5 Things to Check Before Suspecting Insulation Failure When Power Output Is Low

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When low power output continues, people on site tend to become concerned, "Could it be an insulation failure?" Indeed, in solar power generation installations, insulation resistance can decrease due to cable damage, moisture ingress at connections, or internal deterioration of equipment, which can cause protective trips or operational shutdowns. However, insulation failure is not the only cause of reduced power output. There are many items that should be checked first: reduced solar irradiance, shading, soiling, grid-side conditions, curtailment by the power conditioner, and the way monitoring data is interpreted.

Organizing the basic items before suspecting insulation failure reduces unnecessary emergency responses and makes it easier to accurately convey the situation when consulting an inspection contractor. This article explains, in an order that's easy to use on site, five checkpoints to review before considering the possibility of insulation failure, aimed at practitioners searching for "low power generation".

Table of Contents

• Why you shouldn't immediately blame insulation failure for a drop in power output

• Examine the relationship between solar irradiance, weather, seasonal variations, and power generation

• Check for external factors such as shadows, dirt, snow, and weeds.

• Check the power conditioner's display and operating status

• Check for imbalances at the breaker, junction box, and string levels.

• Check for discrepancies between monitoring data and on-site records

• Signs That Indicate Possible Insulation Failure and Precautions for Inspection

• When power output is low, keep records to isolate the cause.

Why you shouldn't immediately assume insulation failure when power generation drops

When you notice low power generation from a solar power generation system, insulation faults are an important item to check. If an insulation fault occurs, there is a risk of leakage current or ground faults, and protective functions of equipment may operate and lead to a shutdown. In particular, after rain, typhoons, snowfall, grass cutting, or damage by birds and other animals, abnormalities can occur in cables and connection points, so this is not a problem to be taken lightly.

On the other hand, it is premature to conclude insulation failure based solely on a decline in power generation. Power output is influenced by multiple factors, such as solar irradiance, ambient temperature, the condition of the panel surface, shading, grid voltage, equipment operation and control, and the communication status of monitoring devices. Even on days that appear sunny but have many thin clouds, on days with high ambient temperatures that cause panel temperatures to rise, or in seasons when shadows from surrounding vegetation or structures lengthen, power generation can decrease even if the equipment is not faulty.

A common mistake in practice is judging a condition as abnormal based solely on a simple comparison with the same month of the previous year or with the previous day. Solar power generation can vary with weather conditions even within the same month. There are clues to the causes in the way generation falls: it may have produced well in the morning but only dropped in the afternoon; even on sunny days it may dip only at certain times; it may be low only on the day after rain; or only a specific power conditioner may be producing less.

Investigating insulation failures requires electrical measurements and safety precautions. It is dangerous if on-site personnel inadvertently touch the inside of panels or terminal sections. Therefore, it is important to first check monitoring data, the site appearance, operation indicators, comparison targets, and the consistency of records, and to clarify the scope of the anomaly and the conditions under which it occurred. With organized information, when consulting a specialist contractor it will be easier to determine whether insulation testing is necessary or whether investigation of other causes should be prioritized.

When addressing a drop in power generation, it's important not to "decide the cause immediately" but to adopt the attitude of "eliminating possibilities one by one." Insulation failure is one of the serious causes, but if you narrow the focus to that from the outset, you are likely to overlook factors such as solar irradiance conditions, curtailment, shading, and missing data.

Examining the relationship between solar radiation, weather, seasonal variations, and power generation

When power generation is low, the first things to check are solar irradiance and the weather conditions. The amount of power produced by a solar photovoltaic system is largely determined by how much sunlight reaches the panels. Even if the equipment is functioning normally, solar irradiance can be reduced by cloudy skies, rain, thin cloud cover, yellow dust, haze, or mist, causing generation to drop. In particular, on days with thin clouds it may look bright to the naked eye, yet the power output can be lower than expected.

If you look only at daily power generation, it’s easy to feel that it’s “lower than yesterday” or “lower than last week,” but when making comparisons you also need to consider the weather conditions. Comparing a clear day with a cloudy day, it’s natural for there to be differences in generation. Days that were sunny in the morning but saw more clouds in the afternoon, or days when a rain cloud passed briefly, will also have lower daily totals. When investigating reasons for low generation, it’s easier to make a judgment if you look not only at the daily total but also at the generation curve by time of day.

A power generation curve on a mostly clear day rises gradually from the morning, forms a peak around midday, and then falls toward the evening. On cloudy days, this peak becomes jagged and can drop sharply even during daytime. If such fluctuations appear similarly across the entire installation, it is natural to suspect the influence of weather or solar irradiance conditions first. Conversely, if only some sections within the same site are significantly lower, factors other than the weather need to be considered.

