Four changes observed in post-rain data when power output is low

When you feel that power generation is low, looking only at data from sunny days to find the cause can lead to oversights. Especially after rainy periods, issues such as dirt on the panel surface, drainage, moisture, insulation condition, and effects at connections tend to become apparent, making it an opportune time to check for equipment weaknesses that are usually hard to see. Rain itself reduces solar irradiance and is a direct cause of reduced generation, but data after the rain has stopped may show changes that are difficult to explain by weather effects alone.

This article explains four changes to look for in monitoring data and inspection records after rainy weather, aimed at practitioners searching for "low power output". Rather than judging based on a short-term drop alone, checking before-and-after rain, the same time of day, adjacent equipment, and past data together makes it easier to determine whether the issue is likely to be improved by cleaning, whether there is suspicion of an electrical fault, or whether an on-site inspection should be prioritized.

Table of Contents

• Do not attribute decreases in power generation after rainy weather solely to the weather.

• Change 1: Observe how power generation returns relative to solar irradiance

• Variation 2: Observe variability per string and per circuit

• Change 3: Observe how insulation, leakage, and shutdown history are displayed

• Change 4: Observe power generation trends caused by dirt removal and residual water.

• Practical considerations when reviewing data after rainfall

• To continuously manage a state of low power generation

Don't attribute a decline in power output after rain solely to the weather

Lower power generation during rainy weather is a natural phenomenon. Because solar power systems generate electricity by receiving solar radiation, generation decreases when clouds are thick, it is raining, or the sky is dark. Therefore, it is premature to conclude there is an equipment fault based solely on output on rainy days. However, if there are changes such as a slow recovery of output after the rain stops, only certain circuits showing low output, alarms or shutdown logs remaining, or output failing to rise as expected even during sunny intervals, there is reason to suspect causes other than the weather.

In practice, it is important not to look at post-rain data in isolation but to check it together with solar irradiance, temperature, power generation, electricity sales, per-circuit data within the equipment, shutdown history, and on-site visual observations. The mere fact of low generation cannot distinguish whether the cause is insufficient irradiance, dirty panels, shading, equipment shutdowns, or wiring/insulation issues. Especially after rain, multiple factors are likely to overlap, so adopting a time-series perspective to observe changes is essential.

The reason to pay attention after rainfall is that you can check the condition of equipment immediately after it has been affected by moisture. Degradation of connections, damage to cable sheathing, water ingress around junction boxes, poor drainage, and changes to racks or the ground—issues that are less likely to surface on dry, sunny days—can show signs after rain. Of course, data alone cannot definitively determine the cause, but it is useful as information for deciding "where to prioritize inspections."

Rain can both wash dirt off the panel surface and cause dirt to be left concentrated in certain areas. Some light dust and pollen may be washed away by rain, but bird droppings, oily grime, mud accumulated at the frame edges, and water marks that remain in low-tilt areas can often persist after rainfall. As a result, some installations see improved power generation after rain, while others may actually show a tendency toward partial decreases.

When power output is low, it is important not to try to explain it solely by a single factor such as rain. If you simply assume that output is low during rainy weather because solar irradiance is reduced, and that it should recover after the rain because the weather has improved, you may overlook early signs of anomalies. Post-rain data provides an entry point for distinguishing the portion affected by weather from the portion that reveals equipment-specific problems.

Change 1: Observe how power generation returns relative to solar irradiance

The first change to check after rainy weather is how much power output has recovered relative to solar irradiance. A determination that generation is low requires looking not only at the raw power numbers but also at how much irradiance there was during that time. Even if the rain has stopped, lingering clouds will make irradiance unstable and cause generation to fluctuate widely. Conversely, if irradiance has returned sufficiently but only the power output fails to increase, that should prompt suspicion of equipment-side causes.

In practice, checking the transitions before, during, and after rain on the same graph or the same time axis makes differences in the recovery patterns easier to see. If solar irradiance suddenly recovers after rain yet the power output rises slowly, possible causes include residual water on the panel surface, dirt, shading, delayed equipment restart, control settings, and the influence of protective functions. However, on days with rapid cloud movement the irradiance itself can change greatly in a short time, so one should avoid judging minute-scale up-and-down fluctuations as abnormal.

