Six humidity and cloud-cover checks to assess reduced power generation during the rainy season

When solar power generation is low during the rainy season, it becomes difficult to determine whether this is due to equipment failure or a temporary drop caused by the weather. Because there are few sunny days, cloud thickness changes day to day, and humidity, condensation, rain-related dirt, and unstable communications can overlap, simply noting "less than yesterday" or "lower than last month" is not enough to narrow down the cause. In practice, when a drop in generation is detected, it is necessary to sort out whether immediate on-site response is required or if it is sufficient to check after the weather improves, and what information should be shared with inspection companies and managers. This article explains, from a practical viewpoint, six aspects on-site personnel should check to judge generation declines caused by humidity and overcast conditions during the rainy season.

Table of Contents

• Isolate the decline in power generation during the rainy season from weather-related factors.

• Reexamine the relationship between solar irradiance and power generation under the same conditions

• Verify that the power generation curve on cloudy days appears natural.

• Check for temporary effects caused by moisture, condensation, and rainwater.

• Check the panel surface soiling and its recovery trend after rain.

• Check abnormalities in the power conditioner and monitoring display separately from weather conditions.

• Narrow down equipment-related factors by comparing multiple days and multiple zones.

• Record the decrease in power generation during the rainy season and use it to inform future countermeasures.

Isolating the decrease in power generation during the rainy season from weather-related factors

When you feel that power generation is low during the rainy season, the first thing to check is whether the decline is within the range expected from weather conditions or whether it indicates a problem with the equipment. Because solar power generation depends on receiving sunlight, it is natural for output to fall when cloudy or rainy weather persists. In particular, during the rainy season not only are there fewer sunny days, but thin clouds, thick clouds, drizzle, heavy rain, and brief sunny intervals often coexist, and even under the same “cloudy” label generation can vary greatly. For that reason, judging that monthly generation is “low” based solely on monthly output can lead to mistaking weather effects for equipment trouble.

In practice, the first step is to check whether the reduction in power generation is occurring simultaneously across the entire system or only in certain sections or specific inverters. If cloudy weather during the rainy season is the main cause, equipment on the same site will generally show a decrease in generation in the same direction. Of course, due to differences in azimuth, tilt, shading, panel capacity, and inverter capacity, the numbers will not be exactly the same, but they often exhibit similar trends overall. On the other hand, if only a particular circuit, a particular inverter, or a particular string shows a large drop, it becomes difficult to explain the phenomenon by weather alone.

Also, the rainy season is also a time when people tend to perceive that “power generation is low.” If you use the high generation during clear weather as a baseline, the output on cloudy days can look extremely low. However, on days covered by thick clouds, generation output can drop substantially compared with sunny days. Even if it isn’t raining, if clouds are thick and solar irradiance is weak, generation will not increase. Conversely, after rain when clouds break and sunlight strengthens, output can rise even for a short time. Judging solely by simple daily totals without accounting for these fluctuations can lead to overlooking the causes.

The first step in isolating the cause is to determine whether the decrease in power generation is linked to the weather. Separate and record rainy days, all-day cloudy days, thinly overcast but bright days, and days with only short sunny periods, and compare these with changes in power output. Because weather-forecast displays alone may not accurately reflect local cloud cover or solar irradiance, where possible make your judgment using on-site monitoring data, pyranometer measurements, trends at nearby power plants, and site photos and work logs. When power output drops during the rainy season, first confirm whether a weather-related cause is plausible and only then suspect equipment issues; this order of checks helps reduce unnecessary on-site responses and incorrect cause determinations.

Reexamine the relationship between solar irradiance and power generation under the same conditions

When determining the causes of low power generation during the rainy season, it is important to reassess the relationship between solar irradiance and power output under the same conditions. In photovoltaic power generation, less irradiance results in less power output. This is a basic relationship, but in practice it is not uncommon to make judgments based solely on the power output. Especially in the rainy season, the difference between clear days and cloudy days is large, and the daily output of the same system can fluctuate significantly. Therefore, if you look only at power output on a low-irradiance day and conclude that it is "too low," you may overestimate the impact of the weather.

