed capacity will naturally result in a smaller absolute generation. Conversely, if installed capacity is large yet generation is only on par with neighboring installations, there may be reason to suspect some factor causing reduced performance.

In practice, it is easier to compare by looking at generation per unit of capacity rather than absolute generation. For example, by dividing the generated energy over a given period by the installed capacity, you can make a comparison that evens out differences in system size. This allows you to understand the situation in a way that more closely reflects differences in generation efficiency than by directly comparing large and small systems. However, this value is also affected by installation conditions and solar irradiance conditions, so it should not be used alone to determine the cause.

When making comparisons, attention must be paid to how equipment capacity is defined. The impression given by the comparison can change depending on whether capacity is considered as the total capacity of the solar modules or the rated output of the power conditioner. In somewhat oversized systems, output can become capped during periods of strong solar irradiance. Conversely, during periods or seasons of weak irradiance, larger module capacity can be advantageous for energy production. When comparing adjacent installations, it is important to use the same basis for capacity.

Also, when using electricity sales statements, check the difference between sold electricity and generated electricity. If the system is operated close to full-feed-in (i.e., nearly all output is sold), the sold electricity will tend to reflect generation patterns more accurately; however, if there is self-consumption, generation may not be apparent in the sold electricity even when power is being produced. If an adjacent installation sells all of its output while your installation includes self-consumption, comparing only sold electricity can make your installation’s generation appear lower.

Comparisons by time of day are also effective. Even if the difference looks small when you only consider the total daily generation, there may be hidden patterns such as being low only in the morning, only in the afternoon, or flattening out around midday. If you view the data with the sense of overlaying the same day's output curve from adjacent installations, you may find declines that are difficult to explain by capacity differences alone. Whether it is low from the morning, suddenly drops around noon, or the gap widens only in the evening, the causes you should suspect will vary.

The important thing when comparing with consistent units is to verify the impression of "low" with numbers. On-site, you may feel from the apparent scale and past experience that "this equipment should produce more." However, that feeling alone is weak as a basis for inspections or reporting. If you align equipment capacity, the target period, units of generation, and comparison targets and then check the differences, it becomes easier to narrow down the causes to investigate next.

Perspective 2: Compare solar conditions and shadow patterns at the same times of day

When comparing the causes of low power output with neighboring facilities, differences in sunlight conditions and shading are important. Because solar power generation is strongly influenced by solar irradiance, even facilities located in the same area as adjacent installations can show differences in energy production due to cloud movement, topography, buildings, trees, utility poles, fences, slopes, and shadows from neighboring structures. This is especially true for large-area installations, where within the same plant sunlight conditions can differ between the east and west sides, between lower and higher elevations, and between valley-side and ridge-side areas.

When making comparisons, it is important not only to look at daily totals but also to examine changes in output during the same time periods. For example, if your system's generation is lower than neighboring systems only in the morning, shadows from the east, morning fog, mountain shadows, building shadows, or differences in orientation may be involved. If the gap widens only in the afternoon, check for shadows on the west side, the low angle of the evening sun, and the influence of nearby structures. If the difference persists around midday, suspect not only shading but also equipment faults, soiling, output curtailment, or differences in temperature conditions.

When comparing shadows, also take into account seasonal differences in the sun's elevation. Shadows that were hard to see in summer can lengthen from autumn into winter and affect specific panel rows or strings. If adjacent installations are not shaded but your installation is shaded at certain times, annual energy production can differ. When investigating causes of low generation, it’s easier to judge if you look not only at the most recent data but also at the same month in the previous year and at seasonal output curves.

When comparing solar irradiance conditions, attention must be paid to the presence and placement of pyranometers. If a pyranometer is installed at a representative point of your facility, its reading does not necessarily fully reflect the conditions of an adjacent facility. Conversely, even if the monitoring data for an adjacent facility includes irradiance, if that pyranometer is located where it is less likely to receive shadows, its readings may differ from the actual irradiance on the panel surface. Even when irradiance values are available, it is important to view them in combination with site photographs and checks of shadows by time of day.

Partial shading can also be difficult to detect in energy production. Even if only part of a panel is shaded, the connection configuration can cause the output of a particular circuit to be affected. If the output curve shows small steps or unnatural dips compared with adjacent equipment, it is worth checking how shading is occurring. Choosing and comparing data from clear-sky days makes it easier to find shading or equipment differences because there is less variability from clouds.

What you should avoid when comparing solar irradiation and shading is assuming "it's next door, so it has the same insolation conditions." Even if an adjacent installation is close, differences in ground elevation, surrounding obstructions, racking height, or panel orientation can cause variations in power generation. Conversely, if insolation conditions are almost the same but only your system consistently shows lower output, you should prioritize checking causes other than insolation.

