Is Low Power Generation a Malfunction? 5 Ways to Distinguish Weather Variations from Abnormalities

When you notice low power generation, the first concern is whether the equipment might be malfunctioning. Solar power generation changes day to day depending on multiple factors such as weather, season, temperature, shading, dirt, and equipment condition, so low generation alone cannot be taken as evidence of a malfunction. On the other hand, assuming it is due to the weather and leaving it unchecked can delay the discovery of abnormalities in the panels, wiring, power conditioner, or measuring instruments.

For practitioners, the important thing is not to judge a decline in power generation by instinct, but to systematically distinguish whether the decline can be explained by weather differences or whether it should be suspected as an equipment abnormality. This article explains five checks to perform before concluding a failure, aimed at site managers and inspection personnel who are searching "low power generation" to investigate the cause.

Table of Contents

• When power generation is low, look at the comparison conditions rather than a single day.

• Confirm whether the decline can be explained by weather differences using solar irradiance and sky conditions.

• Separate the changes in power generation due to seasonality and temperature.

• Check for degradation caused by on-site environmental factors such as shadows and dirt.

• Identify potential anomalies from differences by system and by equipment

When power output is low, look at comparative conditions rather than a single day

When you feel the power generation is low, the first thing you should check is whether you're basing your judgment only on that day's figures. Solar power generation varies greatly between sunny and cloudy days. Even on cloudy days, the results differ depending on whether it's thin cloud cover, thick clouds that remain for long periods, or if it's raining. Therefore, simply viewing it as "lower than yesterday" or "lower than last week" is insufficient grounds to determine whether there is a fault.

In practice, it is important to match the conditions when making comparisons. First, when comparing with past data from the same power plant, look for days in the same season with similar weather and similar solar irradiance conditions. For example, if you compare a cloudy day this month with a clear day last month, a large difference in power generation is not necessarily unnatural. Conversely, if the weather is similar and the daytime cloud cover and solar irradiance trends are also close, but one particular day shows a large drop, it is difficult to explain that by weather differences alone.

Also, when checking power generation, confirm not only the daily total but also the trend by time of day. If generation is generally low from morning through evening, the cause is more likely weather or solar irradiance conditions. Conversely, if output suddenly falls only around midday when it would normally rise, appears to stop at a certain time, or remains low without recovering, these are clues that point to a possible equipment-side malfunction.

A useful basis for comparison is not limited to data from the previous day or the same month of the previous year. Outputs from nearby facilities of a similar scale, other systems within the same site, and other power conditioners within the same power plant are also helpful references. If, among multiple systems that should be subject to the same weather, only some show reduced generation, there may be a problem specific to those systems. Conversely, if the entire site shows similarly low generation, it is necessary to look broadly at weather, solar irradiance, grid-side constraints, and overall environmental changes at the site.

What you should be careful about is not to immediately conclude that a drop in power output is a "fault." In solar power generation, output can decrease even on sunny days because of passing clouds, yellow dust, pollen, snow accumulation, rising temperatures, or nearby shading. However, it's also dangerous to dismiss it too readily as "just a common occurrence." The first step in making a judgment is to place the decreased value back under the same conditions and check whether it falls within the range of natural variation.

In consultations about low power generation, it is common for the discussion to proceed with the question "low compared to what?" left ambiguous. Whether the comparison is to the same day last year, the previous month's average, simulation values, or the forecast value on the monitoring screen changes which points should be examined. Operations staff should clarify the comparison baseline and then sequentially isolate weather differences, seasonal differences, equipment differences, and measurement differences; doing so reduces both unnecessary anxiety and oversight.

Check whether the decline can be explained by weather differences using solar irradiance and sky conditions

A common cause of low power output is the weather. Because solar power generation produces electricity from solar radiation, on days with low solar radiation the power output also decreases. Power output varies under sunny, partly cloudy, cloudy, and rainy conditions, and especially on days with thick cloud cover or during rain it can be significantly lower than on sunny days. Therefore, before suspecting a malfunction, it is essential to check the sky conditions and the level of solar radiation for the day.

However, you should avoid judging based solely on the weather forecast display. Even when it says "sunny," thin clouds may actually be widespread, clouds may increase in the afternoon, or local weather may change along mountain slopes or coastlines. Conversely, even if it says "cloudy," strong sunlight can sometimes come through for a short time. When checking for a drop in power generation, it is important to look not at the forecast category but at how much solar radiation there actually was around the plant during each time period.

