Are low power generation days abnormal? 5 ways to distinguish weather, dirt, and faults

When managing solar power generation equipment, daily output can sometimes vary significantly. If a system has been generating well until yesterday but suddenly seems to be producing less today, it's natural to suspect equipment failure or installation defects. However, a day with low output does not necessarily indicate an abnormality. Solar power generation fluctuates due to multiple factors such as solar irradiance, ambient temperature, clouds, rain, shading, soiling, and the condition of the equipment. What matters is not simply looking at the output numbers, but distinguishing whether the decrease is a natural decline due to the weather, a temporary reduction caused by soiling or shading, or an abnormality that could lead to equipment failure.

Table of Contents

• Reasons not to immediately judge a day with low power generation as abnormal

• How to distinguish 1: Compare weather and solar irradiance under the same conditions

• How to distinguish 2: Determine declines caused by soiling or shading by time of day and location

• How to distinguish 3: Detect panel or wiring abnormalities by differences between strings or systems

• How to distinguish 4: Confirm stoppage or control of power conversion equipment via displays and history

• How to distinguish 5: Isolate discrepancies in measurement conditions and management data

• Practical on-site procedures to check when power generation is low

• Recording and inspection framework to prevent leaving generation declines unaddressed

• Summary: On days with low power generation, assess causes separately

Why we don't immediately consider a day with low power generation to be abnormal

The power output of solar power generation is not the same every day, even when the installed capacity is the same. Because output changes greatly depending on the amount of sunlight hitting the panels, results differ on sunny, cloudy, and rainy days. Furthermore, even on the same sunny day, output can vary due to thin clouds, yellow sand (Asian dust), reflections from the surroundings, temperature, and the presence or absence of wind. For this reason, judging that output is low based on a single day can lead to mistaking normal variation for an abnormal condition.

Operations personnel should be careful not to simply compare power output figures with the previous day or the previous month's average. In solar power generation, it is natural for output to increase on days with high solar irradiance and decrease on days with low irradiance. If the previous day was clear and the current day is partly cloudy, power output will be lower even if the equipment is operating normally. Also, in winter, because daylight hours are shorter and the solar altitude is lower, output tends to be lower compared with summer. Comparing without accounting for seasonal differences can lead to mistaking normal seasonal variation for a fault.

On the other hand, there are declines in power generation that cannot be explained by weather alone. If only part of the nearby equipment is not generating, if the readings from a specific circuit are extremely low, if generation suddenly stops on a sunny day, or if there is a large difference compared with adjacent equipment under the same conditions, you should suspect soiling, shading, wiring faults, equipment shutdowns, or configuration errors. In other words, when you find a day with low generation, instead of immediately assuming an abnormality, it is important to first separate and check natural causes and equipment-related causes.

To correctly assess a drop in power generation, you need to look not only at a single day's generation but also at the weather, solar irradiance conditions, generation curves by time of day, output by system, historical trends, and inspection records. The total generation displayed on the management dashboard alone may be insufficient to identify the cause. By breaking down the phenomenon of low generation into where, when, to what extent, and how it is decreasing, the locations that require inspection become clear.

Identification Method 1: Compare weather and solar radiation under the same conditions

The first point in identifying the cause of low power generation is to check the weather and solar irradiance conditions. Because solar power generation depends heavily on solar irradiance, low output on cloudy or rainy days is not necessarily abnormal. When clouds are thick, direct sunlight is reduced and generation falls compared with clear days. Even thin cloud cover can mean that solar irradiance is more insufficient than it appears. Even if the sky looks bright, if the irradiance reaching the panel surface is weak, power generation will have difficulty increasing.

In practice, when assessing power output, it is important not to rely solely on broad weather categories such as "sunny", "cloudy", or "rainy". Even for days classified as sunny, the power output will differ between a day that is sunny only in the morning and becomes cloudy in the afternoon, and a day that is clear all day. Also, on days when clouds are passing, the generation curve fluctuates in fine detail. Even on days with low output, if the generation curve naturally rises and falls in line with changes in weather, the cause may be solar irradiance variability rather than equipment malfunction.

When comparing, it's basic not to compare only with the previous day but with days that have similar weather and seasons. For example, comparing with a sunny day in the same month, a day with similar sunlight conditions, or the power generation status of nearby facilities makes it easier to judge whether something is abnormal. Rather than simply looking at "lower than yesterday" or "lower than last month," it's important to check whether the power generation is reasonable given the solar irradiance. If records of solar irradiance are available, examining the relationship between power generation and irradiance makes it easier to distinguish whether a decrease is due to weather or to the equipment.

