12 Monthly Benchmarks to Check If You Feel Power Generation Is Low

Table of Contents

• What to check first when you feel your power generation is low

• Estimated monthly electricity generation — 12 months

• 5 perspectives for isolating the causes of low power generation

• Reasons why power generation varies by season

• 10 self-check items when power generation is low

• Cases that are highly likely to be abnormal

• Measures to restore and improve power generation

• Data to prepare before consulting a contractor

• Frequently Asked Questions

• Summary

What to check first when you feel the power generation is low

When using solar power generation, you may sometimes feel "This month's output is lower than I expected," "It seems to be generating less than last year," or "I'm worried that the revenue from selling electricity has decreased."

Especially during periods when rising electricity bills or decreased income from electricity sales are a concern, people tend to become more sensitive to changes in generation.

However, low output alone does not necessarily indicate a fault. Solar power output fluctuates due to many factors, including the season, weather, hours of sunlight, temperature, panel tilt, orientation, shading, dirt, and the condition of the power conditioner. Monthly variations can be especially large, and output tends to be lower in winter and during the rainy season.

The first thing to check is not whether this month's power generation is low, but whether it is unusually low for that month. For example, it's natural for generation in December or January to be lower than in May or August. On the other hand, if, despite many sunny days, generation is more than 30% lower than the same month in the previous year, if there are several consecutive days of extremely low generation, or if daytime generation is nearly zero, you should suspect some kind of abnormality.

This article summarizes month-by-month benchmarks for all 12 months that you should check when you feel your solar power generation is low, and explains how to distinguish causes and countermeasures. By checking seasonal characteristics along with the numbers, you can more easily determine whether the decrease is normal or an abnormality that requires inspection.

Monthly Power Generation Estimates — 12 Months

The electricity generation of a solar PV system is easier to compare when viewed as the monthly generation per 1 kW of installed capacity. Even for common residential systems of 4 kW, 5 kW, or 6 kW, first checking "how many kWh are generated per 1 kW" allows you to assess performance without being affected by differences in system size.

The following are monthly guidelines for a typical residential solar power generation system. Because results can vary depending on region, roof orientation, panel angle, sunlight conditions, manufacturer, and installation age, please treat these only as reference values.

The important thing in this table is to look at the month-to-month differences. For example, it is not uncommon for a household with a 5 kW system that generated around 600 kWh in May to produce about 320 kWh in December. Conversely, if in months that are easy to generate power, such as May or April, a 5 kW system only generates about 200 kWh in a month, you should suspect shading, soiling, equipment malfunction, or metering problems.

There are regional differences even within the same month. In parts of Hokkaido and Tohoku, snowfall affects conditions in winter, and on the Sea of Japan side there are areas where cloudy weather becomes more frequent in winter. Conversely, on the Pacific side there is more sunshine even in winter, making some regions relatively favorable for power generation. You don't need to be worried if output is only slightly below the expected level, but if it has fallen significantly compared with the same month of the previous year or with nearby generation records, you should be cautious.

Reasons for Low Power Generation in January and Points to Check

January is one of the months of the year when power generation tends to be lower. The reasons are that daylight hours are shorter, the sun’s elevation is lower, and the hours of generation in the morning and evening are shorter. Furthermore, some regions are affected by snowfall, frost, and cloudy conditions.

A guideline for monthly generation per 1 kW is about 60–90 kWh. For a 5 kW system, you can expect around 300–450 kWh. Of course, if panels are covered in snow for long periods, actual output may fall below this guideline.

In January, what you should check is whether the system is properly generating power during the daytime on sunny days. Even if the monthly generation is low, if the output is solid between 10:00 and 14:00 on sunny days, it's likely a seasonal factor. On the other hand, consider an inspection if generation is extremely low even on clear days, the power conditioner displays an error, or it appears that only one side of the roof is not generating power.

Reasons for Low Power Generation in February and Points to Check

In February, sunshine hours gradually increase compared with January, and electricity generation also begins to recover. However, solar elevation is still low, and the effects of cold snaps, snowfall, and overcast skies remain, so the month shows large regional variations.

The monthly power generation per 1 kW is approximately 70-100 kWh. For a 5 kW system, use roughly 350-500 kWh as a guideline. Even if output does not increase significantly compared with January, that can be within the normal range when the weather is bad.

In February, you should also be aware of the impact of shadows. Because the sun's elevation is low, shadows from neighboring houses, trees, antennas, utility poles, and roof level differences are more likely to fall on the panels. In particular, if shadows not only lengthen in the morning and evening but also fall around midday, power output tends to decrease. In households where power output drops significantly only in winter, shadows may be the main cause rather than a malfunction.