When examining how power output responds to solar irradiance, it is effective not only to compare absolute power values but also to compare with days under similar conditions. Find days in the same month, the same time of day, with similar temperatures and similar weather-recovery patterns, and check that generation after rainfall is not unusually low. Even when comparing with past clear-sky days, because solar altitude changes with the seasons, it is important not to rely solely on the same calendar day of the previous year or the same day of the previous month as a reference, but to match irradiance and operating conditions.

A common oversight when analyzing data after rainy weather is judging the overall condition of the generation system solely by the amount of electricity sold to the grid. The amount of electricity sold can vary due to self-consumption, battery charging and discharging, output control, or conditions on the receiving side. If you do not separately verify whether the power generation is low, the amount sold is low, or the amount of electricity exported has changed because of charging or consumption, you may misidentify the direction of the cause. This is especially important for systems with co-located batteries, since they may prioritize charging after rain, so it is essential not to confuse power generation with the amount of electricity sold.

Also, after rain, attention to temperature effects is necessary. Solar panels' output varies with temperature conditions. Rain can lower panel temperature, and when solar irradiance recovers afterward, there can be periods where generation efficiency appears better under certain conditions. If generation still does not increase, it is hard to explain by a simple temperature-induced output decline. Viewing solar irradiance, temperature, and generated power together makes it easier to judge whether the decrease is superficial or due to a change in equipment condition.

The point to check is not "generation is low even though the rain has stopped," but "does the power output recover at the same time that solar irradiance returns?" It is natural for power output to be low while cloudiness remains, but if power output does not return proportionally even after solar irradiance comes back, the next step is to look at data by circuit or by equipment. How output recovers after rainy weather is the first entry point for assessing the overall health of the installation.

Variation 2: Examining variations by string and circuit

When generation is low after rainy weather, it is important to look not only at the total output of the entire system but also at variations by string and by circuit. If only the overall generation is low, you cannot tell whether it is a temporary decline due to insufficient irradiance or a problem in some circuits. If multiple circuits drop in the same way, weather or irradiance conditions are likely to be a major factor; however, if only specific circuits are low, that provides grounds to suspect local factors such as panels, cables, connections, shading, soiling, or residual water.

Variations after rain can be more noticeable than on sunny days. For example, rows with poor drainage, surfaces with gentle slopes, locations where shadows from trees or structures are more likely at the low sun angle after rain, and lower-row panels that are prone to mud splatter can behave differently from other circuits. Even if changes appear small in the overall system average, clear differences may emerge when examined at the circuit level.

When assessing variations, simply looking for the lowest circuit is not sufficient. Check whether it was low before the rain, whether the difference suddenly widened after the rain, whether it is low only during specific time periods, or whether it is low throughout the daytime. If it has been continuously low since before the rain, existing shading, dirt, degradation, or poor connections may be involved. On the other hand, if the decline is noticeable only after rain, moisture effects, poor drainage, locations prone to water ingress, or temporary insulation degradation due to humidity may be related.

Comparing adjacent circuits is also effective. Comparing power generation trends between circuits that have the same orientation, the same tilt, and the same installation conditions makes it easier to determine whether an anomaly is present while keeping the effects of weather largely consistent. By contrast, simply comparing circuits with different orientations or tilts can lead to mistaking normal differences for anomalies. When comparing data after rainfall, choose circuits with similar installation conditions and examine changes during the same time of day.

Also, when looking at variations between circuits, pay attention to the measurement units and the timing of data acquisition. Depending on the monitoring device, the displayed items differ, such as per-circuit current, voltage, energy, and instantaneous output. If the data update interval is coarse, temporary changes caused by passing clouds can appear exaggerated. Checking not only instantaneous differences but whether a difference persists as a trend over a certain period can help reduce misjudgments.