When solar irradiance data is available, check power generation in combination with irradiance. Rather than looking at raw generation alone, comparing generation under the same irradiance conditions with past data makes it easier to detect changes in the equipment. For example, if the generation-to-irradiance ratio has dropped significantly compared with the rainy season last year or a cloudy day in the previous month, factors other than the weather may be involved. On the other hand, if irradiance itself has fallen considerably on a given day, low generation may be a natural result.

At sites without a pyranometer, making a complete comparison becomes difficult, but you can still take measures to observe under similar conditions. Use weather records, brightness by time of day, the state of rain clouds, generation trends at nearby facilities, comparisons with other systems on the same site, and so on to align conditions as much as possible when making a judgment. For example, even on the same cloudy day, clouds may be thin in the morning and heavy rain clouds in the afternoon. In that case, looking only at the daily total makes it hard to identify the cause, but by examining time-of-day intervals you can confirm whether changes in solar irradiance and changes in power output are correlated.

Also, during the rainy season you should also pay attention to the effect of temperature. Solar panels generally tend to produce less output as temperature increases, but in the rainy season the temperature effect does not necessarily stand out as much as it does during the intense heat of midsummer. Rather, a lack of solar irradiance due to cloudy skies often has a larger impact. However, on days when the clouds break and sunlight suddenly becomes strong, panel temperature and the inverter’s operating state also influence generation. It is important not to view irradiance, temperature, and generation separately, but to consider them together on the same time axis.

Operational staff should be careful not to change comparison targets casually. Comparing the peak output on a sunny day with the generation on a rainy day does not lead to a correct assessment. You need to compare not only the averages for the same month but also cases with similar weather, similar time of day, similar solar irradiance conditions, and the same equipment category. Especially when you are at the stage of searching 'low power generation' to investigate the cause, it's important to check whether the generation is reasonable given the solar irradiance before suspecting faults or degradation. Carefully examining the relationship between irradiance and generation makes it easier to distinguish between the temporary declines characteristic of the rainy season and ongoing performance degradation.

Check whether the power generation curve on cloudy days has a natural shape

When judging decreases in generation during the rainy season, it is essential to look not only at the daily total generation but also at the generation curve by time of day. A generation curve shows changes in output or generated energy from morning to evening plotted on a time axis. On sunny days it typically forms a mountain-shaped curve: output rises from the morning, peaks around midday, and falls in the evening. On cloudy or rainy days, however, output may fluctuate frequently due to cloud thickness and the passage of rainclouds, or remain generally low. Because these variations are large during the rainy season, it becomes difficult to determine whether conditions are normal or abnormal from the daily total alone.

The natural generation curve on overcast days is lower and smoother than on sunny days, and it may fluctuate up and down with the movement of clouds. When thick clouds persist, output may not rise much at midday and can remain at a low level from morning through evening. On partly cloudy days, output may increase to some extent around midday, although not as much as on sunny days. On days when rain clouds pass intermittently, output may drop suddenly or rise only for short periods. If such variations correspond to the weather, low generation can be more easily explained as an effect of the weather.

What you need to watch for are unnatural curves that cannot be explained by the weather. For example, movements that require checking include output flattening at a constant level from morning until noon; suddenly dropping to near zero from a specific time and remaining there; only certain systems failing to increase output despite sunny intervals; or sharp drops at the same time every day. Rather than overall low output due to cloudy weather, there may be mixed influences from equipment, control, communications, shading, or protective operations. If you assume that low output is simply because it’s the rainy season, you risk overlooking such anomalies.

When looking at generation curves, it is also important not to make a judgment based on a single day alone. During the rainy season the day-to-day weather variation is large, so you should avoid declaring an anomaly from the curve of just one day. If possible, check several days side by side—rainy days, cloudy days, and days with clear spells. Furthermore, compare the curves of multiple power conditioners (inverters) or multiple strings within the same facility. If the whole set shows a similar pattern of decline, the cause is likely weather-related; but if only some parts clearly have a different shape, you need to consider causes limited to that section.

During the rainy season, cloud movement makes short-term power generation fluctuations more likely. Therefore, rather than judging an anomaly by only minute-scale small ups and downs, it is important to look at the overall trend across the time period. Conversely, even if the daily total does not show a large difference, viewing output by time of day can reveal that output drops dramatically during certain periods. The generation curve is an effective clue for distinguishing natural declines caused by weather from unnatural, equipment-related declines. When generation is low, making a habit of checking the shape of the curve as well as the totals improves the accuracy of your judgments.

