6 items to compare when power generation is lower than the previous year

When the power output of a solar photovoltaic system is lower than the previous year, people may become anxious, wondering, "Is there a problem with the equipment?" or "Do we need to inspect it immediately?" Especially for operations staff who monitor generation in monthly reports or internal management, a year-on-year decline for the same month is a change that is hard to overlook. However, it is premature to conclude that lower generation than the previous year automatically indicates equipment failure or poor workmanship. Many factors affect generation, including solar irradiance, temperature, weather, snowfall, shading, soiling, downtime, and measurement conditions.

What’s important is not to judge the difference from the previous year by intuition, but to compare under conditions as similar as possible. Even if the raw generation output appears to have dropped significantly, if the solar radiation was lower than the previous year, it may be explained as a natural variation. On the other hand, if solar radiation conditions have not changed significantly but only the generation output has decreased, you need to check for panel soiling, increased shading, equipment outages, wiring system abnormalities, missing measurement data, and so on.

This article outlines six items to compare when power generation is lower than the previous year, aimed at practitioners who are investigating causes by searching "low power generation". Don't judge based only on year-on-year same-month figures—use this approach to help prioritize inspections.

Table of Contents

• If electricity generation is lower than the previous year, do not judge it based solely on a simple comparison.

• Comparison 1: Solar irradiance and weather conditions compared to the previous year

• Comparison 2 Compare temperature and seasonal factors with the previous year

• Comparison 3: Compare operating days and downtime with the previous year

• Comparison 4 Compare on-site conditions such as shadows, dirt, and snow accumulation with the previous year

• Comparison 5: Compare the power generation trends of power conditioners and each system with the previous year

• Comparison 6: Compare measurement data and aggregation conditions with the previous year

• Practical steps to take after detecting a decrease in power generation

• Summary: Check year-on-year shortfalls in power generation by examining the causes separately.

When power generation is lower than the previous year, do not judge it solely by a simple comparison

When power generation is lower than the previous year, the first thing to check is "what exactly are you comparing?" Simply comparing the total generation for the same month last year with the total generation for the same month this year is insufficient to isolate the cause. Solar power generation is influenced not only by the condition of the equipment itself but also by the amount of solar irradiance during the period, temperature, number of rainy days, number of cloudy days, whether snow has accumulated, whether the equipment was shut down, and how the measurement data were collected.

For example, if the same month in the previous year was mostly sunny and this year has been mostly cloudy or rainy, a drop in power generation itself is not unusual. Conversely, if this year’s solar irradiance conditions have not changed significantly from the previous year yet the generation is clearly lower, there may have been some change in the equipment or the site environment. To tell the difference, you need to look not only at the generation figures but also at the underlying conditions.

Also, when making monthly comparisons, differences in the number of calendar days can have an impact. When comparing months with differing day counts, such as February and March, or even when comparing the same month year over year, discrepancies can arise due to leap years, the aggregation period’s cutoff date, or differences in the timing of data collection. It is important to confirm whether the data represent a complete period from the beginning to the end of the month and to check for any communication outages or missed recordings.

When you find a drop in power generation, don't assume it's a failure from the outset; instead, consider it in categories such as "decline due to natural conditions," "decline due to equipment shutdown," "decline due to changes in the site environment," and "apparent decline due to measurement or aggregation issues"—this approach can reduce unnecessary inspections and oversights. Especially when managing multiple solar power plants, rather than checking every site with the same priority, it is practically important to dig deeper into the sites where the reason for the decline cannot be explained.

The fact that power generation is lower than the previous year is merely an entry point for inspection. Using that figure as a starting point and comparing which conditions have changed one by one makes it easier to pinpoint the cause.

Comparison 1 Compare solar radiation and weather conditions with the previous year

When power generation is lower than the previous year, the first items to compare are solar irradiance and weather conditions. Because solar power generation depends on receiving sunlight, periods with lower solar irradiance tend to result in reduced generation. If there were more cloudy or rainy days compared with the same month last year, the reduction in generation may be explained by weather differences rather than equipment faults.

In practice, the first step is to check how much lower the solar irradiance was in the month in question compared with the same month of the previous year. Rather than looking only at power generation, assessing how much power was produced relative to the solar irradiance allows for a more appropriate assessment. If solar irradiance has fallen substantially and generation has decreased correspondingly, the impact of natural conditions is likely significant. On the other hand, if the decrease in irradiance is small but the decline in generation is large, you should suspect equipment- or site-related factors.