If possible, check the pyranometer and meteorological observation data together with power generation. If irradiance is low during a given period and power generation is also low, it is easier to attribute the change to weather. If irradiance is increasing but power generation does not follow, suspect equipment-side limitations or malfunctions, shading, soiling, measurement errors, or other causes. In particular, by viewing the irradiance waveform and the power output waveform side by side, it becomes easier to tell whether a decrease is a natural decline or an anomalous drop.

If a drop in power generation is caused by weather variability, changes in output tend to be relatively smooth or fluctuate in step with cloud movement. When clouds block the sun, generation falls, and when the clouds pass, it recovers. If such fluctuations occur uniformly across the entire plant, they are likely due to weather. Conversely, if only a particular system remains low and does not recover, if only certain power conditioners have stopped producing output, or if only part of the plant fails to recover even after solar irradiance returns, the situation is difficult to explain by weather alone.

Also, after rain has cleared or a prolonged period of cloudy weather, power generation may fall short of expectations even if it appears sunny. This is because atmospheric water vapor, thin clouds, haze, yellow dust, pollen, and so on can weaken solar irradiance. Even if it looks bright, if the direct solar irradiance is insufficient, power output tends not to increase. When you feel on site that "it's sunny but the power output is low," it's especially important to check not only the apparent brightness but also the solar irradiance and the actual performance on days with the same conditions.

When checking weather-related differences, it is also essential to consider the time of day. In the morning and evening the sun’s altitude is low, so lower power output is natural. Power output tends to be higher around midday, but during hot summer periods output can be suppressed due to higher panel temperatures. If output is low only in the morning or only in the afternoon, you need to check not only cloud movement but also shadows from mountains, buildings, trees, and surrounding equipment.

When determining whether a variation can be explained by weather, you should not rely solely on whether the sky was cloudy; instead, check whether the drop in power generation corresponds to changes in solar irradiance. If the entire plant declines similarly and output returns as irradiance recovers, the likelihood of a fault is relatively low. Conversely, if irradiance conditions have recovered but output does not, only some systems are clearly lower, or output remains flat after a sharp drop, you should proceed to the next checks.

Separating changes in power generation due to season and temperature

When power generation is low, the effects of season and temperature are easy to overlook alongside the weather. Solar power generation is often assumed to always produce at maximum when solar irradiance is strong, but in reality it is affected by solar altitude, sunshine duration, ambient temperature, panel temperature, and other factors. In particular, in summer solar irradiance may be strong while panel temperatures tend to rise, and under some conditions output may not increase as much as expected. In winter, lower ambient temperatures can be favorable for conversion efficiency, but shorter sunshine hours and lower solar altitude mean that the daily total generation is influenced by seasonal conditions.

To isolate seasonal variations, it is effective to compare the same month in the previous year or past data from the same season. For example, during the rainy season and the autumn rainy period, cloudy and rainy weather tends to persist, and monthly power generation may appear lower than planned. Conversely, clear days in spring and autumn are less likely to see extremely high temperatures, and stable generation may be observed. Because which times of the year tend to see increased generation varies by region and installation conditions, it is important to understand past trends for each of your company’s installations.

When considering the effects of temperature, be aware not only of the ambient temperature itself but also of the rise in the panel surface temperature. Solar panels heat up when exposed to sunlight, and under certain conditions their output can decline. Therefore, on a clear midsummer day with strong solar radiation, lower-than-expected power generation does not necessarily indicate a fault. In particular, in installation environments with little wind where heat can become trapped on the back of the panels, temperature-related effects are more likely to occur.

However, there are limits to how much a decline can be explained solely by the effects of ambient temperature. If it is only that the overall output is slightly harder to increase, that can be considered a natural fluctuation; but if output suddenly approaches zero from a specific time, if only a particular system is extremely low, or if the same equipment is underperforming day after day even on clear days, those situations are difficult to explain by temperature alone. Even when citing season or temperature as the reason, it is necessary to separate plant-wide trends from differences among individual pieces of equipment.

