5 Checks to Make If You Feel Your Power Generation Is Low

When operating a solar power plant, there are times when, looking at the monthly power generation or electricity sales, you feel "Isn't this lower than expected?" or "Has it dropped compared to last year?" However, even if the generation appears low, you cannot immediately conclude that the equipment has failed. Weather, season, shading, dirt, measurement methods, equipment condition, assumptions made at the design stage, and other factors overlap to cause generation to fluctuate.

What's important is to avoid relying on intuition alone and to establish an order of checks to isolate the cause. If you can organize the reasons for low power generation early, you can reduce wasted on-site inspections and make requests to inspection contractors and maintenance personnel clearer. This article explains, for practitioners searching for "発電量　低い", the five check items to look at when confirming a drop in generation, from a perspective that's practical for use in the field.

Table of Contents

• What to check first when you suspect low power generation

• Check item 1: Confirm power generation variations due to weather and seasonal changes

• Check item 2: Confirm changes in site conditions such as shadows, dirt, and vegetation

• Check item 3: Confirm abnormalities in panels, wiring, and power conditioners

• Check item 4: Confirm discrepancies in monitoring data, meter readings, and aggregation conditions

• Check item 5: Confirm differences in assumptions between the design's expected generation and actual performance

• Recording and improvement procedures to avoid overlooking declines in power generation

• Summary: Separate the causes of low power generation into data-related and on-site factors

What you should first clarify when you feel the power output is low

When you feel that power generation is low, the first thing to do is to clarify "compared to what" it is low. The items to check differ depending on whether it is lower than the previous month, lower than the same month last year, lower than the design simulation value, or lower than other equipment in the same plant or neighboring plants. For example, even if generation has decreased compared with the previous month, if solar irradiance or weather differs significantly, it may be within the range of natural variation. On the other hand, if it has fallen substantially compared with the same month last year, you need to check for operational changes such as increased shading, soiling, partial shutdowns, or changes in equipment condition.

Also, when reviewing generation it is important not to judge solely by the total for the entire plant. A small decline in the overall figure can hide a large drop occurring in a specific circuit or area. By changing the granularity of checks—total generation, generation per unit of installed capacity, daily trends, output by time of day, comparisons at the power conditioner unit level—you can more easily identify the cause.

A decline in power output can occur suddenly or gradually. If it drops sharply at once, equipment shutdown, breaker operation or tripping, communication failures, or wiring troubles may be suspected. If it decreases slowly, factors such as dirt on panel surfaces, shading from plant growth, aging of components, or contamination fixed by poor drainage may be involved. Simply identifying which type of change it is will alter the inspection priorities.

In practice, it is important to first define the target period and align the comparison targets. Examine not only daily generation but also monthly figures, the same month of the previous year, equipment-level data within the same plant, and trends in similarly sized facilities to determine whether a decline is temporary or persistent. Then, by checking weather and seasonal influences and isolating factors in the order of site conditions, equipment condition, measured values, and design assumptions, it becomes easier to identify the cause.

Checklist item 1: Confirm fluctuations in power generation due to weather and seasons

When you feel the power generation is low, the first thing to check is the impact of weather and the seasons. Solar power generation is greatly affected by solar irradiance, so in months with prolonged cloudy or rainy weather, generation will decrease even if there are no obvious equipment faults. Especially during the rainy season, periods prone to typhoons or low-pressure systems, and in areas where snowfall or fog frequently occurs, you should first confirm whether the decrease in generation is due to weather-related factors.

When looking only at monthly power generation, it can sometimes appear abnormal. However, even in the same month, if the number of sunny days or solar radiation conditions differ significantly from the previous year, generation will change. Therefore, when comparing with the same month of the previous year, it is important to check not only the raw generation figures but also the solar radiation conditions. If solar radiation records are available, examining the relationship between generation and solar radiation makes it easier to determine whether the issue lies with the equipment or with the weather. If a month with low solar radiation also shows low generation, it may be a natural fluctuation. On the other hand, if solar radiation has not changed much but generation has decreased, you should check for changes in equipment or local conditions.