Reasons for Low Power Generation in March and Points to Check

In March, daylight hours lengthen and the sun’s altitude rises, so power generation tends to increase. Compared with winter, it is common to see a clear recovery in generation.

As a rule of thumb, monthly power generation per 1 kW is about 90–120 kWh. For a 5 kW system, around 450–600 kWh is one benchmark. If, even in March, generation is only at January levels, you should check equipment-side factors as well as the weather.

At this time of year, you also need to be aware of panel soiling caused by yellow sand and pollen. Light soiling may be washed away by rain, but if bird droppings, dust, fallen leaves, or sticky dirt remain, power generation efficiency can decrease. Within a range that can be checked safely, rather than going onto the roof, check the monitor and the power generation graph to see whether the generation curve on sunny days is unnaturally low.

Reasons for Low Power Generation in April and Points to Check

April is a month when power generation tends to be higher throughout the year. Because temperatures are not too high and solar irradiance increases, it tends to be a favorable period for solar power generation.

Monthly generation per 1 kW is about 100–130 kWh. For a 5 kW system, you can expect around 500–650 kWh. If generation in April is extremely low, it may be difficult to explain solely by poor weather.

The point to check is a comparison with the same month of the previous year. If last April was 600kWh and this April is about 560kWh, it may not be a major problem. However, if last April was 600kWh and this April is 350kWh, a difference like that means you need to check for a stoppage in power generation, a malfunction of the power conditioner, a breaker fault, dirty panels, increased shading, and so on.

Reasons for Low Power Generation in May and Points to Check

May is one of the months when solar power generation tends to be highest during the year. With long hours of sunlight and temperatures that are not as high as in mid-summer, panel efficiency is relatively well maintained.

The monthly power generation per 1 kW is approximately 110–140 kWh. For a 5 kW system, it may be around 550–700 kWh. If generation in May is low, you should check the cause promptly.

Be especially alert if there were many sunny days but power generation did not increase. Solar panels lose efficiency when they become hot, but May is not yet as hot as midsummer. Therefore, if generation is clearly low in May, possible causes include dirt on the panel surface, faults in the connection system, output curtailment by the power conditioner, or trouble with the measuring equipment.

Reasons for Low Power Generation in June and Points to Check

June tends to have lower power generation due to the rainy season. Although the hours of sunshine themselves remain long, the increase in cloudy and rainy days generally causes monthly power generation to be lower than in May.

The monthly power generation per 1 kW is about 80–110 kWh. For a 5 kW system, you can expect around 400–550 kWh. In June, if there are few sunny days, it may be lower than the guideline.

The key point in judging is the correlation with the weather. It is natural for power generation to be low on rainy or cloudy days. However, check whether power generation has sufficiently recovered during sunny intervals in the rainy season. If the power generation curve on a clear day does not form a peak and instead suddenly drops midway, output curtailment, a temperature rise in the power conditioner, or equipment malfunction may be involved.

Reasons for Low Power Generation in July and Points to Check

July has a high amount of solar radiation, so high electricity generation can be expected when it is sunny. On the other hand, it is also a month prone to impacts such as prolonged rain before the end of the rainy season, typhoons, sudden thunderstorms, and efficiency declines due to high temperatures.

The monthly electricity generation per 1 kW is approximately 90–120 kWh. For a 5 kW system, the guideline is roughly 450–600 kWh. In years with many sunny days it tends to be higher, while in years with a lot of rain it can fall substantially.

What to watch for in July is a decrease in power generation efficiency due to high temperatures. Solar panels generate more electricity the stronger the sunlight, but when panel temperatures rise, their efficiency falls. For that reason, even on a clear midsummer day, the peak output can appear lower than on a clear spring day. This is not necessarily a malfunction.

However, if power generation drops sharply around midday, the power conditioner repeatedly stops and restarts, or error messages appear, there may be overheating or equipment malfunction.

Reasons for Low Power Generation in August and Points to Check

August is a month when the sunlight is strong and it might seem that power generation would be high, but in reality it is affected by efficiency losses caused by high temperatures. In addition, typhoons, evening showers, sudden localized heavy downpours, and cloud formation lead to large day-to-day variations in power generation.

The monthly power generation per 1 kW is approximately 95–125 kWh. For a 5 kW system, expect roughly 475–625 kWh. Even if sunlight is stronger than in May, higher temperatures can cause generation to be lower than in May; this is not uncommon.