When examining weather conditions, pay attention not only to the number of sunny and rainy days but also to periods that were mostly cloudy. In solar power generation, the weather during a few daytime hours greatly affects the amount of electricity produced. Even if it is sunny only in the morning and evening, if thick clouds cover the hours when generation tends to be high, monthly generation will likely decrease. Therefore, if daily generation graphs or hourly trends are available, it is useful to check how the generation curve on sunny days has changed compared with the previous year.

Also, localized weather variations can occur even within the same region. If the power plant you manage is located in mountainous areas, coastal areas, basins, or regions with snowfall, the weather at the nearest observation point may not match the actual on-site conditions. If there are surveillance cameras or patrol records at the site, checking the sky conditions, snowfall, fog, thick clouds, and the presence of yellow sand or dust on days when generation was low will make it easier to explain the solar irradiance conditions.

What is important when comparing solar irradiance and weather conditions is to separate the result "power generation is lower than the previous year" into "a decline that can be explained by worse solar conditions than the previous year" and "a decline that is difficult to explain by solar conditions alone." Being able to make this distinction makes it easier to decide whether to proceed to an on-site inspection or to continue monitoring.

Comparison 2: Compare air temperature and seasonal factors with the previous year

Next, what we want to compare are air temperature and seasonal factors. Solar power generation tends to increase when solar irradiance is higher, but when panel temperature rises, power conversion efficiency tends to decrease. Therefore, in summer, even if daylight hours are longer, increases in air temperature and panel temperature can suppress gains in power output. If temperatures remain higher than in the previous year for a sustained period, power output may appear somewhat lower even with similar solar irradiance.

Especially when comparing year-on-year in summer, you should check not only whether there were more sunny days, but also whether there were many hot days or whether winds were weak, making the panels harder to cool. Because solar panels are installed outdoors, temperature conditions vary not only with ambient temperature but also with ventilation, mounting angle, whether they are roof-mounted or ground-mounted, and how prone the surroundings are to trapping heat. If there were more hot days compared to the previous year and the peak power output is slightly lower, temperature effects may be involved.

On the other hand, winter is a season when, even if air temperatures are low, shorter sunlight hours and a lower solar altitude tend to result in seasonally reduced power generation. Even when comparing the same month year-on-year, differences can arise due to snowfall, frost, morning and evening shadows, and the persistence of cloudy weather. In particular, in snowy regions, if the number of days with snow remaining on the panel surface is greater than in the previous year, the hours during which generation is impossible can increase even when solar irradiance is available. If the installation angle makes snow slow to melt or conditions prevent snow from sliding off easily, these factors can lead to lower power generation compared with the previous year.

Spring and autumn often give the impression of relatively stable power generation, but in reality pollen, yellow sand, fallen leaves, post-typhoon dirt, and prolonged rain can have an impact. If there were periods with more temporary soiling than in the previous year, or times when airborne debris from the surroundings increased after strong winds, differences in power output can occur. In particular, if a decline in power generation has persisted since a specific period, it is important to review whether there were any seasonal environmental changes during that time.

When comparing temperature and seasonal factors, ascertain whether a decline in power generation is caused by an equipment malfunction or by differences in seasonal conditions. Rather than immediately suspecting equipment failure just because output is lower than the previous year, you can make a more realistic assessment by checking factors such as heat, cold, snowfall, seasons prone to soiling, and changes in sunshine duration.

Comparison 3: Compare operating days and downtime with the previous year

When power generation is lower than the previous year, it is necessary to check how much the equipment actually operated. Even if solar irradiance and temperature are fine, an increase in the time the equipment was offline will reduce monthly and annual generation. What is important here is that the outages are not necessarily limited to large, conspicuous failures. Short-duration shutdowns or stoppages of specific components can accumulate and appear as a year-on-year difference.

The first thing to check is whether there were any planned outages or inspection shutdowns during the period in question. If generation was stopped for electrical equipment inspections, nearby construction, grid-side work, maintenance activities, safety checks, or similar reasons, the amount of generation may be lower than the previous year by that amount. If there were no shutdowns in the same month of the previous year but this year experienced shutdowns lasting from a few hours to several days, that can account for the decrease in generation.