What you should pay particular attention to in circuit-level data after rain is whether the circuit that is degrading is the same each time. If the same circuit's recovery is slow after every rain, the same string consistently shows reduced power generation, or alarms occur around the same junction box, on-site inspection should be given higher priority. If the cause is incidental cloud cover, the affected locations tend to change day to day, but if it is a weakness in the equipment, it will often reappear in the same place.

When investigating the causes of low power generation, discussing only the overall values tends to end with "the weather was bad." However, when you look by circuit, you sometimes find that while all are affected by the weather, some circuits behave differently. Post-rain data is a useful resource for finding those differences. If the decline in a specific circuit continues, a realistic workflow is to form hypotheses from remote data and then inspect the panel surfaces, wiring, connections, drainage conditions, and nearby shading on site.

Change 3: Observe How Insulation, Leakage, and Shutdown History Are Displayed

When power generation falls after rainy weather, one thing you must not overlook is the history related to insulation, leakage, and protective shutdowns. Even if there are no issues in sunny conditions, after equipment is wetted by rain the insulation condition can become unstable, causing alarms or shutdowns. If the periods of low power output overlap with the times when alarms or shutdowns occurred, you need to verify whether electrical protective actions — and not just reduced solar irradiance — may be affecting the power output.

In solar power generation equipment, to protect safety, the equipment may shut down or limit output when an abnormality is detected. If generation appears low after rainy weather, it could be that part of the equipment was actually shut down, that it took time to return to normal, or that an alarm condition persisted. Because the generation graph alone cannot reveal the reason for a shutdown, it is essential to check the alarm history, status history, and recovery history together.

After rainfall, what you should check is not just whether an alarm was issued. It is important to confirm when it occurred, which piece of equipment triggered it, how long it lasted, whether it automatically recovered, and whether manual intervention was required. Even if it automatically recovers in a short time, if the same alarm repeats every time it rains, there may be a spot in the equipment that is susceptible to moisture. Conversely, even a single occurrence in the history can have a large impact on generation output if the generation downtime is long.

Particularly, checking whether warnings appear immediately after rain begins, during periods of heavy rain, or some time after the rain has stopped makes it easier to form hypotheses about the cause. If a warning appears when the rain starts, suspect the effects of surfaces or connection points beginning to get wet. If it appears during heavy rain, check for flooding, drainage issues, and water ingress around junction boxes. If it appears after the rain has stopped, focus on places where water pools, areas that are slow to dry, and parts where moisture does not dissipate easily.

However, if there is any suspicion of insulation problems or leakage, it is dangerous for on-site personnel to touch equipment casually to check. After rain, equipment may be wet, which can increase the risk of electric shock or short circuit. If an alarm is confirmed in the data, inspection must be carried out after ensuring safety and in accordance with internal safety rules, the equipment’s operating instructions, and the judgment of the chief electrical engineer or other specialized personnel. You should not push ahead with on-site work simply to restore power generation quickly.

When investigating the causes of low power generation, it is dangerous to assume there is no equipment problem just because no alarms have been issued. Abnormalities not covered by monitored items, or minor unstable conditions that do not trigger alarms, can also affect power output. Therefore, alarm history is an important piece of information, but it needs to be reviewed together with generation data, circuit-level data, and on-site conditions. Even if there is no alarm history, if after rainy weather the same circuit alone shows low output, slow recovery, or an unnatural dip at specific times, it should be subject to on-site inspection.

Post-rain records of insulation, leakage, and shutdowns are important information for distinguishing the causes of reduced power generation from a safety perspective. If the issue is dirt or shading, cleaning or managing the surroundings may improve it, but if electrical anomalies are suspected, the priority of responses and measures to ensure worker safety will change. In practice, checking post-rain data is important not only as an economic matter of low generation but also as an indicator related to safety.

Change 4: Observe power generation trends due to dirt removal and water retention

Post-rain data can show both improvements from dirt being washed off the panel surface and declines caused by residual water or uneven dirt distribution. When it rains, some fine dust and pollen adhered to the surface are washed away, which can slightly improve power output. Conversely, if dirt accumulates where rainwater flows, mud remains along the frame edges, splash-back dirt adheres to lower panels, or marks remain after the water dries, power output may not recover as expected after the rain.