Seasonal differences in solar elevation also need to be checked. In winter the sun’s elevation is lower and shadows tend to be longer, so shadows from buildings, trees, utility poles, fences, and nearby equipment are more likely to fall on the panels. Even with the same layout as in summer, energy generation can drop in winter. Temperature also affects output. Because solar panels tend to lose output as they get hotter, even in sunny summers generation may not reach expected levels depending on temperature conditions. Conversely, in spring and autumn temperatures are relatively lower, and on days with good sunlight they can produce higher energy output.

When checking weather and seasons, it is useful not only to look at the absolute value of power generation but also to examine the daily output waveform. On sunny days, the output tends to form a smooth, gently peaked curve, while on cloudy days the output tends to show more frequent small fluctuations. If output drops at the same time every day, you should consider not only the weather but also the effects of shading or equipment control. Confirming whether generation drops only during specific times or remains low all day makes it easier to narrow down where to inspect next.

What operations personnel should be careful about is assuming an equipment fault without checking weather-related factors. If, after arranging an on-site inspection, it turns out that a decrease in solar irradiance was actually the main cause, you may have misprioritized response actions. First, compile the weather, solar irradiance, temperature, snowfall, typhoons, yellow dust, extended rainfall, and other relevant conditions for the period in question, and confirm whether any unexplained decline remains.

Check Item 2: Confirm changes in site conditions such as shadows, dirt, and vegetation

When a decrease in power generation cannot be explained solely by weather or seasonal factors, the next thing to check is changes in on-site conditions. Because solar power plants are outdoor facilities, the surrounding environment changes even after installation. Factors that affect power generation arise over time, such as vegetation growth, new structures on adjacent land, an increase in nearby equipment, adhesion of soil dust or bird droppings, and accumulation of fallen leaves.

Shadows are an item that is particularly easy to overlook. If part of a solar panel is shaded, that shading can affect the output of the corresponding circuit or string. Shadows do not necessarily remain in the same place all day; they may appear only in the morning, only in the evening, only in winter, or only during specific seasons. For that reason, during on-site inspections it is important not to look at only a single daytime point, but to check against the times when generation drops. If monitoring data shows a pattern such as low output only in the morning or only at certain times in the afternoon, verify whether shadows are present during those times.

Vegetation growth can also cause a decline in power generation. Even in locations that were not problematic at the time of installation, after several months to several years weeds and shrubs can grow and cast shadows on the lower parts of the panels and around the mounting racks. At ground-mounted power plants, if the frequency and extent of mowing are not appropriate, only the lower-row panels may experience reduced power output. Even for rooftop installations, branches of surrounding trees can extend and cast shadows. If decreases in power output recur seasonally, it is advisable to check for the effects of vegetation and fallen leaves.

Soiling is also important. When sand and dust, pollen, yellow dust, bird droppings, fallen leaves, or exhaust-related grime adhere to the panel surface, the incident light is reduced and power generation can be affected. Some dirt is washed away to some extent by normal rain, but it tends to remain on panels with a shallow tilt, at edges where rainwater does not easily run off, in areas where birds tend to gather, and where soil is exposed nearby. Because localized soiling can be hard to detect from overall power output alone, it is effective to check each location using on-site photos and inspection records.

When checking onsite conditions, it's important not only to view the plant as a whole uniformly but also to compare areas with low generation to those operating normally. If, within the same plant, there is a pattern such as only a single row being low, only the area connected to a particular power conditioner being low, or only the edges of the site being low, you should suspect effects from shading, soiling, vegetation, drainage, or topography. Conversely, if the entire plant is uniformly low, you need to consider broader factors such as weather, measurement, receiving-side equipment, and output control.

When conducting on-site inspections, it is also important to take photographs. Recording the position of shadows, the degree of soiling, vegetation height, accumulation of fallen leaves, the condition of panel surfaces, and the area around the mounting racks makes comparisons easier for future inspections. Keeping a chronological record not only of low power generation results but also of the on-site conditions that lead to declines helps prevent recurrence and informs revisions to maintenance plans.

Checklist item 3: Inspect panels, wiring, and power conditioners for abnormalities

If checking the weather and on-site conditions does not reveal the cause, inspect the equipment itself for abnormalities. In a solar power generation system, multiple components — panels, junction boxes, wiring, connectors, circuit breakers, power conditioners (inverters), and monitoring devices — work together to generate electricity. An abnormality in any one part can lead to reduced power output, unstable output, or the shutdown of specific circuits.

