Is low power generation on rainy days abnormal? 3 conditions to check

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Is low power output on rainy days abnormal? Conclusion first

Lower solar power output on rainy days is often not abnormal. Because solar power systems generate electricity from sunlight, if solar radiation weakens due to rain clouds, output drops significantly compared with sunny days. The Agency for Natural Resources and Energy also explains that renewable energies such as solar and wind frequently fluctuate in output depending on the weather.

However, there are cases where it is reasonable to conclude "it's low because of rain" and cases where you should suspect an equipment malfunction. For example, if power output is low only on rainy days and returns to normal on sunny days, that is basically a natural fluctuation. On the other hand, if power output is extremely low despite clear weather, if only one power conditioner shows a drop in its readings, if an error is displayed on the monitor, or if a clearly reduced power output persists compared to before, you should consider having an inspection.

This article explains the three conditions you should check when you feel that "power generation is low" on rainy days, focusing on residential solar power systems. To summarize, the three points to check are "weather and solar irradiance", "comparison benchmarks", and "equipment abnormalities". Checking them in this order makes it easier to distinguish whether the drop is a normal decrease or one that requires inspection.

Three conditions to check when you feel your power output is low

If you're worried that power generation is low, the first three conditions you should check are the following.

The first point is whether the day has conditions that are inherently unfavorable for power generation, such as rain, clouds, dawn and dusk, or winter. Solar power generation does not necessarily increase just because it is bright; it depends on the intensity of solar radiation reaching the panels. The Japan Meteorological Agency defines sunshine duration as "the time during which direct sunlight irradiates the ground" and currently defines sunshine as when direct solar irradiance is at least 0.12 kW/m². On rainy days, the period during which this condition is met tends to be shorter, so the amount of power generated is lower.

The second point is whether the numbers being compared are correct. If you simply compare yesterday’s generation on a sunny day, generation in spring, and the simulated value at installation with today’s generation on a rainy day, it’s only natural that the latter appears lower. The power output of solar power systems varies due to multiple factors: not only the weather but also the season, the time of day, the installation orientation, the roof angle, the temperature, shading, dirt, and the operating status of the power conditioner.

The third is to check whether there are any signs of abnormalities on the equipment side. If the power generation is only lower on rainy days, it is likely a natural decrease, but if there are error displays, shutdown indications, a tripped breaker, unusual noises, a burnt smell, a drop in only one string on the generation monitor, or if it does not recover even on sunny days, you should suspect a malfunction in the equipment or wiring.

In other words, when checking power generation on rainy days, it's important not only to ask "Is today's power generation low?" but also, in order, to check "Is a low output natural given today's conditions?", "Is it lower than on days with the same conditions?", and "Are there any signs of equipment malfunction?".

Condition 1: Check whether solar irradiance is reduced due to rain, clouds, or seasonal changes

The main reason power generation is lower on rainy days is that less sunlight reaches the panels. Solar power systems convert sunlight into electricity. On days with thick rain clouds, dark skies, or continuous precipitation, the light reaching the panels is weaker and power generation drops.

What's important here is that "power generation on rainy days is not necessarily zero, but you can't expect the same output as on sunny days." Even when it's cloudy or raining, if the sky is bright there will be some generation. However, compared with sunny days with strong direct sunlight, the amount of generation drops significantly. Therefore, if the monitor shows low values on a rainy day, that alone does not indicate a fault.

Also, even on the same rainy day, the amount of power generated can vary. A day with light rain and a relatively bright sky and a dark day covered by thick rain clouds produce different amounts of power. Even on a day that is dark in the morning but where the clouds thin in the afternoon, the total daily generation can still seem "low" if you look only at the one-day output. Furthermore, in winter the sun’s altitude is lower and daylight hours are shorter, so even when it’s sunny the generation can be less than in summer or spring.

If you want to objectively confirm solar radiation, the Japan Meteorological Agency's past weather data and NEDO's solar radiation database are useful references. In the Japan Meteorological Agency's past weather data search you can select a location and date to check meteorological data, and NEDO's solar radiation database publishes data by azimuth and tilt angle that can be used for solar radiation and power generation estimates.