As a seasonal factor, changes in shading due to the sun's angle are also important. In winter the sun's elevation is lower, and locations that were not shaded in summer can be affected by shadows. Surrounding trees, utility poles, fences, adjacent equipment, and the slopes of developed land can cast different shadows depending on the season. If periods of reduced power generation occur at the same time every year and there are dips at specific times of day, you should suspect seasonal shading rather than a fault.

On the other hand, even if values are clearly lower than in the same month of the previous year, that does not necessarily mean there is an anomaly. Multiple factors may be involved, such as different weather patterns between last year and this year, differences in the timing or duration of snowfall or rainfall, changes in the surrounding environment, growth of vegetation, or changes in surface reflectivity and drainage due to land development or construction. The important thing here is not to treat the year‑on‑year monthly comparison as absolute. The fact that it is lower than the previous year is only a starting point, and you need to investigate the reasons by checking solar radiation, temperature, site environment, and equipment condition.

When you can separate the effects of seasonality and temperature, the accuracy of explanations for inquiries about low power generation improves. Rather than vague explanations like "it's low because it's summer" or "it's low because it's winter," you can organize points such as "how much lower is it compared with past performance in the same season," "is the output unreasonably low given the solar irradiance," and "is the decline system-wide or localized?" This organization directly leads to distinguishing between weather-related differences and actual anomalies.

Confirm degradation due to on-site conditions such as shadows and dirt

The causes of low power generation are not limited to equipment failure. Changes in site conditions can cause generation to gradually decline. Typical examples are shading and soiling. Even shading over just part of a panel can, depending on the connection configuration, affect output at the system level. Similarly, soiling—such as dust, bird droppings, fallen leaves, pollen, yellow sand, snow, and mud splatter—can block the light-receiving surface and lead to reduced power generation.

When checking for shadows, it is important to correlate the time periods of low power output with the times when shadows occur. If output is low only in the morning, shadows from trees or buildings on the east side may be involved. If output is low only in the afternoon, obstructions on the west side may be affecting it. If there is a localized drop around midday, narrow shadows from utility poles, mounting racks, surrounding equipment, or inspection structures may be casting for a limited time. On-site visual inspections are most effective when carried out during the time periods when power output is reduced.

Regarding soiling, it is necessary to distinguish between overall, uniform soiling and localized soiling. If the issue is general dust or pollen, similar patterns may appear throughout the power plant. If it is localized bird droppings, fallen leaves, or mud splashes, the impact may be concentrated on certain strings or specific panels. Some soiling will be naturally washed away by rain, but not all types of soiling are removed by rain. Regular inspections are especially necessary at sites where soil and debris tend to accumulate at the lower positions of panels or where there are many trees nearby.

At power plants, grass growth can also be a factor in reduced power output. Even if there is no problem immediately after installation, as the seasons progress weeds can grow and cast shadows on the lower parts of the panels or on the front rows. In particular, grass growth varies by location near low-mounted racks, on slopes, or close to the slopes of developed land. Check where on the site the systems with low power output are located and see whether they correspond to shadows from grass, the terrain, or surrounding structures.

Changes in on-site conditions can be difficult to detect from monitoring screens alone. Even if a decline is detected in monitoring data, it is hard to determine whether the cause is shading, dirt, wiring, or the equipment itself without confirming the actual site conditions. If a drop in power generation is localized to a particular area, check the area's photos, inspection records, cleaning history, weeding history, and whether any nearby construction has occurred. When performing on-site checks, it is important to stay within safe limits, avoid touching equipment unnecessarily, and request confirmation from a specialist as needed.

A decrease caused by shading or soiling does not necessarily mean power generation stops completely, unlike a failure. Rather, it manifests as a gradual drop in power output, slowed gains even on sunny days, or lower output only during certain seasons or times of day. For this reason, it’s important to be aware that detection can be delayed. Even if no alarm like an equipment fault is triggered, continuously monitoring power generation trends will let you notice something is off at an early stage.

When checking site conditions, it is important not only to perform a visual inspection but also to keep records. If you document with photos and notes when, in which area, and to what extent there were shadows or dirt, you can later compare them with power generation data. Confirming how power generation changed before and after weeding or cleaning will also help inform future management decisions. Rather than searching for the cause of low power generation only once, linking and accumulating site conditions with generation data leads to more stable management.