Next, check the history of abnormal or automatic shutdowns. Look for occurrences such as power conditioner shutdowns, disconnection from the grid due to grid abnormalities, overvoltage, overtemperature, communication faults, and protective actions. Here, not only the number of shutdowns but also the time periods when they occurred are important. If a shutdown occurred around midday when generation is high, even a short interruption can have a large impact. Conversely, a shutdown in the early morning or evening when generation is low may have a relatively small effect on monthly generation.

In a power plant made up of multiple pieces of equipment, attention must be paid not only to total shutdowns but also to partial shutdowns. Even if the drop in overall generation appears small, specific circuits or particular pieces of equipment may not be producing power. Because such situations are easy to miss in the aggregate figures, it is effective to compare with the previous year at as fine a granularity as you can manage—equipment-level, system-level, or string-level.

The time to recovery is also a point of comparison. If last year anomalies were restored in a short time but this year detection and response are delayed, resulting in longer downtime, operational issues may be contributing to the reduction in power generation. Rather than attributing low power generation solely to the equipment itself, it is important to review the management system, including monitoring, notification, on-site inspection, and recovery procedures.

By comparing operating days and downtime, you can distinguish whether a decline in power generation is due to natural conditions or to lost operating opportunities. If you can determine how much of the lower generation compared with the previous year is attributable to downtime, it becomes easier to prioritize inspections and improvements.

Comparison 4: Compare on-site conditions such as shadows, dirt, and snow cover with the previous year

If power generation is lower than the previous year, changes in the on-site environment are also an important comparison factor. Because solar power generation equipment is installed outdoors, it is affected by surrounding trees, buildings, topography, grass, dust, bird droppings, fallen leaves, and snow. Even if the equipment itself has no faults, an increase in shadows or dirt on the light-receiving surface can lead to a reduction in power generation.

The impact of shading is something that tends to change from year to year. Even slight changes on site—such as surrounding trees growing, structures being installed nearby, increased periods of shadow from fences or utility poles, or changes in the frequency of vegetation management—can affect power generation. Especially in the morning and evening, when the sun's altitude is lower, shadows that were not a problem in the previous year may become noticeable due to seasonal or local environmental changes.

Soiling should also be compared with the previous year. If soil dust, pollen, yellow sand, bird droppings, fallen leaves, mud splashes, or similar adhere to the panel surface, they can obstruct light reception. Some dirt is naturally washed away by rain, but in locations with a gentle tilt, near agricultural land or unpaved roads, where birds tend to gather, or where drainage and mud splashing are likely, dirt can remain. If there were fewer rain events than the previous year or increased dust due to strong winds, a performance drop caused by soiling is suspected.

In snowy regions, we also compare snowfall and how long snow remains. We check whether days with low power generation coincide with days of snowfall, how many days snow remained on the panels, and whether snow has accumulated on lower-row panels or walkways after it slides off. Differences in power generation can occur not only when there was more snow than the previous year, but also when melting is delayed due to differences in temperature or sunlight.

The impact of grass should not be overlooked. At ground-mounted power plants, if the timing of grass growth or the timing of weeding differs from the previous year, shadows may fall on the lower parts of the panels or on some rows. Even a temporary increase in grass height can cause power output to drop during certain periods. Information such as fewer weedings compared with the previous year, delayed maintenance timing, or areas left uncut can help explain reductions in power generation.

When comparing site conditions, photo records are effective in addition to power generation data. Comparing photos of the site from the same period last year with this year’s photos makes it easier to identify changes in trees, grass, dirt, drainage, snow cover, and nearby construction. If photo positions and angles vary each time, comparisons become difficult, so keeping a practice of recording from fixed reference points makes investigations into causes of reduced power generation smoother.

Comparison 5: Compare power generation trends per power conditioner and per system with the previous year

When total power generation is lower than the previous year, it is important to compare not only the entire plant but also generation trends by power conditioner and by system. Even if the aggregate numbers make it hard to identify the cause, examining at the equipment level can reveal that only specific locations have declined. Whether the decline is occurring evenly across the whole plant or is concentrated in certain parts will change which causes should be suspected.