Especially for installations with gentle slopes and for installations surrounded by soil, gravel, farmland, unpaved roads, or trees, the way they become soiled after rain tends to vary. Rather than assuming that rain has cleaned them, it is important to check which rows improve after the rain and which remain low. If low power output is caused by soiling, the entire system will not necessarily improve evenly after rain. In some places, the rain may have merely moved the dirt, leaving it on the parts that affect power generation.

On the data side, comparing generation trends by circuit before and after rain makes it easier to infer the cleaning effect of the rain. If overall output was muted before the rain and power generation improved across multiple circuits afterward, surface soiling may have had some impact. Conversely, if only certain circuits remain low after the rain, the panels corresponding to those circuits may have persistent dirt, puddles, mud splashes, or localized shading. Of course, you should not conclude soiling based on data alone; it is necessary to combine the data with on-site photos and visual inspections.

Residual water is also an element to check in post-rain data. When water remains on the panel surface, even for a short time, it can change how light enters and may affect output. It usually resolves as the panels dry, but if there are locations where water tends to remain, it becomes a prompt to check for dirt along the frame edge, panel tilt, the condition of the racking, drainage routes, and warping of the mounting surface. If, after rain, certain circuits consistently take longer to recover to the same time of day, residual water or slow drying should be considered as one hypothesis.

Also, the appearance of shadows can change after rainy weather. On mornings and evenings after rain, the sun angle is low, and the shadows of nearby trees, fences, utility poles, buildings, weeds, and so on stretch long. Reflections from wet ground and panel surfaces can cause effects to appear in the data that are difficult to perceive by visual inspection. If periods of low power generation are concentrated in the mornings and evenings, it may not be simply the effect of rain, but rather the overlap of reduced post-rain solar irradiance and shadows.

On-site photographic records are useful when checking dirt removal and water residue. If you have photos taken from the same position, same angle, and same time of day, it becomes easier to compare how the panel surface and the surrounding environment changed before and after rain. Even when judging based solely on remote data, cross-referencing with previously taken inspection photos and cleaning records makes it easier to explain a reduction in power generation. If cleaning was performed, keeping the data from before and after cleaning will serve as reference material for future decisions.

What you should avoid when assessing soiling after rain is assuming that "cleaning is unnecessary because the rain washed it away." Some dirt is removed by rain, but other types are not easily washed away. Also, the effects of soiling may not appear uniformly across the entire installation and can be concentrated on certain panels or circuits. If low power generation persists, it is practical to separate the amount of improvement after rain from any remaining downward trend and consider the need for cleaning, weed control, drainage improvements, or an on-site inspection.

Practical considerations when checking data after rainfall

When using post-rain data to confirm a decline in power generation, it is important to first align the comparison conditions. Simply comparing before and after the rain can leave solar irradiance, temperature, cloud cover, wind, season, and equipment operating status too different to make a correct judgment. You should select time periods with similar solar irradiance conditions, circuits with the same orientation, the same equipment groups, and the same monitored items, and adopt an approach of narrowing down the causes of change one by one.

What you should be especially careful about is not being swayed by short-term data. After rain, clouds tend to move quickly and solar irradiance can suddenly rise or fall. For that reason, looking only at instantaneous output can make even normally functioning equipment appear to show large fluctuations. To determine whether generation is low, it is important to look not only at instantaneous values but also at averages over a certain period, daily cumulative values, and comparisons under the same conditions.

Be careful about gaps in monitoring data. When data are missing during rainy weather or when communications are unstable, the generated output can appear lower than it actually is, or it may look as if the system has stopped. If you do not distinguish between missing data and a true lack of generation, you may be led to unnecessary on-site responses. It is important to check not only the numbers on the monitoring screen but also the timestamp, update interval, communication status, and the instrument-side history.