If you notice low power generation in August, first check not only the peak generation on sunny days but also the total daily generation. Even if the daytime peak is slightly lower, if generation is steady from morning through evening it may be within the normal range. On the other hand, if it’s sunny but generation only occurs in the morning, if output plunges in the afternoon, or if generation suddenly drops from a particular day, those cases should be inspected.

Reasons for Low Power Generation in September and Points to Check

September is the period when summer transitions into autumn, and it is a month that is prone to the effects of typhoons and the autumn rainy season. Sunny days generate electricity, but if cloudy or rainy weather continues, monthly power generation decreases.

Monthly power generation per 1 kW is about 80–110 kWh as a guideline. For a 5 kW system, it's around 400–550 kWh. In years with many typhoons or prolonged rainy seasons, it can be even lower.

September is an important time to carry out checks after typhoons. Strong winds may have caused flying debris to strike the panels, leaves and branches to accumulate around the panels, or the mounting racks and wiring to be affected. Avoid going up onto the roof to inspect because it is dangerous; instead, check the power generation monitor, the power conditioner display, breakers, any unusual noises, and any burnt smell from a safe distance.

Reasons for Low Power Generation in October and Points to Check

October is a month when temperatures fall, and in regions with many sunny days power generation tends to be more stable. However, because sunlight hours are shorter than in summer, it is generally not as high as in August or May.

A guideline for monthly power generation per 1 kW is approximately 80–110 kWh. For a 5 kW system, use around 400–550 kWh as a reference when checking.

If power output is low in October, check for autumn rains, post-typhoon dirt, fallen leaves, and changes in shading. As the sun's altitude begins to decrease, shadows from trees and buildings that were not a problem in summer may start to fall on the panels from autumn onward. In particular, if power output drops dramatically only in the afternoon, it may be caused by shadows from the west, shadows cast by neighboring houses, or the growth of garden trees.

Reasons for Low Power Generation in November and Points to Check

In November, hours of sunshine become shorter and the sun's altitude decreases, so power generation gradually declines. It is natural for it to be lower than in October.

A rough guideline is that monthly electricity generation per 1 kW is about 65–95 kWh. For a 5 kW system, expect around 325–475 kWh.

If you feel that power generation is low in November, it is often due to seasonal factors and shading. In particular, roofs that are not south-facing, roofs with a shallow pitch, and houses with surrounding buildings or trees tend to experience a decline in power generation from autumn to winter.

However, if it is significantly lower than the same month of the previous year, please also check the condition of the equipment. The lifespan of a power conditioner is generally shorter than that of solar panels, and there are cases where faults appear around ten years after installation. If the decline in power generation is increasing year by year, consider aging-related degradation or equipment malfunction as possible causes.

Reasons for Low Power Generation in December and Key Points to Check

December is a month when annual power generation tends to be lower. With shorter daylight hours and lower solar altitude, the periods during which electricity can be generated are limited.

The monthly power generation per 1 kW is approximately 55–85 kWh. For a 5 kW system, that would be around 275–425 kWh. In some regions it can be even lower due to cloudy weather or snowfall.

You don't need to worry in December just by looking at the monthly generation. What matters is whether generation on sunny days is normal. Even in winter, if it's clear you should see a steady amount of generation around midday. If it's sunny but generation is almost zero, the power conditioner has stopped, or the monitor shows abnormal indications, it's likely a problem rather than a seasonal factor.

5 Perspectives for Isolating the Causes of Low Power Generation

When power output is low, it is important not to assume a single cause but to isolate possible causes one by one. There are five main perspectives.

1. Degradation due to weather

The most common cause is the weather. On cloudy or rainy days, electricity generation falls substantially compared with clear, sunny conditions. Light cloud cover still allows for some generation, but if thick clouds or rain persist, monthly generation drops sharply.

Especially in June, September, and on the Sea of Japan side in winter, weather-related variability is large. If you feel the power generation is low, first review the number of sunny days in that month. It is natural for power generation to be low in months with poor weather.

2. Seasonal decline

Power output changes with the seasons. Spring, when daylight hours are long and temperatures are not too high, tends to produce more electricity, while in winter, when daylight hours are short, output decreases. In summer, although solar irradiance is greater, panel temperatures rise and efficiency drops, so output can be lower than in spring.

If you think "it should generate the most in summer," you're likely to feel that August's power generation is low. However, in reality some households have higher power generation in May or April.