Perspective 3: Compare differences in orientation, tilt, and installation environment

When comparing the power output of adjacent installations, differences in orientation and tilt should always be checked. Solar power systems’ generation patterns by time of day and by season change depending on which direction the panels face and at what angle they are installed. Even when system capacities are similar and the weather is alike, if orientation or tilt differ, the difference in output may be a natural result of the installation conditions.

For example, east-facing systems tend to generate more electricity in the morning, while west-facing systems tend to generate more in the afternoon. Systems that face closer to south tend to have more stable daytime generation, but site conditions may prevent installation in the optimal orientation. When comparing a neighboring system with your own, look not only at the daily total but also at the morning and afternoon generation balance to determine whether the difference is due to orientation or to an anomaly.

The tilt angle also affects seasonal power generation. Installations with a shallow tilt can be advantageous for summer solar irradiance in some seasons and regions, but in winter they can be affected by the sun’s low altitude and by how soiling persists. Installations with a steep tilt may generate more easily during periods of low solar altitude, but the installation environment, wind effects, and racking/mounting conditions should also be taken into account. When comparing adjacent installations, if the way differences appear changes by season, it can sometimes be explained by checking differences in azimuth and tilt.

Differences in installation environments must not be overlooked. Factors that affect power generation—ground reflectance, surrounding grass height, soil dust, snowfall, sea breeze, fallen leaves, bird damage, and dust from farmland or roads—vary for each installation. If a neighboring installation is farther from the road and the installation itself is in a position more prone to receiving dust, differences in dirt accumulation can occur. At installations with a lot of surrounding vegetation, the effects of shade and fallen leaves can become significant depending on the season.

Additionally, differences in mounting height or layout can change ventilation and temperature conditions. Because photovoltaic modules tend to lose output at high temperatures, installations that dissipate heat easily and those prone to heat buildup can produce different amounts of power even under the same solar irradiance conditions. If the gap compared with adjacent installations widens during hot sunny periods and narrows in cooler seasons, the temperature environment and ventilation conditions should also be checked.

Comparing orientation, tilt, and the installation environment provides the basis for distinguishing whether a drop in power generation should be considered an "anomaly" or a "difference in conditions." If you clarify the range that can be explained by condition differences, it becomes easier to align understanding among inspection personnel, managers, and owners. Conversely, when a sudden drop that is difficult to explain by installation conditions, or a decline limited to a specific circuit, is observed, it is necessary to proceed to check the equipment for faults.

Perspective 4: Comparing stop history, output control, and communication abnormalities

When comparing low power generation with adjacent facilities, you need to check not only the facility's generation capacity itself but also its shutdown history, any output control, and whether there were communication abnormalities. Some causes that make generation appear low are not a decline in the performance of the panels or power conditioners, but cases where the facility was stopped for a certain period, output was curtailed, or monitoring data was missing.

First, what you should check is the shutdown history of the power conditioner, junction box, and monitoring equipment. If, compared with adjacent facilities, only your installation shows periods during daytime when output suddenly falls to zero, verify equipment shutdowns, protective trips, maintenance activities, or grid-side impacts. Even a short interruption can have a large effect on daily energy output if it occurs during periods of strong solar irradiance. When looking at monthly generation, the figures may merely appear low, but by tracing the time-of-day history you may find that a shutdown on a specific day was the primary cause.

Output curtailment is also important. When output curtailment occurs, generation output and electricity sales volume may be reduced even if the equipment is not faulty. When comparing with adjacent facilities, check whether a similar capping of output appears at the same times. If adjacent facilities are similarly curtailed, the cause may be regional or grid conditions. If only your facility shows low output while adjacent facilities are generating normally, you need to check the individual facility’s settings, communications, equipment condition, and the handling of control signals.

Communication failures or measurement errors can also appear as a drop in power generation. If the monitoring screen shows low generation but the power sales statement or the on-site meter does not show a significant decrease, the problem may be with data acquisition rather than generation itself. Temporary stoppage of communication equipment, malfunction of measuring devices, delays in data transmission, configuration changes, or time offset can cause the figures on the monitoring screen to differ from reality. When comparing with adjacent facilities, also check whether the data were obtained using the same monitoring method.

Inspection work, weeding, and construction work that cause temporary shutdowns should also be checked. If neighboring facilities were operating normally on a given day while your facility alone was stopped for maintenance, the difference in power generation is not an anomaly but due to an operational shutdown. By cross-checking daily reports and work logs with power generation data, you can identify the cause more quickly. Especially at sites where multiple staff are involved, if records of shutdown operations and recovery confirmations are insufficient, it becomes difficult to trace the cause of a later drop in power generation.