Confirm temporary effects caused by moisture, condensation, and rainwater

A feature of the rainy season is not only cloudy skies and rain but also prolonged high humidity. While moisture itself does not necessarily reduce power generation as directly or as much as insufficient solar radiation, condensation, accumulation of rainwater, faults in terminal and connection parts, or insulation-related alarms can make it necessary to check the power generation status. In particular, when humidity is high in the early morning or at night and there is a temperature difference, condensation can occur around the equipment. If the influence of condensation or rainwater is suspected, check not only the generated power but also alarm history, shutdown history, and the timing of recovery.

An important point when checking issues related to humidity is to distinguish whether a drop is temporary or occurring repeatedly. For example, if an alarm is triggered temporarily during a period of heavy rain or in the early morning and then clears on its own, the influence of humidity or rainwater may be suspected. However, just because an alarm is present does not mean you should assume humidity is the sole cause. Factors that may require inspection include deterioration of connections, damage to wiring, water ingress into equipment, and a decrease in insulation resistance. Because these issues can affect safety, if on-site confirmation is necessary, do not attempt forcible operations or open equipment; follow the prescribed procedures and contact the appropriate specialist.

During the rainy season, humidity remains high even during periods without rain, and areas around equipment stay damp and are slow to dry. Outdoor-installed equipment, junction boxes, cable routes, cable entry points, and locations with poor drainage tend to retain moisture. If there are problems at these points, faults that were not noticeable on sunny days may surface only during the rainy season. If a drop in power generation repeatedly occurs during the rainy season and does not fully recover after the weather clears, equipment-related factors associated with moisture and rainwater should be suspected.

However, in actual operations, care is needed when confirming things on site. Electrical equipment in rainy or wet conditions can be hazardous. Generation equipment involves both DC and AC sides, and even if it appears to be stopped, voltage may still be present. Therefore, avoid touching wet equipment, opening covers, or moving cables unnecessarily. On-site personnel should limit their checks to appearance, alarm indications, monitoring data, the presence of unusual noises or odors, and puddles or drainage conditions that can be safely inspected; electrical inspections should be performed by personnel with the appropriate qualifications and procedures.

When assessing the effects of moisture, condensation, and rainwater, it is useful to record the relationship between the time of occurrence and the weather. Organize whether alarms tend to be triggered from night through early morning, whether the system stops immediately after heavy rain, whether output becomes unstable only on high-humidity days, or whether it recovers after clear, dry weather. With such records, it becomes easier to determine whether the issue is a temporary weather effect or an equipment fault that becomes apparent during the rainy season. Moisture is a factor that is difficult to see, but by linking power generation trends, alarm history, and on-site conditions you can reduce oversights.

Confirm panel surface soiling and post-rain recovery trends

During the rainy season, a decline in power output also warrants checking for dirt on the panel surface. Although rain may seem to wash dirt away, in reality rain alone does not necessarily remove all contaminants. If yellow sand, pollen, soil dust, bird droppings, fallen leaves, or dust from nearby construction or farmland adhere to the panel surface, some may be washed off by rain while dirt can remain at the edges or in lower areas. Because surfaces are slow to dry during the rainy season, moisture can mix with dirt and remain as streaks or cause partial deposits to persist.

When judging whether a drop in power generation is due to soiling, it is important to check whether output recovers after rain. It is natural for output to be low on cloudy or rainy days, but if, after the rain stops and solar irradiance conditions improve, output does not return to previous levels, you should suspect soiling or other equipment-related factors. In particular, if only some arrays on the same site recover more slowly, check localized factors such as panel surface contamination, drainage, tilt angle, nearby shading, vegetation growth, and bird damage.

During the rainy season, rain both washes away dirt and can spread and leave it behind. Heavy rain can remove general dust, but fine mud and pollen may remain as water droplet marks. If the panel tilt is shallow or water tends to pool near the frame, dirt is more likely to remain at the bottom edge. Also, bird droppings and sticky grime may not be removed sufficiently by rain alone. These can act like partial shading and may affect power generation.