Seasonal differences are also important. In winter the sun’s altitude is lower and daylight hours tend to be shorter, so depending on the region and installation conditions, power output may not match that of spring or summer. Shadows from surrounding mountains, buildings, trees, and rows of racking also change with the seasons. Shadows that had little effect in summer can stretch longer in the morning or evening in winter and appear as reduced power generation. Even when comparing with the same month of the previous year, differences in weather, snowfall, prolonged rain, and the timing of the rainy season must be taken into account.

Ambient temperature is another easily overlooked factor. While solar panels tend to generate more power the stronger the sunlight, their output generally falls as panel temperature rises. On a clear midsummer day, if generation doesn’t increase as much as expected, it isn’t necessarily abnormal. Even with sufficient solar irradiance, losses due to elevated panel temperature may be occurring.

Before suspecting insulation failure, first confirm whether the days of low power generation truly met the conditions that would warrant suspecting an equipment fault. Comparing weather, solar irradiance, season, temperature, and time-of-day generation curves makes it easier to distinguish natural variability from an equipment-side fault.

Check for external factors such as shadows, dirt, snow, and weeds

Next, what we want to check are external factors related to the panel surface and the surrounding environment. Causes of reduced power output include shadows, dirt, fallen leaves, bird droppings, snow accumulation, weeds, soil/sediment, and mud splashes. These are not insulation faults, but they can lower power output. In particular, if the impact is concentrated on some panels or strings, it can affect the power generation efficiency of the entire system.

Shadows vary by time of day and season. Low sun in the morning and evening, nearby buildings, utility poles, fences, trees, slopes, adjacent equipment, and rows of racking can cast shadows on solar panels at specific times. If time-of-day power generation data shows a drop at the same time every day, shading is a likely cause. If there is no alarm history and the drop is easily reproducible at specific times on sunny days, first check the locations and timing of the shadows.

Dirt can also cause a reduction in power generation. When sand and dust, pollen, yellow sand, bird droppings, fallen leaves, or splashed mud adhere to the panel surface, they block light. If there is a thin layer of dirt across the entire surface, overall power output may drop slightly, whereas localized dirt can have a disproportionate effect on particular circuits. When performing a visual inspection, rather than just viewing the whole array from a distance, check how each row is soiled, look for mud splashes on panels in low positions, and identify spots where drainage is likely to flow onto the panels.

Weeds should not be overlooked. In ground-mounted installations, grass can grow from spring through summer and cast shadows on the front of the panels. Even if it had been cut by the time of a site visit, grass may have grown during periods when power generation was low. By combining the date mowing was carried out, photos before and after weeding, and changes in power generation, it becomes easier to assess the impact of weeds.

Snow cover and frost can be major factors in some regions. When there is snow, the time during which generation is not possible increases. Even if snow remains only in part of the array, power output can become unbalanced from string to string. After snowfall, there can be a time lag before output recovers. Even after the snow melts, mud or dirt may remain on the panel surface, so if generation does not recover immediately a visual inspection is necessary.

When evaluating external factors, it's important to imagine the site conditions at the same time of day as on the low-output day. Even if you see no shadows at the site at midday, there may have been shading in the morning. Even if dirt isn't noticeable on a sunny day, mud splash may have spread after rain. Before suspecting insulation failure, check site photos, past inspection records, grass-cutting records, cleaning records, and weather records together to help narrow down the cause.

Viewing the power conditioner display and operating status

When power generation is low, the operating status of the power conditioner is something you should check. The power conditioner converts the direct current generated by the solar panels into alternating current and is a device that greatly affects the overall output of the system. If it is stopped, on standby, curtailed, experiencing a fault, or having communication problems, the power generation will decrease.

The first thing to check is whether any alarms or errors are displayed on the display or monitoring screen. If an error is shown, record its details and the time it occurred. However, the presence of an error display does not necessarily mean an insulation fault. There are multiple possible reasons for a power conditioner to stop, such as grid voltage, frequency, temperature, communications, input voltage, or internal equipment protection. It is important to record the display contents accurately and use them to consult the operating manual or to support the judgment of maintenance personnel.

There may be cases where generated power is low but no error is displayed. For example, when output curtailment or overvoltage suppression occurs, the equipment may reduce output even if it is not faulty. Especially if the top of the generation curve flattens around midday on sunny days, or if the output does not rise above a certain level, check for possible curtailment. In such cases, it may be related to grid-side conditions or control settings rather than an insulation fault.

It is also useful to compare generation on a per-power-conditioner basis. If multiple power conditioners are installed on the same site, the interpretation changes depending on whether all units are similarly low or only a specific unit is low. If all units show a similar downward trend, consider the weather, solar irradiance, grid curtailment, and overall system impacts. If only one specific unit is low, you need to check the strings connected to that unit, the connection/combiner boxes, the input circuits, and the condition of the unit itself.