In particular, if you want to determine whether a drop in power generation is due to the weather, it becomes easier to understand if you consider it as follows.

If the same area experiences prolonged rain, it is natural for power generation to remain low for several days. During the rainy season, around typhoons, or in the autumn rainy period there are fewer sunny days, so monthly power generation may be lower than expected. Conversely, if generation recovers on days when the weather improves, it is more likely that the cause was reduced solar irradiance rather than an equipment malfunction.

On the other hand, if nearby areas are sunny, the daytime sky is bright, power generation is significantly lower than past data for the same month, and generation does not recover even on the day after a sunny day, check for factors other than the weather.

Condition 2: Confirm whether the baseline for the compared power generation is correct

When you feel that power generation is low, what many people tend to overlook is "compared to what." Because power generation can vary greatly from day to day, using the wrong baseline for comparison can make perfectly normal generation look abnormal.

For example, in a household that produced 20kWh on a sunny day, if it only produced 2kWh or 3kWh on a rainy day, looking at the numbers alone it feels like a large drop. However, if the solar irradiance has fallen significantly due to rain, that difference can fall within the range of natural variation. Conversely, if similarly sunny days have continued but on one day production suddenly drops to less than half, an inspection of the equipment is necessary.

When comparing, we recommend looking at them in the following order.

First, compare days with the same weather. Rather than comparing a sunny day with a rainy day, it's important to compare "similar rainy days", "similar cloudy days", or "sunny days in the same season." If you have past records from generation monitors, HEMS, your power company's visualization service, or app history, compare several days within the same month instead of comparing single days.

Next, compare within the same season. Solar power generation varies in how easily it produces electricity depending on the season. In winter, days are shorter and the sun’s altitude is lower, so daytime generation hours are shorter. In summer, while daylight hours are longer, efficiency can drop due to rising panel temperatures. Therefore, it is not appropriate to compare "a sunny day in May" with "a rainy day in December" and judge one as lower.

Additionally, care is needed when comparing against simulation values. The power-generation simulation provided at installation should be viewed as an annual or monthly guideline. It does not guarantee that the system will generate that amount of power every day. Data such as NEDO’s solar radiation database can also be used as a reference for estimating generation, but actual power output varies depending on the day’s weather, installation conditions, shading, and the condition of the equipment.

Also, it is important not to confuse the amount of electricity sold with the amount generated. The amount generated is the total electricity produced by the solar power system. In contrast, the amount sold is the electricity sent to the utility company because it could not be consumed within the household. Not only can generation be low on rainy days, but on days when people are at home for long periods and electricity consumption is high, the amount sold may appear even lower.

For example, on rainy days, if you use many appliances such as the washer-dryer, air conditioner, dishwasher, induction cooker, and electric water heater, the electricity you generate is consumed at home and less is available to sell. In this case, a low amount of electricity sold does not necessarily mean that the power generation equipment is malfunctioning. By separating generation, consumption, sold electricity, and purchased electricity, it becomes easier to determine the cause.

Condition 3: Confirm that there are no signs of equipment trouble

When judging whether a drop in power generation on rainy days is natural, the most important thing is signs of equipment trouble. If you still have concerns after checking the weather and reference values, check the power conditioner, power generation monitor, circuit breakers, and the area around the panels.

The first thing to check is the displays on the power generation monitor and the power conditioner. Check whether any indications such as error codes, stop, grid-interconnection stop, islanded/standalone operation, abnormalities, or curtailment are displayed. If an error is shown, check the owner’s manual for details and consult your dealer or the manufacturer if necessary. Writing down the error code will make it easier to explain the situation when you contact them.

Next, when there are multiple power conditioners or multiple systems, check for imbalances in the readings. For example, if only one of two power conditioners is not generating, if only one circuit is extremely low, or if only certain strings have low output, there may be a problem with the equipment, wiring, the junction box, or some of the panels. Because overall generation is low on rainy days it can be hard to notice, but the differences often become clear on sunny days.