However, you should avoid overestimating the impact of dirt. The primary cause of reduced power generation during the rainy season is often insufficient solar radiation. To determine whether dirt is causing the decline, check days with clear skies and the recovery before and after the end of the rainy season to see if there are differences that cannot be explained by weather alone. When performing on-site visual inspections, work on roofs, at heights, or on wet scaffolding is dangerous. If a visual check is necessary, limit it to what can be confirmed from a safe position, and plan cleaning and detailed inspections with safety management as a prerequisite.

Combining power generation records with appearance photos makes it easier to assess soiling. By linking which areas retain dirt after rain, which arrays show lower output, and how much they recover during sunny periods, you can make judgments based on practical evidence rather than mere intuition. Because weather variability is high during the rainy season, it is important not to attribute effects to soiling alone but to evaluate by combining solar irradiance, power-generation curves, comparisons within the same site, and visual inspections.

Check inverter and monitoring-display abnormalities separately from weather conditions

Even when power generation is low during the rainy season, you should not blame everything on the weather; you need to check for abnormalities in the inverter and the monitoring display. It is natural for generation to decrease on cloudy or rainy days, but situations such as the inverter being stopped, errors appearing, communications being interrupted, or measured values not being updated require action separate from the weather. Especially during the rainy season, humidity and rain can make equipment faults surface more easily, and relying solely on monitoring data can cause a drop in generation to be mistaken for a communication failure.

The first thing to check is whether the power output is actually low or whether the data is not being acquired correctly. Even if the monitoring screen shows generation near zero, the system may actually be generating but the communication is interrupted. Conversely, the communication may be normal while the inverter (power conditioner) is stopped. Check the timestamp of the displayed values, communication status, operating status of each inverter, alarm history, and recovery history, and distinguish whether it is a decline in the generation equipment itself or a display problem on the monitoring side.

When suspecting an inverter malfunction, it’s important to know whether all units are showing similarly low output or only some. On cloudy days overall output is often lower, but if only specific inverters have stopped or only certain inputs are extremely low, you need to check individual equipment factors. During the rainy season, temporary protective actions can occur due to rain and humidity, but if the same alarm repeats or a unit stops again shortly after restarting, it’s safer not to postpone inspection.

Also, you need to pay attention to the units and aggregation periods shown on the monitoring display. If instantaneous output, daily generation, monthly generation, and cumulative generation are confused, you may misjudge the cause of low generation. It is natural for instantaneous output to be low on cloudy days, but if output does not increase even when there are sunny intervals, you should check. Even if daily generation is low, it may be within a natural range if it rained all day. On the other hand, if monthly generation has dropped significantly, check not only for reduced insolation due to the rainy season but also the number of outage days and the duration of any alarm events.

When checking the display on site, record not only whether there are alarms but also when they occurred and when they were cleared. During the rainy season, declines in power generation can be caused by both weather and equipment conditions at the same time, so simply noting “error present” or “low generation” will not narrow down the cause. Organizing the time of occurrence, weather, affected equipment, display details, restoration status, and the range that declined simultaneously will make subsequent investigation and reporting smoother. Checking the power conditioner and monitoring displays is an important check to avoid attributing low generation during the rainy season solely to weather.

Narrow down equipment-related factors by comparing multiple days and multiple zones

During the rainy season, decreases in power generation are difficult to judge from only a single day of data. Because the weather is unstable and solar irradiation varies greatly from day to day, looking at the power generation for one day alone makes it hard to tell whether the decrease is normal or abnormal. Therefore, it is important to narrow down equipment-related causes by comparing multiple days, multiple sections, and multiple devices. Even if you feel the power generation is low, arranging data for several days will reveal whether the decrease is linked to the weather or limited to specific locations.

First, compare within the same installation. If there are multiple inverters (power conditioners) or arrays on the same site, check whether the reduction in power generation is occurring uniformly across the whole. If capacities or orientations differ, you cannot compare by raw generation alone, so compare generation per unit of installed capacity or compare sections with the same configuration. If the whole site is similarly low, it is likely due to insufficient solar irradiance caused by cloudy or rainy weather. On the other hand, if only some sections remain consistently low, you need to check for shading, soiling, wiring, connections, equipment shutdowns, or input faults.

Next, examine trends over multiple days. During the rainy season, day-to-day weather varies greatly, so compare days with some sunshine, days that were cloudy all day, and days with heavy rain separately. If the same section is low on every day, factors other than weather are suspected. Conversely, if the magnitude of the drop varies across the board with the weather, it is more likely that insufficient solar irradiance is the primary cause. If there is a large drop only on a particular day, check that day's alarms, shutdowns, communication outages, nearby work, grid-side stoppages, heavy rain, lightning, and other events.