Ambient temperature also affects power generation. Solar panels tend to produce more electricity with stronger sunlight, but their output generally decreases as panel temperature rises. For this reason, even on hot summer days with intense sun, generation efficiency can be lower than on clear spring or autumn days under certain conditions. If you notice low power generation on a hot day, the decrease in output may be due to elevated temperature rather than equipment failure.

Snow cover, frost, condensation, yellow sand, pollen, and dust stirred up after strong winds also affect solar irradiance conditions. On the day after rain, dirt may be washed away and power output may recover, while contaminants in the rainwater can dry into streaks and locally hinder power generation. To determine whether a decline is due to weather, you need to check not only the power output but also the state of the sky that day, the temperature, the weather in the days before and after, and the condition of the panel surface.

The characteristic of weather-related factors is that they tend to affect the entire facility relatively uniformly. If the plant's overall power generation falls in line with the weather and no particular system is unusually low, first suspect natural causes. Conversely, if only some circuits are low under the same solar irradiance conditions, if there is a sharp drop only at a specific time, or if output plateaus despite clear skies, you should proceed to check for factors other than the weather.

Identification Method 2: Judge Decreases Caused by Dirt or Shadows by Time of Day and Location

Dirt and shading on the panel surface are also common causes of reduced power output. Because solar panels generate electricity when sunlight strikes their surface, dirt or shadows will lower the amount of power produced. Although dirt and shading may seem like minor issues at first glance, they can affect power generation depending on their extent and location. In particular, if part of a panel is shaded, the shading can affect not only the shaded area but also the power output of panels in the same circuit, so caution is required.

Performance declines caused by soiling can be identified by visually inspecting the panel surfaces. Bird droppings, fallen leaves, dust, yellow sand (Asian dust), pollen, dust from nearby construction, water stains, and contact with weeds can all cause reduced power generation. If the entire surface is lightly soiled, power output across the whole plant may gradually decrease. In contrast, localized soiling such as bird droppings or fallen leaves tends to manifest as a decline concentrated on specific panels or circuits.

Declines caused by shading are characterized by the tendency for power generation to vary by time of day. If power generation is low only in the morning, suddenly drops only in the evening, or the decline becomes noticeable in certain seasons, check for shadows from surrounding buildings, trees, utility poles, mounting racks, fences, and adjacent equipment. Because the sun’s position changes with the seasons, shadows can lengthen and affect power generation in winter even if there is no problem in summer. Shadows are especially likely to be longer than expected during periods of low solar altitude.

To distinguish dirt from shading, it is useful to look at the power generation curve by time of day. If dirt is widespread, power generation tends to be slightly lower throughout the daytime. With shading, output drops during the periods when the shadow occurs and may recover after that period. For example, if a particular string is low only in the morning and recovers around noon, it may be a morning shadow. Conversely, if the same string is low throughout the daytime, you need to check for dirt, panel faults, wiring faults, and so on.

During on-site inspections, it is important to view the panel surface at the times when power generation is low. If you inspect at a different time on the day after output has dropped, you may not be able to find the cause of the shading. Because shadows move over time, check the monitoring data to identify the times when the decline occurs, and observe the site during those times to more easily pinpoint the cause. When taking photographs, it is also important to record the time of the decline, the azimuth, the extent of the shadow, and nearby obstructions so they are clear.

Dirt and shading are not equipment failures, but if left untreated they can cause a sustained decrease in power generation. During periods when weeds grow, when fallen leaves increase, after strong winds or construction, or in locations with frequent bird damage, a system for regular inspections is necessary. Also, when performing cleaning or weeding, safety should be prioritized, and the work must be carried out in a way that does not damage the panels or wiring. Performance losses due to dirt and shading are relatively easy to address if detected early, but if overlooked they can lead to long-term generation losses.

Identification Method 3: Detect Panel and Wiring Abnormalities by Differences Between Systems

When there is a reduction in power generation that cannot be explained by weather or soiling alone, check for abnormalities around the panels and wiring. Photovoltaic systems generate power with multiple panels connected in series and parallel, routed through connection boxes and power conditioners. Therefore, if you only look at the total output of the entire system, it can be difficult to notice localized faults. To distinguish a drop in power generation, it is important to check not only the overall system figures but also output differences by string, by circuit, and by device.

If there are abnormalities in panels or wiring, the power generation of a particular system may be lower despite the same solar irradiance conditions. For example, if only one of several circuits has an extremely low output, if part of the generation curve looks abnormal compared with adjacent equipment, or if the output is interrupted midway on a sunny day, suspect panel damage, poor connector contact, a wiring break, a fault inside the junction box, or the operation of protective devices.