When comparing shutdown histories and output control, the important thing is not to immediately assume that low generation is due to "insufficient capacity." You need to distinguish whether the equipment was unable to generate, generated but could not sell the power, or was generating but the output was not recorded. Making this distinction clarifies which documents should be checked before an on-site inspection and makes it easier to reduce unnecessary work.

Please translate the following input into English.

Perspective 5: Compare how dirt, deterioration, and localized defects manifest

If your installation’s power generation is consistently lower than neighboring installations, check for soiling, degradation, or partial faults. In particular, if the difference persists for a long time and cannot be explained by weather, capacity, orientation, or downtime history, some part of the system may be hindering generation. What’s important here is not to view the entire installation as a single number, but to compare in as much detail as possible—by sections, inverters, circuits, strings, and so on.

Soiling is one factor that easily causes differences between adjacent installations. If there are roads, fields, cleared land, factories, trees, or places where birds tend to gather nearby, certain installations may accumulate more soiling. On installations with a shallow tilt, rain may not wash the soiling away easily, leaving dirt on the bottom edge of the panels. If a system is not cleaned at the same time as neighboring installations, or is located where it is likely to receive dust after grass cutting, this may show up as a difference in power generation.

Regarding deterioration, it can appear not as a sudden drop but as differences that gradually widen over years. However, aging-related deterioration does not necessarily occur uniformly across all equipment; variations arise depending on the installation environment and the condition of each unit. If adjacent equipment began operating at different times, comparing them without considering the difference in years can lead to incorrect assessments. Also, if there are past replacement or repair records, the items cannot necessarily be treated as being the same age.

In partial faults, they can appear as only certain power conditioners showing low output, only certain strings having low current, or only certain sections displaying abnormal output curves. After overall differences become apparent by comparing with adjacent installations, moving on to comparisons within the facility makes it easier to narrow down the cause. Even if total generation appears low, a fault in a single circuit may actually be pulling down the overall average.

In on-site inspections, we separate abnormalities that are visible to the eye from those that are difficult to see except in the data. Items that can be visually confirmed include dirt on panel surfaces, cracks, burn marks, fallen leaves, bird droppings, contact with vegetation, abnormalities in cable appearance, and the condition around junction boxes. On the other hand, internal connection faults, deterioration inside equipment, records of protective actions, and abnormalities in measured values may not be discernible from appearance alone at the site. For safety, detailed inspection of electrical equipment must follow the procedures of qualified personnel or specialists.

When comparing soiling, degradation, or partial faults with adjacent equipment, it is useful to keep photos taken on the same day and data recorded under the same conditions. Recording the condition of panel surfaces, grass height, shadow positions, and the surrounding environment photographed at the same time of day on a sunny day makes it easier to later explain their relationship to differences in power generation. By accumulating facts such as noticeably heavier soiling than adjacent equipment, grass shadows appearing only in specific sections, or lower output compared with circuits of the same configuration, it becomes easier to prioritize inspections, cleaning, and repairs.

Approach to determining inspection priorities from comparison results

Once differences become apparent in comparison with neighboring installations, the next step is to decide inspection priorities. The cause of low power output is not necessarily a single factor. Shading, dirt, shutdown history, output control, communication errors, and equipment malfunctions can overlap. Therefore, rather than concluding a single cause from the comparison results, it is important to organize possible causes in order: those with the greatest impact, those that are easiest to check, and those that can be checked safely.

First, prioritize items that have a large impact on power generation and are easy to verify. For example, long daytime shutdowns, communication outages, and sudden drops of output to zero are relatively easy to detect in the data and can have a large impact on generation differences. Next, check items that are easy to assess on-site, such as shading and soiling. These are easy to document with photos and inspection records and are information that can be readily shared among stakeholders.

On the other hand, internal faults or electrical abnormalities in equipment require safety management and professional inspection. Even if a decrease in power generation is suspected, on-site personnel should not attempt to work on electrical equipment. It is safer to assess possible abnormalities using monitoring data and visual inspections, then hand the case over to specialists as needed. If you submit a request with comparison results against adjacent equipment, the scope of checks will be clarified and it will be easier to carry out the necessary responses.

When deciding inspection priority, consider not only the magnitude of the discrepancy but also its persistence. If it is low for only one day, check temporary factors such as weather, shutdowns, output control, and measurement outages. If it continues for several days to several weeks, suspect shading, soiling, settings, or partial equipment faults. If the discrepancy has been widening over several months to years, also examine the installation environment, cleaning frequency, aging, and differences in equipment configuration.

Also, confirming the effects after inspections is important. After carrying out cleaning, mowing, settings checks, equipment restoration, and so on, verify whether the difference in power generation compared to adjacent installations has narrowed. Without a before-and-after comparison, it is difficult to determine whether the actions taken led to an improvement in reduced power generation. Recording the date of the work, the work performed, the weather, and the power generation of the comparison target together will provide reference material for future decisions.