The start-up and shutdown times are also points you should check. If there are temporal patterns—such as a slow morning start-up, an early evening shutdown, temporary stops during the day, or stopping only after rain—it becomes easier to narrow down the causes. If insulation-related alarms occur after rain, that may indicate moisture ingress into cables or connection points, but it can also simply mean that weaker sunlight caused a delayed start-up.

When on-site personnel carry out checks, they should not touch inside panels or terminal areas, and should focus on displays, indicator lamps, monitoring screens, and records. Electrical measurements and inspections of equipment internals are tasks that should be requested of personnel who hold the necessary qualifications and follow required safety procedures. When power generation is low, rather than hastily operating on-site equipment, it is important to first record the operating condition and verify whether the time the abnormality occurred coincides with the timing of the power output decline.

Check for imbalances at the breaker, junction box, and string levels

Before suspecting insulation failure, confirm whether the drop in power generation is occurring across the entire system or only in some circuits. A solar power generation system is composed of multiple panels, strings, junction boxes, power conditioners (inverters), breakers, and so on. If any part is offline, the overall power output may appear low.

First, if you have a monitoring environment that can compare generated power and current values at the power conditioner unit, junction box unit, and string unit, check for any imbalances. If a circuit that should be installed under the same conditions shows a significant difference, that circuit may have shading, soiling, poor connections, broken wires, blown fuses, open or tripped breakers, or equipment faults. In addition to insulation failure, consider causes such as no current flow, no input, or disconnected/loose connections.

If a breaker has tripped or some switches are open, this can also lead to reduced power generation. Possible causes include incomplete restoration operations after inspection or construction, protective functions activating after strong winds or thunderstorms, or some abnormality causing the circuit to be interrupted. However, it is dangerous to simply reclose a breaker without confirming the cause of the trip. Whether it is safe to reclose should be determined by on-site safety procedures and the judgment of qualified personnel.

Around the junction box, visible abnormalities can also provide clues. If the door does not close properly, there are signs of water ingress, excessive condensation, traces of insect or small-animal intrusion, damage at the cable entry, or suspected cable damage from a grass trimmer or animals, these may lead to electrical faults. While these can also cause insulation failures, it is important to first document the exterior appearance from a safe distance and report the situation to a qualified professional.

When evaluating string-level imbalances, you should compare circuits that have the same orientation, the same tilt, the same number of modules, and the same installation conditions. Simply comparing circuits with different conditions can lead you to mistake normal differences for anomalies. For example, it is not unusual for energy output to differ between east- and west-facing arrays, between rows that are prone to shading and open rows, or between sections with different tilt angles.

To isolate the cause of low power generation, it’s important to determine whether it’s an “overall drop” or a “partial drop.” If the whole system is low, prioritize solar irradiance, curtailment, and monitoring-data issues; if only part is low, focus on circuits and on-site environmental conditions. If you suspect insulation failure alone without this sorting, the scope of checks can become too broad and response may be delayed.

Checking for discrepancies between monitoring data and on-site records

The basis for determining that power generation is low is often monitoring data and monthly reports. However, if the monitoring data itself has gaps or communication problems, it can appear lower than the actual generation. Before suspecting an insulation failure, it is important to verify the data acquisition status and the consistency with on-site records.

Even if the monitoring screen shows the power output near zero for a period, that does not necessarily mean the equipment actually stopped. Data may not be recorded correctly due to communication device malfunctions, temporary line outages, power anomalies in data acquisition devices, misconfiguration of measuring instruments, clock drift, and so on. If data are missing only for specific times, if readings are recorded in bulk after communication is restored, or if the values shown on the screen do not match the on-site meter readings, the monitoring system — rather than the power generation equipment itself — needs to be checked.

Be careful when looking at monthly power generation figures. If there is missing data partway through the month, the total generation for that month will appear low. If the equipment has not stopped but the daily report shows low generation, first check the daily data, hourly data, and equipment-specific data. Clarifying which period of data is missing and whether the missing range affects the entire facility or only some equipment makes it easier to determine whether the issue lies with the monitoring side or with the generation side.

Cross-checking with on-site records is also useful. When you view inspection dates, power outage dates, construction dates, mowing dates, cleaning dates, snowfall dates, typhoon passage dates, alarm activation dates, restoration work dates, and so on alongside power generation data, you can sometimes see the reasons for a decline. For example, a day when operations were temporarily halted for inspection may be mistaken for an abnormal day, or a stoppage caused by a power outage may be misinterpreted as equipment failure.