Past data is also useful for comparisons. Comparing with the same month last year, the previous month, the most recent sunny day, a cloudy day before the start of the rainy season, etc., makes it easier to determine whether the current decline is within seasonal variation or due to a change in equipment condition. However, because weather varies year to year, you should avoid drawing definitive conclusions from a simple comparison with last year’s generation for the same month. Compare not only generation but also weather, solar irradiance, downtime, equipment changes, and changes in the surrounding environment. For example, if surrounding vegetation has grown and shading is more likely, the decline can become especially noticeable when combined with low irradiance during the rainy season.

When narrowing down equipment-related causes, it is important to organize the comparison results in a way that can be explained. Classifying them as "overall low", "low only in certain parts", "low only on rainy days", "doesn't recover even during clear weather", or "drops only at specific times" clarifies which areas should be checked next. Simply knowing that power generation is low leaves the scope of response too wide, but by narrowing the target through comparison, the accuracy of on-site checks and inspection requests improves. Because judgments are especially difficult during the rainy season, comparing under multiple conditions rather than relying on a single impression is a basic practice in the field.

Document power-generation drops during the rainy season to inform future countermeasures

To correctly assess reductions in power output during the rainy season, it is important to record what you have checked. Low power output can result from multiple overlapping factors such as weather, insufficient solar irradiance, humidity, dirt, equipment outages, communication failures, and partial shading. Even if you think you understood the cause on site, without records you cannot make comparisons when a similar drop occurs later. Because the rainy season in particular is a seasonal factor that occurs every year, accumulating records will enable quicker assessments in subsequent years.

The records should include more than just the generated power. Organize together the weather, whether it was raining, cloud thickness, impression of solar irradiance, characteristics of the power generation curve, alarm history, inverter status, communication status, on-site appearance, presence of dirt or puddles, the time of restoration, and the inspection details. Site conditions that numbers alone don't reveal can be preserved as photos or notes and serve as decision-making material. However, regardless of whether photos are used in the main text or reports, as practical records it's easier to compare if they are taken from the same angle, the same section, and the same time of day.

The purpose of recording is not merely to keep a history but to speed up subsequent decision-making. For example, if the same inverter issued an alarm after rainy weather during last year’s rainy season, this year’s identical alarm should be treated not only as an effect of the weather but also as a recurring equipment-related issue. Conversely, if total power generation drops in a similar way every rainy season and recovers on sunny days, it is easier to explain it as a seasonal decline. Having past records also helps with reporting to managers and prioritizing inspection plans.

Also, keeping records can reduce discrepancies in understanding among stakeholders. When multiple parties are involved — power producers, maintenance personnel, on-site staff, management companies, etc. — the expression "power generation is low" can be interpreted differently. If you organize and share information on which day, which equipment, to what extent it declined, what the weather was like, and whether there were any alarms, it becomes easier to make the response concrete. This is especially true during the rainy season, when declines caused by weather and equipment faults are easily confused, so the granularity of the records affects the quality of the response.

It is not easy to make a definitive judgment at a single glance about reduced power generation during the rainy season. That is precisely why it is important to define the items to check and to observe them continuously from the same viewpoints. If you sequentially check the relationship between solar irradiance and power generation, the power-generation curve, the effects of humidity and rainwater, dirt on panel surfaces, inverters and monitoring displays, and comparisons across multiple days and multiple sections, it becomes easier to separate weather-related factors from equipment-related ones. Rather than rushing to judge when you first notice low generation, proceeding with checks based on records reduces unnecessary actions and makes it less likely that you will miss required inspections.

The rainy season is also a good opportunity to reassess the condition of solar power equipment. Precisely because generation can appear reduced due to humidity and cloudy skies, you can check trends in alarms that are easily overlooked, communication instability, differences between sections, and post-rain recovery. Rather than simply looking at daily generation data, organizing it into decision-making material that can be used on site leads to more stable operation. If you want to efficiently confirm generation declines and grasp the condition of a solar power plant, it is important to review the flow of on-site inspections, monitoring data, inspection records, and photo management, and to establish a recording system that allows stakeholders to make judgments under the same conditions.