3. Degradation due to shading

Shadows have a significant impact on power generation. Solar panels can experience reduced overall output even if only a portion is shaded. Causes of shading include neighboring houses, trees, utility poles, antennas, chimneys, changes in roof level, mountains, and buildings.

The impact of shadows changes with the seasons. Even if there’s no problem in summer, the sun’s altitude drops in autumn and winter and shadows become longer. Therefore, if you see that only winter has lower power generation, it’s worth suspecting shading.

4. Degradation Due to Dirt

When dirt adheres to the panel surface, power generation efficiency decreases. Some dirt may be washed away naturally by rain, but bird droppings, yellow sand, pollen, mud, fallen leaves, and tree sap can remain.

Performance declines due to soiling usually occur gradually rather than suddenly. However, if bird droppings or similar contaminants concentrate on some panels, they can affect the power generation of the entire circuit. Working on a roof is dangerous, so avoid climbing up to clean them yourself and consult a professional contractor when necessary.

5. Decrease due to equipment malfunction

If power generation is lower than can be explained by weather or seasonal factors, there may be an equipment malfunction. Typical causes include failure of the power conditioner, a tripped breaker, wiring faults, junction box problems, and abnormal measurements from the monitor.

In particular, if the power output suddenly drops to near zero, falls sharply from one day to the next, an error code is displayed, or there are unusual noises or odors, please request an inspection promptly.

Reasons Why Electricity Generation Varies by Season

Solar power generation is not simply "the hotter it is, the more power is generated" or "the stronger the sunlight, the more power is generated." Power output is influenced by a combination of factors: solar irradiance, hours of sunshine, solar altitude, temperature, and weather.

Spring is a season when daylight hours lengthen and temperatures are not too high, so power generation efficiency tends to improve. Therefore, April and May tend to have higher power generation within the year.

In summer, solar irradiance is strong and daylight hours are long, but panel temperatures rise. Because solar panels’ conversion efficiency decreases as temperature increases, prolonged periods of extremely hot days make it harder for power output to reach its peak. Typhoons, sudden evening downpours, and high-humidity clouds also affect performance.

In autumn, temperatures fall and power generation efficiency tends to recover, but the hours of sunlight become shorter. Typhoons and the autumn rainy season also have an impact, causing monthly variations in power output.

In winter, because temperatures are low, panel efficiency is not necessarily worse; however, shorter daylight hours and a lower sun angle mean total power generation is reduced. In snowy regions, panels may not generate electricity while they are covered with snow.

As such, there are many natural reasons for decreases in power generation. By checking monthly guidelines together with seasonal factors, you can reduce unnecessary worry.

10 Self-Checks to Perform When Power Generation Is Low

If you feel that power generation is low, check the following 10 items in order. Avoid dangerous tasks such as going onto the roof, and make sure to perform checks only within a range where you can do so safely.

1. Compare with the same month of the previous year

First, check the power generation for the same month of the previous year. Since solar power generation varies greatly from month to month, it's difficult to judge based only on a comparison with the previous month. It's natural to see large differences when comparing May and June, or August and December.

If the difference compared with the same month of the previous year is around 10%, it may be due to weather variations. If it's 20% or more lower, check the weather and shading; if it's 30% or more lower, consider inspecting the equipment as well.

2. Viewing the power generation graph on a clear day

We look at not only the monthly power generation but also the power generation graph for a sunny day. Under normal conditions, generation starts in the morning, reaches a peak around midday, and then falls toward the evening, forming a mountain-shaped curve.

If the output suddenly drops midway, if it does not generate during the daytime, if the output is extremely jagged, or if it remains almost flat despite sunny conditions, possible causes include shading, output curtailment, or equipment malfunction.

3. Check the power conditioner display

Check the power conditioner for any error messages or warning lights. Because the displayed information varies by manufacturer, refer to the user manual or the manufacturer's website to confirm the meaning of the error codes.

If an error occurs, please do not disassemble or touch the internal parts yourself. There is a risk of electric shock and of worsening the malfunction.

4. Check that the circuit breaker hasn't tripped

If the breaker for the solar power system has tripped, even if it is generating electricity, it may not be reflected on the household side or the export (sell-back) side. Check the distribution board and the dedicated breaker to see whether they are in their normal positions.

However, if the breaker keeps tripping, there may be a fault. Do not keep forcing it back on; consult the installation company or an inspection/maintenance contractor.

5. Check monitors and apps for measurement malfunctions

Even if the power generation appears low, it may actually be caused by a fault in the measurement monitor or communication equipment. If the app loses connection, the monitor has a display error, or data updates are delayed, the power generation will not be displayed correctly.