The first thing to check is the power generation per power conditioner unit. If the systems have the same capacity, orientation, and tilt, they will usually show similar generation trends. If only one power conditioner has low generation, you need to inspect the panel strings connected to that unit, the DC-side wiring, the AC-side equipment, device settings, and shutdown history. If only one unit among several is extremely low, it may appear as a small difference in total generation but could represent a significant loss in the affected area.

Next, check the output at the string and circuit levels. A string is a unit formed by connecting multiple solar panels in series. If there are differences in voltage or current between strings, partial panel failure, connector faults, open circuits, shading, soiling, or connection errors may be suspected. Not all strings will show identical values, but if one is clearly lower compared with circuits under the same conditions, it should be inspected on site.

Possible abnormalities of the panel itself include cracks, discoloration, burn marks, hotspots, frame deformation, damage to the glass surface, abnormalities on the rear side, and faults in the connections. Some anomalies are difficult to detect by visual inspection alone, but combining power generation data with on-site verification can help narrow down the suspicious areas. In particular, localized heating and connection failures affect safety, so they are important maintenance checks not only for reduced power output but also for safety.

Do not overlook the condition of wiring and connectors. Outdoors, wiring can develop problems due to wind and rain, ultraviolet exposure, temperature changes, animal damage, or bending and improper fastening during installation. Situations such as cables hanging down, loose connections, damaged protective conduits, or conditions that allow water ingress can cause reduced power output or shutdown. When inspecting on site, there is a risk of electric shock or equipment damage, so verification by a qualified person or specialist contractor is required when appropriate.

For the power conditioner, check the shutdown history, error history, temperature rise, cooling failures, clogging of filters and ventilation openings, settings, and whether output curtailment is in effect. Even if the unit has not completely stopped, output can be curtailed by temperature conditions or protective functions. If output levels only flatten out during specific times in the high temperatures of summer, the unit’s cooling environment and installation location should also be checked. You should also check that nothing is blocking ventilation around outdoor panels or the equipment and that there is no direct sunlight or heat buildup.

When checking for equipment abnormalities, it is more effective to narrow the scope using data rather than inspecting every location at once. Organize power conditioners with low output, strings with low output, the day the decline began, and the time periods when the decline occurs, and cross-check them against on-site shading and soiling. If anomalies in the data match on-site conditions, it becomes easier to identify the cause. Conversely, if the data appear abnormal but no on-site issues are found, it is necessary to recheck, including potential measurement and communication problems.

Checklist Item 4: Verify discrepancies between monitoring data, meter readings, and aggregation conditions

When you feel that power generation is low, it does not necessarily mean there is a problem with the equipment or on-site conditions. Differences between monitoring data, meter readings, and aggregation conditions can make generation appear lower than it actually is. Especially at power plants managed using multiple data sources, failing to clarify which figures are being used as the reference can lead to incorrect judgments.

First, what you should confirm is which type of generation figure you are looking at. In solar power generation there can be multiple figures, such as the power output reported by the power conditioner (inverter), the generation totaled by the monitoring device, the meter reading for sold electricity, and the measurements on the incoming power equipment side. Because the measurement locations and aggregation methods differ, they may not match exactly. It is important to check where the figure you consider to be low was measured.

Next, check for discrepancies in the aggregation period. When comparing monthly power generation, the figures will differ depending on whether you aggregate by calendar month or from meter-reading date to meter-reading date. For example, if the monitoring screen shows generation from the first to the last day of the month while the amount of electricity sold is reported for a meter-reading period, the figures for the same month will not match. If the number of days differs, the generation will of course change. Before comparing, you need to align the start date, end date, and the number of days covered.

Also check for communication errors and data loss. If the monitoring device’s communication is temporarily interrupted, data for that period may be missing or later backfilled. Even if the display shows low power generation, it may actually have been generating. If daily or hourly data contain unnatural gaps, suddenly drop to zero, or generation data are missing despite no record of equipment shutdown, suspect a problem on the communication or data-collection side.

It is also important to distinguish between generated electricity and electricity sold. In systems with self-consumption, part of the generated power is used within the facility, so looking only at the amount sold can make generation appear lower. Conversely, even in systems that sell all output, differences can arise between the power conditioner (inverter) readings and the meter readings due to conditions on the receiving side, measurement location, in‑facility consumption, transformer losses, and so on. When confirming a decline in generation, you need to be clear whether you are referring to generated power, sold power, or the surplus remaining after onsite use.