Also, the surface of the panels and the surrounding environment are inspection points. You don't need to climb onto the roof, but from a safe location such as the ground or a balcony, check within the visible range for fallen leaves, bird droppings, yellow sand, pollen, dust, accumulated snow, shadows from trees, shadows from nearby buildings, and shadows from antennas or power lines. In particular, if trees have grown or new buildings or installations that were not there before increase shading, power generation may decrease.

One thing to be careful about is not to assume that "rain will wash the panels, so dirt isn't a problem." Light dust may be washed away by rain, but bird droppings, fallen leaves, oil-containing grime, caked-on mud, yellow sand, and the like may remain. Also, dirt can accumulate near gutters or at the lower edge of the panels, which can affect power generation efficiency.

Typical signs that should prompt suspicion of equipment abnormalities are as follows:

• Extremely low power generation despite sunny conditions

• Power generation does not recover even after the rain stops.

• Monthly power generation is significantly lower compared to the same season in previous years.

• An error code is displayed on the power generation monitor.

• The power conditioner has stopped.

• The circuit breaker has tripped.

• Only one of the power conditioners has lower readings

• There is a burning smell, unusual noises, and abnormal heating.

• Damage and heavy soiling are visible on some parts of the panel.

• Power output decreased after a typhoon, lightning strikes, and heavy snowfall.

If any of these signs are present, do not attempt to restore them yourself; consult the installer, retailer, manufacturer, or a maintenance/inspection provider. Solar power generation equipment is electrical equipment, and work on roofs or inspecting the inside of the equipment involves risks.

Points to Note When Checking Power Generation on Rainy Days

When checking power generation on rainy days, it is important not to judge solely by the total daily generation. Power output fluctuates hour by hour with changes in the weather. A day that was rainy from morning until evening and a day that was only rainy in the morning and cleared in the afternoon will produce different amounts of electricity, even though both are "rainy days."

What you should look at is the power generation curve. On sunny days, the generation curve tends to rise from the morning, peak around midday, and fall in the evening, forming a mountain-like shape. By contrast, on rainy or cloudy days the peak is lower and, depending on the thickness of the clouds, the curve becomes jagged. Even if the output is low, if it varies slightly with the time of day, it may be responding to changes in light and generating power.

Conversely, if the generation remains close to zero during the day, suddenly drops abruptly and does not recover, or fails to rise even when there are sunny intervals, you should check whether the power conditioner has stopped, the breakers, grid-side issues, or equipment malfunctions.

Also, on rainy days you may be more concerned about the amount of electricity purchased than the amount generated. On days with little solar power generation, much of the electricity used in the home will be purchased from the utility. As a result, on days with low generation the amount purchased increases and your electricity bill may feel higher. This is not necessarily due to a failure of the generation equipment, but may occur when rain-related undergeneration coincides with increased household consumption.

For households that also use battery storage, the situation looks different. If rainy days continue, the amount that can be charged into the battery during the daytime decreases, and the electricity available at night also decreases. As a result, not only the power generation but also the battery’s remaining charge and the amount of electricity purchased are affected. A low battery remaining charge does not necessarily mean that the solar power system is malfunctioning.

7 Self-Check Items When Power Generation Is Low

If you notice low power generation, check the following seven items in order. It is important to focus only on checks that can be carried out within a safe range.

1. Check today's weather and the brightness of the sky

First, check whether the weather is one that makes power generation difficult, such as rain, cloudy conditions, fog, snow, yellow dust, or before and after a typhoon. Even on rainy days, power output differs between when the sky is bright and when clouds are so thick that it’s dark even during the daytime. Don’t rely only on the weather forecast; also look at the actual brightness of the sky when making your assessment.

2. Check sunshine hours and solar radiation data

By using the Japan Meteorological Agency’s past weather data search and NEDO’s solar radiation database, you can objectively verify local weather conditions. Rather than looking only at your home’s power generation, checking whether there were low solar radiation days across the entire area will improve the accuracy of your assessment.