Checking multiple sources—such as the electricity sales statement, the power company's data, and the power conditioner's built-in display—makes it easier to make a judgment.

6. Check that shadows are not increasing

Seasonal changes, a neighbor's new house, the growth of garden trees, or the installation of a storage shed can increase shadows that weren't there before. Shadows are especially long in the morning and evening, and during autumn and winter.

If possible, on a sunny day check the area around the roof from the ground at 9:00, 12:00, and 15:00 to confirm that no shadows are cast on the panels.

7. Check for dirt and fallen leaves on the panels

From the ground, check the panels within your line of sight for any significant dirt, fallen leaves, branches, bird droppings, or similar. Avoid going onto the roof because it is dangerous.

If the dirt is widespread or does not improve even after rain, consider having it cleaned by a professional.

8. Confirm the possibility of output suppression

Depending on the region and contract terms, output curtailment may be applied due to the condition of the power grid. When output curtailment is in effect, generation may be reduced even when it is sunny.

A curtailment indicator may appear on the power conditioner or monitor, so check the displayed information. If power generation drops only during specific time periods, the possibility of output curtailment should also be considered.

9. Confirm the installation year

As years pass after installation, the performance of solar panels and power conditioners may decline. Solar panels are long-lasting equipment, but they are not immune to deterioration.

In particular, power conditioners may need to be replaced or repaired after around 10 years. If it has been more than 10 years since installation and power generation has declined, it is worth having an inspection.

10. Confirm the warranty details

We will check the contents of the manufacturer warranty, installation warranty, output guarantee, and natural disaster coverage. If it is within the warranty period, you may be able to reduce the cost burden for repairs or replacements.

If you organize warranty certificates, contracts, the contractor's contact information, the manufacturer's name, model numbers, and so on, inquiries will go smoothly.

Cases with a High Likelihood of Abnormality

A decrease in power output does not always indicate a malfunction, but in the following cases an abnormality is more likely.

First, the case where power generation is almost zero despite clear weather. If the system is not generating power even though it is clear and sunny during the day, possible causes include the power conditioner stopping, a breaker malfunction, or wiring trouble.

Next, consider the case where, from a certain day onward, power generation suddenly falls to less than half. If this is not due to a change in the weather, part of the equipment may have stopped. If only some of the multiple circuits are not generating power, the overall power output will drop significantly.

Also, pay attention if an error code appears on the power conditioner. Depending on the error, it may be temporary, but if it appears repeatedly or does not clear, an inspection is necessary.

If you notice a burning smell, unusual noises, abnormal heating, water leakage, or damage, do not immediately decide to continue using the equipment; consult a qualified professional. Problems with electrical equipment can also affect safety.

Measures to Recover and Improve Power Generation

After identifying the cause of low power generation, we will take measures appropriate to the cause.

If the cause is weather or seasonal factors, no major countermeasures are necessary because there is no problem with the equipment. Monitor the situation, using monthly benchmarks and year-on-year comparisons for the same month to confirm whether the variation is natural.

If shading is the cause, pruning garden trees, moving unnecessary items, and checking the positions of antennas and equipment can sometimes improve the situation. However, shadows from neighboring houses or nearby buildings may be beyond your ability to resolve on your own. You can also consider operational improvements that assume fluctuations in power generation, such as installing a new storage battery or shifting electricity use to daytime.

If dirt is the cause, check whether rain will improve the situation, and if it does not, consider having the panels cleaned by a professional. Avoid climbing onto the roof to clean them yourself, as there is a high risk of a fall and you may damage the panels.

If equipment malfunction is the cause, consult the installation company, the manufacturer, or the inspection/maintenance provider. If it is within the warranty period, it may be covered by the warranty. Even if the warranty has expired, repairing it promptly can reduce losses from power sales and from self-consumption.

If deterioration of the power conditioner is the cause, replacement may be more appropriate than repair. Replacing an older model with a newer one can also improve power conversion efficiency and monitoring capabilities.

Data to Prepare Before Consulting a Contractor

When consulting an inspection company or contractor, organizing your data in advance will make it faster to identify the cause. Have the following information ready.