Output curtailment or grid-side constraints can also appear as a reduction in power generation. If, despite sunny conditions, output plateaus at a fixed value; if multiple facilities experience similarly suppressed output on specific days or at specific times; or if generation does not increase even though there are no equipment abnormalities, check for control factors other than equipment failure. Review control histories and operating rules to determine whether the decrease is due to a malfunction or suppression by external conditions.

When checking monitoring data, it is important to view it as a time series, not just for a single month. Check whether it dropped suddenly from a certain day, declined gradually, fell only at specific times, or showed different patterns only on weekends and holidays. Changing the way you look at the data makes it easier to distinguish between on-site anomalies, equipment faults, and problems with aggregation conditions. Rather than judging based on numbers alone, confirming where those numbers came from is essential for identifying the cause of a decline in power generation.

Checklist Item 5: Confirm the differences in assumptions between the expected power generation at the design stage and actual performance

One reason you may feel the power generation is low is that the expected generation at the design stage does not match the actual operating conditions. The simulated power generation is calculated based on a set of assumptions. Many factors are combined: solar irradiance, weather data, orientation, tilt, shading, loss rates, panel capacity, power conditioner capacity, temperature conditions, soiling, cable losses, downtime periods, and so on. Therefore, when comparing with actual results, you need to check not only the assumed values themselves but also the assumptions under which those values were produced.

The assumed power generation at the design stage is useful as an annual reference, but it does not necessarily match the actual monthly output. Actual weather varies from year to year, and solar irradiance also fluctuates. Even if a single month falls short of the assumption, the annual total may still approach the estimate. Conversely, if several months continuously fall below the assumption, there may be differences between the assumed and the actual equipment or site conditions.

One thing to check in particular is the shading assumptions. If surrounding buildings or trees were not adequately accounted for in the design, actual power generation may be lower than expected. The same applies if the surrounding environment changes after installation. Structures that did not exist at the time of design, grown trees, or newly installed equipment can cast shadows and widen the gap between expected and actual generation. If the discrepancy is large in winter, it is worth rechecking the shadows during the period of low solar altitude.

Assumptions about loss rates are also important. In solar power generation, various losses occur, such as soiling of the panel surface, temperature rise, wiring losses, conversion losses, equipment downtime, degradation over time, and mismatch. If losses were underestimated at the design stage, actual performance may appear low. Conversely, if on-site management is good, generation close to the assumed level can be achieved. When evaluating the gap between expected and actual generation, it is necessary to verify that the assumed loss conditions reflect the on-site reality.

Care must also be taken in how equipment capacity is interpreted. If the panel capacity and the power conditioner capacity differ, the output may plateau at a fixed value during sunny conditions. This is not necessarily an abnormality; it can result from the capacity balance in the equipment design. However, if this effect was not properly accounted for in the assumptions, comparisons with actual performance may feel inconsistent. When judging a decrease in power generation, it is important to look not only at the instantaneous maximum output but also at daily, monthly, and annual cumulative totals.

Also, the condition of equipment changes between immediately after start-up and several years later. Solar panels gradually change in performance with long-term use, and the mounting structures, wiring, connections, and surrounding environment also change over time. When comparing with the design assumptions, you need to take into account the elapsed years and the maintenance status. When determining that generation is low, rather than simply comparing the initial year's expected output with current actuals, an evaluation that includes aging-related changes is necessary.

In practice, reviewing the design documents, as-built documents, inspection records, monitoring data, and site photographs together makes differences in assumptions easier to see. Even when there is a gap between expected power generation and actual performance, distinguishing whether that gap is due to weather, differences in design conditions, on-site changes, or equipment abnormalities clarifies what actions should be taken next.

Recording and Improvement Procedures to Avoid Overlooking Power Output Declines

What is important when responding to a decline in power generation is not to treat it as a one-time inspection. A solar power plant is equipment operated over a long period, and changes in power generation can manifest gradually over time. Therefore, it is important to manage by linking daily monitoring, monthly comparisons, regular on-site checks, and records of inspection results.