If the power output has fallen by the same proportion across the board, common factors affecting the entire plant—such as solar irradiance or weather—should be considered. On the other hand, if only a specific power conditioner, a specific circuit, or only certain rows have declined significantly compared with the previous year, suspect anomalies or environmental changes confined to that range. For example, candidates include faulty connections, cable damage, circuit breaker operation, shading over a specific area, localized soiling, or panel damage.

When comparing, it is easier to judge by looking at the generation curve on clear days, not just daily or monthly totals. In systems that are close to normal, power output on sunny days often shows a smooth, mountain-shaped profile. Changes such as a lower peak compared with the previous year, a drop only in the morning, a drop only in the afternoon, generation cutting out for certain periods, or a shape that is clearly different from other systems can provide clues to narrow down the cause.

Also, verify when the decline in power generation began. If it dropped suddenly on a particular day, it may be related to a specific event such as equipment shutdown, setting changes, construction work, anomalies after lightning strikes or strong winds, or problems with cables or connections. Conversely, if it has been decreasing gradually, consider slowly progressing factors such as dirt accumulation, tree growth, shading from grass, or the aging of equipment. However, even when citing aging as the reason, avoid drawing a definitive conclusion without checking for other factors.

When equipment within the same power plant can be compared, in addition to year-over-year comparisons, side-by-side comparisons for the same period this year are also useful. If, among adjacent units that should be exposed to the same solar irradiance, only a particular system shows lower generation, it becomes easier to suspect a problem unique to that system. Even when managing multiple power plants, comparing trends with nearby plants helps determine whether the cause is a regional weather factor or an issue at an individual site.

Comparing power conditioners and individual systems is an important step in linking the causes of reduced power generation to on-site inspections. Rather than judging solely by total generation, identifying the range in which the decline is occurring allows you to narrow down inspection locations and improve the efficiency of your response.

Comparison 6 Compare measurement data and aggregation conditions with the previous year

When power generation is lower than the previous year, you should check not only for an actual decline in generation but also for apparent declines caused by differences in measurement data or aggregation conditions. Even if the equipment is generating normally, power output may be shown as lower if there are communication outages, missing records, failures to obtain meter readings, differences in aggregation periods, unit mix-ups, or similar issues.

First, check whether the data for the target period are missing. If communication with monitoring devices or measuring instruments was temporarily interrupted, some of the power generation may not have been recorded. Review the daily data to see if there are days with extremely low generation or days that suddenly drop to near zero. If those days coincide with bad weather or a shutdown, that can be explained, but if there is no record despite clear weather, you should suspect a problem on the measurement side.

Next, check whether the aggregation period is the same as the previous year. In monthly reports and internal documents, data may be aggregated from the beginning to the end of the month, or aggregated based on meter-reading dates or billing cutoff dates. If, for example, the previous year covers 31 days and this year covers 30 days, a simple total comparison will show differences. In particular, if there have been changes to the cutoff date, a switch in the measurement system, or changes in management practices, you should review whether the comparison is being made under the same conditions as the previous year.

Careful attention is needed to differences in units and displayed items. Generated energy, amount of electricity sold, surplus after consumption, AC-side output, and estimated DC-side values may look similar but have different meanings. If you looked at generated energy last year but are looking at amount sold this year, the comparison is inappropriate. In systems with self-consumption, generated energy and amount sold may not match. Do not judge by the names on documents or screens alone; confirm what the aggregated numbers actually represent.

Changes in installed capacity, partial additions or removals, or configuration changes also alter the conditions for comparison. If the equipment configuration differs between last year and this year, comparing only total power generation will not yield an accurate evaluation. It is necessary to compare generation per unit of installed capacity and to align the scope of equipment that was in operation. If there were partial construction works or temporary shutdowns, it is also important to match the scope of the comparison.

Checking measurement data and aggregation conditions is a basic task that should be performed before on-site inspections. If you misinterpret the data, you may judge something that is not actually abnormal as abnormal, or conversely overlook an actual fault. Especially when you feel the power generation is low, it is important to first check for missing data, the aggregation period, the comparison targets, the units, and the equipment configuration, and to establish a sound basis for comparison.

Practical steps to take after detecting a decline in power generation

When you have confirmed that power generation is lower than the previous year, the next step is to narrow down the causes in order. In practice, before heading to the site, it is important to organize information that can be checked at your desk. By reviewing solar irradiance, weather, outage history, generation by each piece of equipment, gaps in measurement data, and past inspection records, you can clarify which areas should be prioritized on site.