Identifying Potential Anomalies from System- and Device-Level Differences

If a drop in power generation remains difficult to explain even after checking weather, season, temperature, shading, and soiling, we examine differences by system and by equipment in detail. If you only look at the plant’s total generation, some anomalies can become buried within the whole. Especially in large facilities, it may look like the plant is generating overall, yet specific power conditioners, strings, combiner boxes, or metering systems can be experiencing problems.

The basic approach when suspecting an anomaly is to compare equipment that should be subjected to the same conditions. If there are multiple sections on the same site with the same orientation, the same tilt, and similar solar irradiation conditions, compare the power generation and output trends of each. If one device is consistently lower, its output suddenly falls from a certain time, or its peak on sunny days is clearly lower than the others, focus attention on that device or system.

You should also check for power conditioner shutdowns and output limitations. If it has shut down due to some protective action or fault detection, power generation will fall significantly. Even if it has not shut down, output may be limited by temperature, voltage, or grid-side conditions. Check the monitoring screen and equipment displays for fault histories, shutdown histories, alarms, and operating status, and look for signs that are not simply due to weather variations.

Abnormalities in wiring and connection points can also lead to reduced power generation. If there is a broken wire, poor contact, connector faults, abnormalities inside junction boxes, or operation of protective devices, only certain circuits may experience reduced output or stop. However, because these issues affect safety, operational staff should avoid casually touching exposed parts or checking live components. If an abnormality is suspected, request confirmation from personnel with the appropriate qualifications or authority, or from maintenance and inspection specialists.

Faults in measurement instruments or communications must not be overlooked. Even if power generation itself is occurring, if monitoring data are not being correctly acquired, the displayed output may appear low on the screen. Communication outages, data loss, delays in updating measured values, or sensor malfunctions can cause the actual generation and the displayed values to diverge. When generation is suddenly shown as low, check not only the operating status of the equipment but also whether the data are being acquired normally.

An anomaly is more likely when a drop in power generation is localized, persists, and does not recover with improving weather. For example, if the plant’s overall output rises with clear skies but only some systems remain low, suspect shading, soiling, system faults, or equipment shutdowns. Also, if output suddenly falls at a particular time and does not recover afterward, it may indicate equipment stoppage or connection abnormalities. Conversely, if the entire plant fluctuates in step with cloud movement, first consider weather effects.

What’s important here is not to determine the presence or absence of an anomaly from a single numerical value. We判断 by combining power generation, solar irradiance, system-specific output, equipment condition, alarm history, site photographs, and past performance. Looking at only one item can easily lead to misunderstanding, but if multiple pieces of information point in the same direction, it becomes easier to prioritize responses. Breaking down a condition of low power generation into whether it is an overall decrease or a partial decrease, and whether it is momentary or sustained, is the quickest way to detect anomalies.

In responding to a decline in power output, it is important not to leave the cause unclear. A temporary decline due to weather differences may not become a major problem, but if it is caused by equipment abnormalities or deterioration of the site environment, leaving it unaddressed will continue to result in lost generation opportunities. Record the date and time you noticed the decline, the power output, the weather, solar irradiance conditions, the extent of the decline, and the evidence you confirmed, and keep the records in a state that can be handed over to a specialist as needed.

When keeping records, note not only the power generation figures but also the flow of your judgment. For example: "Judged to be lower due to overcast skies," "Requested inspection because only some arrays were low relative to solar irradiance," "Confirmed shadow from grass, so recheck after weeding." Making the rationale clear in this way helps with later explanations and improvements. Because the sequence of events can be followed even if the person in charge changes, it reduces the need to repeat the same checks multiple times.

Determining how to distinguish low power generation is not a simple fault diagnosis. Some decreases can be naturally explained by weather variations, while others may result from multiple overlapping factors such as seasonal and temperature effects, shading or soiling, communication failures, and equipment malfunctions. That is why, in practice, it is important to check in order whether the issue affects the whole system or only part of it, whether it is temporary or persistent, whether it is correlated with solar irradiance, whether there have been changes in the site environment, and whether there are differences between pieces of equipment.

To detect declines in power generation quickly and accurately, it is essential to have a system that links daily data with on-site conditions. By organizing generation output, solar irradiance, site photos, inspection records, and status by system, it becomes easier to judge whether differences are due to weather or to an anomaly. First, standardize comparison criteria and verification procedures for each of your plants, and establish a management system that, when signs of decline are found, allows checks in the order of weather, season, site environment, and equipment condition.