First, it is essential to record monthly power generation in a consistent format. Keeping records of monthly generation, generation per unit of installed capacity, year-on-year comparison for the same month, deviations from expected generation, downtime, abnormality history, and whether any on-site work was performed will make it easier to trace causes later. In particular, if work such as grass cutting, cleaning, equipment replacement, setting changes, or nearby construction took place, you should be able to verify how generation changed before and after those activities.

Next, it is useful to establish criteria for judging when generation output seems low. If decisions are made based only on staff intuition, responses can be delayed or, conversely, unnecessary inspections may increase. By combining multiple criteria—year-on-year (same month) comparison, comparison to expected values, differences between devices within the same plant, number of consecutive days of decline, etc.—and defining conditions that require verification, operations become easier. However, because criteria vary depending on the plant’s scale, region, installation conditions, and monitoring accuracy, it is important to review them according to your company’s operational track record.

In on-site inspection records, it's also important to standardize how photos are taken. If you shoot from different angles and locations each time, it becomes difficult to compare vegetation growth, dirt, and changes in shadows. If you establish the primary shooting positions, shooting directions, and time windows in advance, seasonal changes and the progression of abnormalities become easier to see. Continuously recording the panel surface, under the racking, site boundaries, nearby trees, drainage conditions, and around the equipment cabinet makes it easier to detect early signs of power generation decline.

When a drop in power generation is detected, it is important not to assume a single cause. In actual field situations, adverse weather, soiling, vegetation, shading, partial shutdowns, communication failures, output curtailment, and other factors may overlap. If you stop after addressing only the most obvious factor, the decline may not be resolved. It is preferable to proceed step by step: confirm trends in the data, inspect conditions on site, and carry out specialized inspections as necessary.

After implementing improvement measures, verifying their effectiveness is essential. When mowing, cleaning, repairing equipment, or fixing wiring, check how much power generation has recovered afterward. If weather conditions differ before and after the work, a simple comparison is not possible, but by looking at output waveforms on similarly sunny days or comparing with equipment under the same conditions, it becomes easier to grasp the improvement effect. If no effect is observed, another cause may remain.

To streamline power generation management, it is important to manage by linking on-site conditions with data. If you can organize where drops in power output are occurring, at what times they appear, and how they correspond with on-site photos and inspection records, isolating the cause becomes faster. As the number of power plants increases, managing based only on staff memory becomes difficult, so it is helpful to establish an operational system that allows confirming generation data, location information, and on-site records together.

In on-site inspections and record management, it is important to choose a method suited to the scale of the power plant and its management structure rather than limiting yourself to specific products or systems. Whether you use paper inspection sheets, spreadsheet software, photo logs, or cloud-based management tools, it is essential to keep records in a form that allows later cross-checking of generation data with on-site conditions. By linking data with on-site records so that causes of generation decline are not left ambiguous, it becomes easier to prioritize inspections and explain the situation to stakeholders.

Summary: Separate the causes of low power generation into data-related and on-site factors

When you feel the power output is low, it is important not to immediately conclude there is a fault, but to decide on an order and systematically isolate the causes. First, clarify what you are comparing it to and check for natural variations due to weather and season. Then, on site, check for shadows, dirt, vegetation, and changes in the surrounding environment, and inspect equipment-side abnormalities such as panels, wiring, and power conditioners. Furthermore, confirm there are no discrepancies in the aggregation conditions of monitoring data and meter readings, and clarify any differences in assumptions between the design-stage expected generation and actual performance.

The causes of a drop in power generation are not necessarily limited to a single factor. Multiple factors can overlap—for example, dirt in addition to poor weather, low output in some circuits in addition to shading from vegetation, or communication failures causing readings to appear lower than they actually are. For this reason, checking only the data or only the on-site conditions is insufficient. Combining power generation trends, output by time of day, comparisons by equipment unit, on-site photos, and inspection records makes it easier to approach the true cause.

In operating a solar power plant, it is important not only to respond after noticing a decline but also to build systems to detect declines early. By continuously comparing monthly generation, keeping fixed-point records of on-site conditions, organizing inspection histories, and verifying the effects after improvements, you can reduce missed detections of generation declines. When you feel the generation is low, rely on records and comparisons rather than intuition, and use them to trigger necessary inspections and improvements—this leads to stable power plant operation.