3. Compare with days in the same season and the same weather

When comparing power output, compare rainy or cloudy days with similar conditions rather than sunny days. Furthermore, it is important to compare within the same season. Judging the output to be low by comparing a sunny day in spring with a rainy day in winter would not be correct.

4. View power generation and electricity sold separately

Even if the amount of electricity sold is low, it may simply be that much of the electricity generated is being used within the home. Check the generated electricity, consumption, electricity sold, and electricity purchased separately. In particular, on rainy days the use of dryers, air conditioners, and lighting tends to increase, which can make the amount sold appear low.

5. Check the displays on the monitor and power conditioner

Check for any indications of errors, stoppages, abnormalities, or curtailment. If an error code is displayed, take a photo or make a note of it. When contacting support, providing the date and time the issue occurred, the displayed message, the weather, and any changes in power generation will help isolate the cause more smoothly.

6. Check that the circuit breaker hasn't tripped

Check whether the electrical panel or the breaker for the solar power system has tripped. However, if the breaker trips repeatedly, or if you notice a burning smell, unusual noises, or overheating, do not repeatedly attempt to reset it yourself; consult a professional.

7. Check for changes in shadows, dirt, and the surrounding environment

From a safe location, check around the panels for any shadows or dirt. Trees may have grown, a neighbor may have installed new equipment, shadows from antennas or bird deterrents may have increased, or leaves may have accumulated — the surroundings can change after installation. Avoid climbing onto the roof to check, as it is dangerous.

Actions You Must Not Take Yourself

Even if you feel worried when power output is low, avoid undertaking dangerous work based on your own judgment. In particular, actions such as climbing onto the roof, removing panels, opening the inside of the power conditioner, touching wiring or junction boxes, washing by spraying water, or touching electrical equipment while wet are dangerous.

Solar photovoltaic systems carry electrical current while generating power during the daytime. Because they can generate electricity even on rainy days if there is light, they cannot be considered completely safe. Touching equipment with wet hands or opening the interior of outdoor units can pose a risk of electric shock or malfunction.

Also, take care when cleaning the panels. Using a household pressure washer too forcefully can put stress on the panels, sealants, and wiring. Depending on the detergent’s ingredients, it may also have adverse effects on the panel surface or metal parts. If you are concerned about dirt, consult the installer or a professional service.

The operation of the circuit breaker should also be carried out with caution. If it trips only once and can be safely reset following the instruction manual, there may be no problem, but if it trips repeatedly, stops again shortly after being reset, or there are unusual noises or smells, there may be an issue with the equipment. Repeatedly attempting to reset it in that condition may worsen the problem.

Guidelines for When to Consult a Retailer or Installer

A decrease in power generation on rainy days is often a natural fluctuation, but you should consult a professional in the following situations.

First, consider the case where power output does not recover on sunny days. If it is only low on rainy days, this can be attributed to reduced solar irradiance, but if power output is clearly lower than before even on sunny days, you need to check for equipment faults, shading, soiling, or aging-related degradation.

Next, this applies when an error appears on the power generation monitor or power conditioner. Error codes can provide clues to identifying the cause. It's a good idea to write down the displayed information and also take photos.

A prolonged decline in power generation is also a guideline for seeking consultation. If the decrease continues not just for 1 day but for 1 week or 1 month, check your monthly power generation history and compare it with the same month of the previous year and the same season in past years. The Japan Photovoltaic Energy Association’s basic knowledge page shows an example assuming an annual generation of 1,000 kWh for a 1 kW system, but actual generation varies depending on installation environment and regional conditions. Do not rely only on the guideline; it is important to compare with your home’s past data.

Furthermore, if power output drops after a typhoon, lightning strike, heavy snowfall, earthquake, hail, or strong winds, hidden damage may have occurred. Cracks in the panels, loosening of the mounting structures, damage to wiring, or faults in connection points may not be detectable from ground-level observation alone. If you notice any abnormalities after a disaster, do not attempt to inspect it yourself—request a professional inspection.

When asking for advice, it helps to have the following information prepared.