When examining differences between systems, you need to consider not just simple generation output but also installed capacity and installation conditions. If you compare systems with different capacities as-is, you may mistakenly interpret a normal difference as an anomaly. Also, if orientation or tilt differ, output patterns can vary even at the same time of day. As a rule, choose comparison targets whose capacity, orientation, tilt, and surrounding conditions are as similar as possible. If, despite similar conditions, only a particular system shows lower output, the case for suspecting equipment-related causes becomes stronger.

There are panel abnormalities that can be identified by appearance and others that are difficult to judge from appearance alone. Cracks, discoloration, burn marks, delamination, frame deformation, and wiring damage may be detectable by visual inspection. On the other hand, internal degradation or poor connections may not be apparent from appearance alone. Therefore, even if a visual inspection finds no abnormalities, if there is a clear discrepancy in power generation data, electrical measurements or specialized inspections may be necessary.

Faults in wiring and connection points can lead not only to reduced power generation but also to safety issues. Poor contact can cause heating and should not be ignored. In particular, if a drop in power output is accompanied by abnormal shutdowns, warning indications, a burnt smell, discoloration around junction boxes, or breaker operation, you should not force continued operation and must perform appropriate checks. If it is difficult for on-site personnel to make a judgment alone, it is important to consult the installer or maintenance staff and clarify the scope of inspection.

Daily accumulation of data is useful for detecting abnormalities in panels and wiring. By routinely checking the power output and generation curves for each system, you can determine when a discrepancy began, whether there was a sudden drop, or whether it has been declining gradually. A sudden drop often points to causes such as a broken wire or equipment shutdown, while a gradual decline calls for checking factors like dirt, degradation, or expanded shading. To distinguish the causes of low power generation, you need to look at changes over time as well as the current figures.

Identification Method 4: Confirm converter shutdowns and controls using displays and logs

Stops or output control of conversion equipment, such as power conditioners, are also important causes of low power generation. Even if the solar panels are generating power, if the conversion equipment is stopped or a protective function is active, the values recorded as power generation will be low. In practice, people tend to focus only on the panels and postpone checking the condition of the conversion and protective equipment. However, when power generation suddenly drops or is interrupted for a certain period of time, it is necessary to check the conversion equipment’s display and operating history.

When power generation decreases related to power conversion equipment, characteristic patterns can appear in the generation curve. Examples include output suddenly dropping to near zero despite sunny conditions, output not rising above a certain level, repeated stops and restarts during the daytime, or low output only in the areas connected to specific converters. In such cases, possible causes include protective actions due to temperature rise, control based on voltage conditions, grid-side conditions, internal equipment faults, and problems with configuration values.

If warning or error logs remain on the display or in the monitoring data, compare their occurrence times with the times when power generation declined. If the time of the power drop matches the warning timestamp, the likelihood that equipment operation is the cause increases. However, the absence of a displayed warning does not prove there is no equipment problem. Communication failures, temporary control actions, or insufficient checking of logs can make the cause difficult to identify. Rather than relying solely on the display, it is important to check the power generation curve, output by grid, and on-site operating conditions together.

The surrounding environment of the power conversion equipment should also be checked. If the equipment’s intake or exhaust vents are blocked, objects are placed around it, it is exposed to direct sunlight or high-temperature conditions, or dust has accumulated inside, the equipment temperature is more likely to rise. If output is limited at high temperatures, this can lead to a situation where generation does not increase despite clear skies. If generation plateaus during summer daytime, check not only solar irradiance but also the equipment temperature and the installation environment.

Also, depending on the equipment, output may be controlled according to external conditions. In this case, the reduction in power generation may be due to operational control rather than equipment failure. To determine whether the decrease is caused by control or by equipment malfunction, check management records, operating history, whether control commands were issued, and the status of other equipment under the same power-receiving conditions. If control is the cause, inspecting the power generation equipment itself may not reveal any abnormalities.

When inspecting around conversion equipment, you must take adequate safety precautions. Tasks that involve opening the interior for inspection or performing electrical measurements may require specialized knowledge or qualifications. It is practical for on-site personnel to first check the display, warning history, operating status, appearance, surrounding environment, and communication status, and, if necessary, hand the matter over to maintenance personnel. To quickly identify the cause of reduced power output, it is important to record which equipment showed what indications and when.

Identification Method 5: Isolate Discrepancies in Measurement Conditions and Management Data

When you feel that power generation is low, you tend to suspect a problem with the equipment itself, but in reality the cause can sometimes be the measurement conditions or how the management data is being viewed. For example, if the aggregation period on the monitoring screen is shifted, the date changeover time is different from what was assumed, some data are missing, the latest values are not reflected due to communication delays, or the registered equipment capacity value is incorrect, the generation can appear low. Because there may be no actual issue with the equipment and the low reading may only exist in the data, checking the measurement conditions is indispensable.