The comparison results are used to reinforce on-site impressions with numbers and records. By organizing how much lower it is compared to adjacent equipment, since when it has been low, which time periods show differences, and which sections exhibit large differences, you can reduce wasted inspections and begin checks from locations closer to the cause.

Decision-Making Mistakes to Watch Out for When Comparing Adjacent Equipment

Comparing neighboring installations is useful, but if used incorrectly it can lead to misjudgments. In particular, be careful not to assume something is abnormal simply because it is lower than a neighbor’s without checking the conditions of the comparison. If there are differences in system capacity, installation orientation, tilt, commissioning date, output control conditions, or whether electricity is sold or used for self-consumption, differences in energy output can naturally occur. Comparison is merely a starting point for investigating causes, not a conclusion in itself.

Conversely, it is dangerous to over-rely on the idea that “it’s not a problem because the neighbor is low too.” On days when solar irradiance was low over a wide area or when output curtailment occurred, adjacent facilities can also show similarly low values. However, if the adjacent facility is affected by the same poor maintenance or the same seasonal shading, both may be operating at low levels. Just because the comparison target is low does not necessarily mean there is no problem with your own facility.

Be careful about time offsets in the data. If the clocks of monitoring devices or recording systems are off, you may think you are comparing the same time periods but are actually looking at data from different times. Time offsets especially affect judgments when comparing short-term cloud passages, output control events, or shutdown histories. When reconciling daily reports, monitoring data, on-site meters, and power sales statements, it is important to align the target dates and times.

Be careful in selecting the comparison period. If you compare only periods with many cloudy or rainy days, differences between installations can be hard to see. Conversely, judging based on a single sunny day can cause you to overstate the impact of temporary shading or outages. To narrow down the causes, check from multiple angles—sunny days, cloudy days, monthly totals, and the same month in the previous year—to reach a more reliable conclusion.

If adjacent facilities are managed by other parties, you must not enter the premises, touch the equipment, or obtain detailed information without permission. Information used for comparison must be limited to data obtainable within your authorized scope or information that is publicly available or permitted to be shared. When taking site photos, also pay attention to property boundaries, personal information, and management rules. In practice, both the validity of comparisons and the way information is handled are important.

When comparing adjacent installations, the important thing is that, after finding a difference, you sequentially check which factors can explain it. By examining step by step whether it can be explained by capacity differences, solar irradiance or shading, shutdown history, or whether dirt or malfunctions should be suspected, you improve the accuracy of your judgment.

Summary: Improve the accuracy of comparisons to more quickly narrow down the causes of reduced power generation

When investigating the causes of low power generation, comparing with adjacent installations can provide useful clues. By comparing with installations operating in the same area and under similar weather conditions, it becomes easier to determine whether the decline is occurring only in your installation or is common across the surrounding area. However, rather than judging solely by the generation figures, it is important to check, in order, the installed capacity, unit, period, azimuth, tilt, shading, downtime history, output control, communication status, soiling, degradation, and partial faults.

First, align the units of system capacity and generated output, and take a step beyond a simple comparison of magnitude. Next, compare solar irradiance conditions and the occurrence of shadows during the same time periods to identify factors causing reductions in the morning, evening, or across seasons. Furthermore, review differences in orientation, tilt, and installation environment to determine whether the variations can be explained by natural condition differences. Then check shutdown history, output curtailment, and communication anomalies to distinguish whether the system is not generating, not selling power, or not being recorded. Finally, check for soiling, degradation, and partial failures, including comparisons within your own facilities.

The results of comparisons with adjacent equipment provide a basis for prioritizing inspections and improvements. They make it easier to determine which sections to inspect first, which time periods’ data to review, whether on-site verification is necessary, and whether to hand the case over to a specialist. When addressing a drop in power generation, getting to the site quickly is important, but clarifying the comparison criteria beforehand can reduce missed checks and rework.

If low power output is left unaddressed, losses of generation opportunities can accumulate without the cause being identified. Conversely, repeatedly performing cleaning and inspections on weak grounds makes effects hard to see and explanations to stakeholders difficult. Leveraging comparisons with adjacent equipment and organizing numerical data, photographs, records, and on-site conditions together is the quickest way to narrow down the causes of a decline in power output.

If you want to set up a system to detect, compare, and record drops in power generation on site, it can also be effective to use management tools that streamline equipment-specific condition checks and the organization of on-site information. By establishing processes that allow smooth comparison with adjacent equipment and effortless sharing of inspection records, you can more easily improve the accuracy of root-cause investigations.