• Date when you felt the power generation was low

• The day's weather

• Power generation figures

• Comparison with the same season in previous years

• Presence or absence of an error code

• Power conditioner display

• Breaker status

• Recent events such as typhoons, lightning strikes, and heavy snowfall

• Changes in shadows and dirt around the panel

• Years since installation and manufacturer name

By conveying these, the contractor will be better able to isolate issues such as "performance degradation due to weather," "equipment shutdown," "problems on the panel side," "wiring/connection box problems," and "monitor display problems."

Frequently Asked Questions

Is it a malfunction if power generation is nearly zero on rainy days?

On days when rain clouds are thick and it’s dark even during daytime, power generation can drop significantly. However, if output remains close to zero throughout the day, does not increase even when a break in the clouds appears, or does not recover even on a sunny day the following day, check whether the power conditioner has stopped, whether a breaker has tripped, or whether there is an equipment malfunction.

Do they still generate some electricity on rainy days?

Even on rainy days, solar panels can generate electricity if the sky is bright and light reaches them. However, compared to sunny days, the amount of power generated drops significantly. The fact that power generation is lower on rainy days is a natural characteristic of solar power generation.

Why is the amount of electricity sold also low even though power generation is low?

The amount of electricity sold is the portion of the electricity you generated but did not use in your home. On rainy days, power generation tends to be lower, and if household consumption increases—because people are at home longer, a dryer or an air conditioner is used, or lights are left on for longer—the amount of electricity sold will decrease further. Check generated electricity and sold electricity separately.

If the power output is lower than last year, does that mean it's a malfunction?

It doesn’t necessarily mean there’s an immediate fault. There are multiple factors such as weather, hours of sunlight, temperature, shading, dirt, household electricity usage, output control, and how the monitor is read. However, if performance is substantially lower when compared with the same season and similar weather, or if the decline persists for an extended period, consider having it inspected.

I think the panels are dirty. Will the rain naturally clean them?

Light dust may be washed away by rain, but bird droppings, fallen leaves, yellow sand, pollen, mud stains and the like may remain. Climbing onto the roof to clean them yourself is dangerous. If a decrease in power generation due to dirt is suspected, consult the installer or a professional contractor.

What should I do if the power conditioner sounds different than usual?

If there are unusual noises, a burning smell, abnormal heat, or error messages, consult your retailer or the manufacturer before continuing to use it. Avoid opening it up yourself or touching it while it is wet.

Is there any way to prevent reduced power generation on rainy days?

You cannot prevent the decrease in solar irradiance caused by the weather itself. However, by reducing shadows around the panels, not leaving dirt on them, regularly recording power generation, not overlooking error messages, and having inspections as needed, it becomes easier to detect decreases in power generation due to equipment problems at an early stage.

Summary

Low power generation on rainy days is, in many cases, not abnormal. Solar power generation is influenced by the amount of sunlight, so on days with thick rain clouds, short hours of sunshine, or a dark sky, generation falls much more than on sunny days. The Agency for Natural Resources and Energy also explains that renewable energy such as solar power fluctuates in output depending on the weather.

However, whether you can be reassured should be checked against three conditions. The first is whether the decrease in solar irradiance is due to rain, clouds, the season, etc. The second is whether you are comparing it to a sunny day or a different season, or confusing generated output with exported (sold) electricity. The third is whether there are any signs of equipment abnormalities, such as error messages, shutdowns, a drop only in one string/system, breaker trips, shading, or dirt.

If you're unsure, check the power generation not only on rainy days but also on sunny days. If generation recovers on sunny days, it's likely a natural reduction due to the weather. On the other hand, if output remains low even when it's sunny, errors are appearing, or the generation is clearly lower than before, contact your retailer, installer, or the manufacturer as soon as possible.

When power output is low, the important thing is not to rush to climb onto the roof or to touch the inside of the equipment. Calmly check the generation monitor readings, the weather, past generation history, and any error codes, and systematically narrow down the cause within a safe range. Low generation on rainy days is often normal, but a decline that does not recover even on sunny days is a sign that an inspection is needed.