Particular attention should be paid to how the comparison targets are aligned. When comparing power generation, you need to be clear whether you are looking at daily, monthly, or hourly figures. For daily generation, the day's aggregation period must be the same to make a valid comparison. If you compare today's generation in the morning with yesterday's full-day total, it will of course appear lower. Similarly, if you compare the current month's generation partway through the month with the entire previous month, it will look deficient even though there is no anomaly.

Loss of communication data is also a factor that can easily be mistaken for a decrease in power generation. If the system is generating power on site but values fail to reach the monitoring screen due to communication problems, the displayed generation may appear low on the screen. In such cases, checking on-site cumulative totals or records stored on the equipment may reveal discrepancies with the monitoring data. Do not judge based solely on the numbers on the monitoring screen; by cross-checking the on-site equipment displays and other records, you can distinguish between an equipment malfunction and a communication failure.

Mistakes in registering equipment capacity or system configuration also make judgment difficult. If the actual equipment capacity differs from the capacity in the management data, evaluations of expected power generation and generation efficiency will be skewed. Also, if management information is not updated after expansions, renovations, or equipment replacements, correct comparisons with past data cannot be made. Before concluding that power generation is low, it is important to confirm that management information matches the equipment capacity, circuit configuration, number of devices, start date of operation, outage periods, renovation history, and so on.

Discrepancies in measurement conditions can sometimes arise from differences in awareness between the field and the office. If part of the equipment was taken offline for inspection on site but management compared it against normal operation, the power generation may appear to be low. Conversely, if an abnormal event occurs on site but that information is not communicated to management, it may be overlooked as merely bad weather. To correctly assess a drop in power generation, a system is needed to share not only the data but also the operational status and work records.

Thus, the causes of low power generation are not limited to equipment failures. By checking the data aggregation conditions, communication status, registration information, outage records, and comparison methods, you can avoid unnecessary on-site inspections and incorrect judgments. Especially when managing multiple power plants, making uniform judgments based only on the figures on the management screen can make isolating the cause take longer. When you find a decline in generation, it is important to first check whether “the generation is truly low” or “it only appears to be low.”

Practical procedures to check on-site when power output is low

When it is found that power output is low, it is important not to perform ad hoc inspections starting from whatever comes to mind, but to decide on an order of checks and proceed. The first thing to check is the extent of the power reduction. The suspected causes change depending on whether the entire installation is low, only part of the system is low, or only specific time periods are low. If it is low overall, possible causes include weather, solar irradiance, output control (curtailment), or a total shutdown. If only part is low, possible causes include soiling, shading, wiring, connections, or equipment-level faults.

Next, check the power generation curve. The power generation curve is important information for distinguishing causes. For declines due to weather, output may fluctuate with cloud movement. For declines due to shading, there may be drops at specific times of day. In the case of equipment shutdowns, output may suddenly fall to near zero. If the output plateaus at a constant value, check temperature, controls, settings, equipment capacity, and so on. Rather than looking at a simple daily total, examining changes by the hour makes it easier to infer the cause.

On-site inspections should start by observing only those areas that can be checked safely. Check the panel surface for dirt, fallen leaves, bird droppings, weeds, shading, nearby obstructions, the condition of the mounting racks, the appearance of wiring, and the labels or indicators on junction boxes and power conversion equipment. Power generation equipment is electrical equipment, and forced contact or disassembly is dangerous. Even when an abnormality is suspected, it is necessary to distinguish between what on-site personnel can handle and what should be referred to specialists.

It's also important to correlate the times when power generation is low with on-site conditions. Monitoring data may show a decline in the morning, but when you inspect the site in the afternoon you may not be able to identify the cause of shading. Conversely, if the drop occurs only in the evening, you need to check shading and the surrounding environment during that time. When keeping site photos, record the date and time, shooting direction, the equipment shown, and the extent of any shading or soiling so these are clear. By creating records that stakeholders can evaluate later, you can speed up identification of the cause.

Also, during the on-site inspection, check the operation logs and work records. If there were maintenance shutdowns, mowing, cleaning, equipment replacements, nearby construction, power outages, warnings, or the like, they may be related to the drop in power generation. If on-site work records are not linked to the generation data, identifying the cause takes longer. When you find a day with low generation, it is important to make a habit of checking what happened on that day.