• Installed capacity

• Installation year and month

• Solar panel manufacturer and model number

• Manufacturer and model number of the power conditioner

• Electricity generation over the past 12 months

• Electricity generation in the same month of the previous year

• When you began to notice a decrease in power generation

• Presence or absence of error codes or warning displays

• Power generation graph for a sunny day

• Electricity sales statements and power company data

• Presence or absence of shadows and dirt

• Triggers such as typhoons, lightning strikes, and construction work

What is especially important is “when it started to drop.” Whether the decline has been gradual or whether it suddenly fell on a particular day changes the causes you should suspect. If it has been decreasing gradually, consider dirt, shading, or age-related deterioration. If it dropped suddenly, suspect equipment shutdown, the breaker, wiring, or a problem with the power conditioner.

Judgment mistakes to avoid when power generation is low

When you feel that your power generation is low, there are common mistakes in judgment.

The first is comparing only with the previous month. For example, if power generation decreases from May to June, it may naturally decline due to the rainy season. If you are going to compare, the basic approach is to use the same month of the previous year.

The second is judging solely by the amount of revenue from electricity sales. Sales revenue depends not only on generation output but also on self-consumption, the selling price per unit, and electricity usage. Treat generation output as generation output, and check it separately from sales revenue.

The third is assuming that summer produces the most electricity. Although sunlight is strongest in midsummer, panel temperatures rise and efficiency drops. May or April can sometimes produce more electricity.

The fourth is concluding that it’s a fault based solely on the monitor display. Communication errors or display glitches can make the power output appear low. Check the power conditioner unit itself and the utility company's data as well.

The fifth is attempting to climb onto the roof yourself to check. Inspecting and cleaning solar panels involve working at height and are dangerous. For any work that goes beyond what you can safely inspect, hire a professional contractor.

Frequently Asked Questions

Which months have low power generation?

In general, December, January, and February tend to be months with lower power generation. This is because daylight hours are shorter and the sun's altitude is lower. Also, June can be lower due to the rainy season, and September can be lower due to typhoons and autumn rains.

Which month has the highest power generation?

April and May tend to be months with higher solar power generation. The longer hours of sunshine and temperatures that are not too high make them well suited to solar power generation. Depending on the region and the weather, generation in August can also be high, but in midsummer efficiency drops due to high temperatures.

Is 300 kWh per month low for a 5 kW solar power system?

It depends on the month. In December or January, a 5 kW system can produce around 300 kWh in a month. On the other hand, if it only produced 300 kWh in a month in April or May, that may be somewhat low. Check the generation graphs for the same month in the previous year and for sunny days.

How much electricity is generated on rainy days?

There can be a small amount of power generation even on rainy days, but it decreases significantly compared with clear, sunny conditions. On days when thick clouds and rain persist, the amount of power generated can be considerably lower. Avoid concluding that there is a fault based only on the power generation on rainy days.

Will cleaning the panels restore power generation?

If the issue is caused by dirt, cleaning may improve it. However, if the reduction in power output is due to weather, shading, or equipment malfunction, cleaning alone will not resolve it. It is important to first check for causes other than dirt.

What should I do if power generation suddenly drops to zero?

If the power output is close to zero even during a clear daytime, possible causes include faults in the power conditioner, breakers, monitor, wiring, and so on. Check for error indications and the breaker status, and if it does not recover, consult the installation company or a qualified technician.

Does power generation decrease after more than 10 years since installation?

Solar panels are equipment that can be used for long periods, but they do experience performance degradation with age. Also, power conditioners may develop faults after around 10 years. If it has been more than 10 years since installation and power generation has noticeably declined, consider having an inspection.

Summary

When you feel the power output is low, first compare it with the monthly benchmarks to confirm whether the decline is natural for that month. Solar power generation tends to be higher in April and May and lower in December and January. The rainy season in June, typhoons and autumn rains in September, and efficiency losses from high temperatures in midsummer also affect power output.

The basic rule for assessment is to compare with the same month of the previous year, not with the previous month. Furthermore, by checking in order the power generation graph for clear days, the power conditioner display, the breakers, shading, dirt, and the installation age, it becomes easier to isolate the cause.

For a 5 kW system, monthly generation varies greatly — around 550–700 kWh in May and around 275–425 kWh in December. If output is only low in winter or during the rainy season, that may be within the normal range; however, if the system produces no power even on sunny days, is more than 30% lower than the same month the previous year, experiences a sudden drop after a certain day, or displays an error message, having it inspected promptly is advisable.

A drop in power generation does not necessarily indicate a malfunction. However, if left unaddressed, you may lose revenue from selling electricity and the benefits of self-consumption. Check from three perspectives—monthly benchmarks, the same month of the previous year, and graphs for sunny days—and if an anomaly is suspected, do not try to judge it yourself; consult a professional.