The first thing to do is to determine the extent of the decline. Separate whether the entire power plant is low, only some equipment is low, only specific days are low, or the entire target period is low. If the whole plant is similarly low, prioritize checking the weather, solar irradiance, and aggregation/summary conditions. If only part is low, prioritize checking shading, soiling, shutdown history, and connection status in that area.

Next, confirm when the decline began. If the decline was sudden, investigate what happened immediately before and after. Inspection work, construction, power outages, typhoons, lightning strikes, snowfall, or changes in the surrounding environment can be candidates for the cause. If the decline is gradual, consider accumulation of dirt, growth of vegetation, aging of equipment, or insufficient inspection frequency. In this way, the pattern of the decline changes the direction of the inspection.

When proceeding to an on-site inspection, it is important to keep photographic records. Record, in a way that allows later comparison, the soiling on panel surfaces, the way shadows fall, the condition of vegetation, drainage status, the remaining snow, and the condition around equipment. Photos can also serve as documentation to explain causes of reduced power output and are useful when sharing the situation among stakeholders. If possible, take photos from the same positions each time so comparisons can be made with the previous year or the last inspection.

When addressing a decline in power generation, it is also important not to assume a single cause. In practice, a reduction in solar irradiance, soiling, short-term shutdowns, missing data, and other factors can overlap. For example, if the weather was worse than the previous year and, in addition, some power conditioners had stopped, the decline in generation may result from multiple factors. By separating and organizing the different factors, you can distinguish between issues that require action and those that only need to be monitored.

In internal reports and explanations to customers, it is important not only to state that "it is lower than the previous year" but also to show which items were compared and how thoroughly they were checked. Organizing whether solar irradiance was lower than the previous year, whether there were records of shutdowns, whether there were changes in site conditions, and whether there are any gaps in the measurement data will make the explanation more persuasive. In power generation businesses in particular, because a decline in power output affects revenue and maintenance decisions, comparisons backed by evidence are required.

To detect declines in power output early, how you keep daily records is also important. In addition to power output, recording the weather, inspection dates, weeding dates, cleaning dates, outage history, site photos, and records of responses to anomalies makes year-over-year comparisons easier. If last year's information is lacking, it's difficult to determine whether a decline this year is within a natural range. Accumulating comparable records itself leads to improved quality in power plant management.

Summary: Confirm the causes separately for year-on-year declines in power generation

When power generation is lower than the previous year, it is important not to judge based only on the total amount generated, but to separate and check the specific items for comparison. Looking at solar irradiance and weather conditions makes it easier to determine whether the decline is due to natural factors. Checking temperature and seasonal factors allows you to sort out season-specific impacts such as high temperatures in summer, snow accumulation in winter, and increased soiling in spring and autumn. By comparing operating days and downtime, you can confirm whether the time the equipment was unable to generate has increased compared with the previous year.

Furthermore, comparing on-site conditions such as shadows, dirt, snowfall, and vegetation makes it easier to identify causes external to the equipment. Comparing power conditioners and generation trends by system allows you to determine whether the issue is a whole-system problem or only affects part of it. Finally, by checking measurement data and aggregation settings, you can prevent apparent declines and discrepancies in comparison conditions.

When you feel that power generation is low, the important thing is not to draw immediate conclusions. Organize the differences from the previous year from multiple perspectives—solar irradiance, temperature, stoppages, site environment, equipment-specific trends, and data conditions—and separate declines that can be explained from those that are difficult to explain. If unexplained declines remain, on-site inspections and specialized verification should be carried out.

When managing a solar power plant, it is important not only to check daily power generation but also to keep records that can be compared over time, as they serve as material for future decision-making. By managing site photos, inspection logs, outage histories, records of weeding and cleaning, and weather notes together with generation data, you will find it easier to trace the cause when generation is lower than in the previous year.

Operations that allow generation data to be cross-referenced with on-site conditions are useful for detecting declines in power generation early and for organizing their causes with supporting evidence. If solar irradiance conditions, shutdown history, generation trends by equipment, site photos, and inspection records can all be reviewed within the same workflow, it becomes easier—when generation falls below the previous year's level—to determine whether the change is due to natural conditions or a decline that requires on-site inspection.