If the cause cannot be pinpointed to a single factor, isolate possibilities in order of likelihood. Check in this order whether the issue can be explained by the weather; whether there is a difference compared with other equipment under the same conditions; whether the problem is concentrated in particular circuits; whether the power generation curve has an unnatural shape; whether dirt or shadows can be confirmed on site; and whether there are warnings in the equipment history—doing so reduces the chance of oversights in the investigation.

In practice, it is efficient to verify a low power-generation phenomenon by separating it into the factors of weather, dirt, shadow, wiring, equipment, and data.

Recordkeeping and Inspection System to Prevent Overlooking Declines in Power Generation

Even if there are days with low power generation, if it’s only a single day it may be overlooked as a weather-related effect. However, if similar drops occur repeatedly or if only specific equipment continues to produce low output, it is necessary to check the cause promptly. Leaving a decline in generation unaddressed not only reduces electricity sales and self-consumption, but can also lead to missed equipment faults. Even a small decline, if it continues for a long time, can result in significant losses.

Daily records are essential for early detection of declines in power output. Not only recording the power output itself but also keeping records of the weather, temperature, inspection details, dates when cleaning or weeding were performed, equipment warnings, downtime, and changes in the surrounding environment makes it easier to isolate the causes. In particular, without knowledge of past normal conditions, you cannot determine whether current power output is low. Having baseline data from normal operation is the first step in detecting anomalies.

When managing multiple power plants or a large site, on-site location information and equipment-specific management data also become important. If it is unclear which area or which circuit is experiencing a decline, where a site photo was taken, or what scope cleaning and inspections covered, reconfirmation takes time. To find the cause of a drop in power generation quickly, it is effective to link data with site locations and ensure that stakeholders can view the same information.

In an inspection regime, it is easier to operate if routine checks and specialized inspections are treated separately. Routine checks should verify changes in power generation, the power output curve, warning displays, the presence of dirt or shadows, and changes in the surrounding environment. Specialized inspections involve electrical measurements, internal equipment inspections, checks of connection points, and deterioration diagnostics. If you try to have on-site personnel handle everything, the burden becomes large and items may be overlooked. It is important to separate abnormalities that should be detected routinely from those that require specialized judgment.

Also, deciding in advance the criteria for when a day with low power generation should be considered will speed up response. For example, having a shared internal view that flags for inspection cases such as when there is a clear difference compared to other systems under the same conditions, a sudden shutdown on a sunny day, a decline that continues for several days, or when there is a warning history will reduce variation in judgments. If responses vary depending on the person in charge, abnormalities may be missed and unnecessary on-site inspections may increase.

Declines in power output are an important sign of the equipment’s condition. If they are natural fluctuations caused by the weather, the problem is minor, but dirt, shading, wiring, equipment faults, or data management issues may be hidden. Viewing daily generation data as a flow rather than as isolated points, and linking it with on-site information when making judgments, makes it easier to identify causes and implement countermeasures.

Summary: On days with low power generation, determine the causes separately

Just because there are days with low power generation does not mean you should immediately conclude there is an equipment malfunction. Solar power generation is heavily influenced by weather and solar irradiance conditions, so it is natural for output to decrease due to cloudy skies, rain, thin clouds, high temperatures, and seasonal variations. However, if there is a decline that cannot be explained by weather alone, you should check in order for soiling, shading, faults in panels or wiring, stoppage or control of conversion equipment, and discrepancies in measurement conditions or management data.

The basic principle for distinguishing issues is not to judge based solely on total power generation. By looking at the generation curve by time of day, output differences by system, comparisons with historical data, on-site dirt and shading, equipment displays and histories, and work records together, it becomes easier to determine whether there is an anomaly. In particular, it is important to first distinguish whether the entire installation is underperforming or only part of it. A widespread decline suggests weather or control issues, while a decline in only part suggests dirt, shading, wiring, or equipment-level problems.

When responding to a drop in power generation, what you should avoid is leaving it unaddressed without confirming the cause and assuming it is a fault without sufficient isolation. If the variation is natural, excessive measures are unnecessary, but if there is actually an equipment abnormality, prompt action is required. To distinguish correctly, it is important to record daily data, link it with on-site photos and inspection histories, and establish a system in which stakeholders can make judgments using the same criteria.

In managing solar power facilities, you need not only to know that generation is low but also to be able to read the underlying causes. If you can distinguish between weather, soiling, and faults, you can reduce unnecessary on-site inspections while more quickly identifying the anomalies that truly require action. To efficiently grasp declines in generation, it is effective to organize solar irradiance conditions, time-of-day data, output by system, site photos, and inspection history, and to establish operational procedures that sequentially isolate the causes.