When judging a drop in power generation after rainy weather, it is also necessary to take into account differences in equipment capacity and installation conditions. Even when comparing adjacent installations, differences in panel capacity, orientation, tilt, shading conditions, equipment configuration, and start of operation can naturally lead to differences in power generation. Rather than simply comparing absolute generation amounts, examining generation per unit capacity and trends among circuits with similar conditions improves the accuracy of the assessment.

When conducting on-site inspections, prioritize safety management. After rain, roofs, slopes, around mounting racks, and around cables can be slippery, and mud or puddles may remain. Even when you want to quickly determine the cause of low power generation, avoid entering areas unnecessarily or making inadvertent contact with wet electrical equipment. If electrical alarms are present, it is important to seek the judgment of personnel with specialized knowledge and follow the necessary procedures.

How records are kept also has great significance in practical work. For days when power generation was low after rain, keeping records of the weather, solar irradiance, alarm history, the circuits that declined, on-site photos, actions taken, and the restoration time will allow for quicker judgment when a similar drop occurs next time. Rather than treating it as a one-off anomaly, ensuring that you can check whether the same trend appears after each rainfall will help prevent recurrence.

Checking data after rain is not a task intended to immediately conclude a drop in power generation. Rather, it is a process to organize and prioritize multiple possible causes — weather effects, soiling, shading, drainage, electrical protective operations, and communication failures. By deciding the order in which to review the data, you can reduce variation in judgments among personnel and cut unnecessary on-site responses.

To Continuously Manage Low Power Generation

To address low power output, it is important not to treat post-rain periods as special cases but to manage them in connection with normal-condition data. If you routinely understand the baseline generation trends for each piece of equipment, it becomes easier to determine how much recovery after rain constitutes the normal range, which circuit’s recovery is slow, and which alarms are recurring. Conversely, if you look only at post-rain data without a baseline from normal conditions, it becomes difficult to assess the extent of the decline.

What operations personnel should first put in order is a state that allows comparison. If power generation, solar irradiance, temperature, amount of electricity sold, per-circuit data, outage history, inspection photos, and cleaning history are managed separately, you have to re-gather information every time you look for the cause. Because the drop in power generation after rainy weather changes over time, the longer it takes to collect information, the harder it becomes to understand the relationship between the on-site conditions and the data.

One major advantage of post-rain checks is being able to immediately see whether "it was the same last time." If the same circuit in the same installation shows a drop after similar rain, you are more likely to suspect an equipment-specific cause rather than a coincidental weather effect. Conversely, if the locations or times of the drops vary each time, cloud cover or temporary fluctuations in solar irradiance may be the main cause. In this way, having a continuous history makes it easier to rationally prioritize inspections.

Also, post-rain data can be used to review maintenance plans. If there are places where dirt remains after every rain, they can provide a basis for considering cleaning or drainage improvements. If certain equipment tends to trigger alarms after rain, inspections of connection points and the condition of waterproofing should be included in the plan. If a tendency for lower power generation only in the morning and evening becomes stronger after rain, that can be an opportunity to reassess changes in shading and the management of surrounding vegetation. It is important not to let data end as mere records but to link them to subsequent maintenance decisions.

When power generation is low, the person in charge wants to know the cause quickly. However, if you pin the cause on a single factor, countermeasures can miss the mark. After rainy weather, checking four changes in sequence—the return of solar irradiance, variations by circuit, insulation, leakage and shutdown history, and dirt being washed off and residual water—can improve the accuracy of your assessment. Form hypotheses from the data, connect them to the necessary on-site checks, and keep records to inform the next assessment; creating this workflow is the practical response to reduced power generation.

Going forward, in managing solar power generation facilities it will be increasingly important not to rely solely on on-site inspections but to use data to detect signs of abnormalities early. By continuously tracking changes in power output and not missing timings when equipment weaknesses are likely to appear—such as after rainy periods—you can reduce the neglect of declines and delays in response. If you want to organize the reasons for low power generation with data rather than intuition, it is important to establish a management system that allows centralized review of power output, solar irradiance, per-circuit data, alarm histories, and inspection records, and to continuously monitor trends for each facility.