Also, the choice of comparison targets is important. Year-on-year for the same month, month-on-month, versus planned values, versus nearby facilities — the way you compare changes how things look. Even if performance is lower than the same month last year, if this year’s weather was poor it may be a natural decline. Even if it is lower than the planned value, the plan’s assumptions may not match the site conditions. When making comparisons, you need to consider solar irradiation conditions, equipment capacity, number of operating days, downtime, and curtailment time together.

Monitoring data is useful, but it is dangerous to conclude the cause from it alone. When power output is shown as low, distinguish and check whether the system is actually not generating, is generating but not being recorded, or is recorded but the assumptions used for comparison are different. By taking this step, you can reduce unnecessary insulation testing and emergency responses, and more easily focus on the investigations that are necessary.

Signs Indicating Insulation Failure and Precautions When Inspecting

If, after these checks, the cause remains unclear or a specific alarm is present, consider the possibility of insulation failure. Because insulation failure involves safety, on-site personnel should not pursue it on their own; they should request confirmation from specialists who have the necessary qualifications and procedures.

Signs that may indicate insulation failure include abnormalities that appear more easily during rain or just after rain, protective trips when specific circuits are connected, recorded alarms related to leakage or earth faults, traces of water ingress or damage around junction boxes or cables, a drop in power generation after grass cutting or animal damage, and the same alarms recurring even after restoration. If several of these occur together, insulation failure should be prioritized when checking the cause of reduced power generation.

However, investigations of insulation failures may require shutting down equipment, operating switches or circuit breakers, using measuring instruments, and narrowing down the measurement scope. Incorrect operations can lead to electric shock, equipment damage, fire, or secondary failures. For safety, on-site personnel should limit their checks to recording displayed readings, photographing the appearance, noting the time of occurrence, the relationship with weather, and identifying the equipment or circuits that showed abnormalities.

When consulting a specialist, rather than simply saying 'the power generation is low,' organize and report when it became low, which equipment is affected, whether it is related to rainy weather, what alarm messages are displayed, whether it is recurring, and whether there are any visible abnormalities on site—this will speed up the initial response. Photos, screenshots of the monitoring screen, daily and hourly generation data, alarm history, and maintenance/work history help narrow down the cause.

Insulation faults are an important issue not only because they reduce power generation but also for safety management. However, low power output alone is not sufficient to determine this. It is practical to check solar irradiance, shading, soiling, curtailment, breakers, and monitoring data, and only proceed to investigate insulation faults if there are still unexplained reductions or alarms.

When power generation is low, keep records and isolate the cause

The thing to avoid most when responding to low power generation is proceeding with on-site measures after assuming the cause. Insulation failure is an important cause, but many factors can underlie a drop in power generation. By checking, in order, the weather, the season, shading, dirt, weeds, snow accumulation, power conditioner derating, breaker condition, and missing monitoring data, you can reduce unnecessary work and more easily proceed to the inspections that are actually needed.

What operations personnel should do first is not to rely on subjective impressions of a drop in power generation, but to record and organize it. Keeping the date of occurrence, time of occurrence, weather, affected equipment, generation curve, alarm displays, site photos, and work history makes it easier to see patterns of causes when reviewing later. In particular, decreases that recur at the same time of day, abnormalities that appear only after rainy weather, decreases affecting only specific power conditioners, and decreases that began after grass cutting or construction are clues to identifying the cause.

Addressing a decline in power output may not be completed with a single inspection. On sunny days, cloudy days, after rain, or during seasonal transitions, the way it appears can change as conditions change. By continuously recording data and understanding trends across the entire system, you can notice an "unusual decline" more quickly. This can lead to early detection of insulation faults and help reduce preventable losses such as shading, weeds, and dirt.

Keeping on-site information centralized also helps with internal explanations and coordination with maintenance contractors. When explaining the causes of low power generation, conveying the situation verbally alone is difficult and decision-making tends to become subjective. If photos, locations, power generation data, and inspection notes are compiled, staff who have not visited the site can more easily understand the situation. As a result, it becomes easier to prioritize inspections and reduces the risk of missed actions.

When power generation is low, before suspecting insulation failure it is important to first check solar irradiance conditions, external factors, equipment operating conditions, imbalances at the circuit level, and the consistency of monitoring data. If, based on that, alarms following rainy weather, circuit abnormalities, or visible damage are confirmed, proceed to safe measurement and inspection by qualified personnel.

To carry out these checks consistently at each site, a system that can link and manage power generation data, on-site photos, inspection locations, anomalies, and work history is effective. By standardizing the format of records and recording the date and time when the decline occurred, the affected equipment, what was checked, and the next inspection to request, it becomes easier to isolate causes, including insulation failures.