First check whether it is actually too little – weather and season explain a lot, and a sudden drop often turns out to be a creeping trend in the monitoring. If a genuine decline remains, the most common causes are: an interrupted data connection, increased shading, an inverter fault or a failed string. Narrow it down from the ground, then the specialist company comes. (As of: July 2026)
Key points in brief
- The first step is not a tool, but a clean comparison: the same month with the previous year, not summer with winter.
- Much of what looks like a defect is normal – weather, season, summer heat and a minimal annual loss of performance are part of it.
- The most common «fault» is not one at all: the data connection is gone, the app shows nothing, the system produces flawlessly outside.
- Genuine causes in order: connection, shading, inverter, failed string – dirt and module defects are rare.
- No one belongs on the roof without necessity: you can narrow it down from the ground and from the app, the specialist company takes over the rest.
Is your system really delivering too little – or does it just look like it?
Before you look for a cause, clarify whether there is a problem at all. The most common reason for the impression of «too little» is a skewed comparison. A solar system delivers a fraction of the June yield in December – this is not a fault, but the course of the sun. The comparison only becomes meaningful when you measure the same month with the previous year, not the current week with the last.
Add to that the weather. A gloomy month can be significantly below average, a sunny one above, without anything being different about the system. Only over several years and in a monthly comparison does it become apparent what is a trend and what is weather noise.
And then there is the trickiness of perception: a «sudden» drop is rarely sudden. If you look into the monitoring after the first anomaly, the decline often shows up as a creeping curve over months – only nobody noticed it because nobody looked. That is exactly why the year-on-year comparison is the most important diagnostic tool of all.
The harmless explanations first
A large part of the «reduced yield» cases vanishes into thin air as soon as the normal effects are known. They belong at the beginning of every search for a cause, because they do not need a tradesman.
| Effect | Impact on the yield | normal? |
|---|---|---|
| Season | Winter delivers a fraction of summer | yes – winter yield |
| Weather | individual months strongly above/below the average | yes |
| Summer heat | hot modules deliver minimally less | yes – temperature effect |
| Ageing | slight decline of less than one per cent per year | yes – degradation |
| Snow | failure for days until it slides off | yes, seasonal |
Guideline values for classification, As of: July 2026. The comparison of your own system over several years is decisive.
The order is important: only if these explanations do not cover the decline – i.e. if a sunny month is significantly below the previous year, without snow, without a heatwave – is it worth looking for a genuine cause. In short: not every weak month is a case for a service visit.
What are the most common genuine causes?
If a decline remains that cannot be explained by weather and season, work through the causes from frequent to rare. The order in this table corresponds to our experience from service cases, not a gut feeling – the classic is rarely the broken module.
| Cause | typical sign | first step |
|---|---|---|
| Data connection interrupted | App shows nothing or old values, but utility bill is correct | Check router/Wi-Fi, read meter |
| Shading (grown) | Decline over the year, at certain times of day | Check tree/structure, shading |
| Inverter fault | Fault message, error code, system is completely at a standstill | Read message, inverter |
| String failed | Part of the system delivers zero, rest normal | String comparison in monitoring |
| Dirt | slow decline, visible deposits | rare – only with stubborn dirt |
| Module defect | single module permanently weak | rare – specialist company checks |
Frequency as an empirical value, As of: July 2026. The specific finding results from the monitoring of the individual system.
Two things stand out. Firstly, the data connection is right at the top – not because it paralyses the system so often, but because it so often creates the impression of a failure, while everything is running outside. The failure is then the app, not the technology. Secondly, the inverter is the component that is most likely to actually cause a fault – it is the component with the highest probability of failure over the life of the system and is typically replaced once. An inverter replacement sounds more dramatic than it is: announced by the fault message, done in hours.
How to narrow down the cause yourself
You get surprisingly far without stepping onto the roof – and that is exactly where you should stay. These steps narrow down most cases:
1. Compare correctly. Same month, previous year. If the value is within the usual range, everything is fine. 2. Check the app against reality. Does the app show nothing, but the system obviously has sun? Check the last utility bill or the meter – often the system is producing, only the data connection is missing. 3. Look for a fault message. Check the inverter display or portal for an error code and fault message. A clear message takes away half the search work. 4. Read the string comparison. If part of the system delivers zero while the rest runs normally, a string or inverter problem is likely. 5. Note the time of day. If the yield always drops at the same time of day, this indicates shading – a tree or structure that has grown over the years.
What you should never do in the process: climb onto the roof yourself. The roof belongs to the professional, the ground to you – the risk of falling, sensitive modules and warranty conditions make any DIY work up there a bad deal. If the narrowing down leaves a genuine cause or a fault message remains, calling the specialist company is the right next step – with your observations in your luggage, which make the work easier for the company.
A special case also belongs here: the system produces flawlessly – and yet the credit on the bill has shrunk. This is not a technical reduced yield, but mostly the fallen feed-in tariff: anyone who feeds in a lot and has little self-consumption notices the price drop directly. The lever is then not a service visit, but more self-consumption – for example with a retrofitted battery. We clarify whether this pays off at your location in the free battery check.
The service calls on the subject of «too little yield» paint a surprisingly uniform picture for us – just a different one than most expect. The most common case is not the defective module, but the system that «suddenly» delivers less, whereby the suddenness then turns out to be three-quarters of a year in the monitoring. Right after that comes the app, which has shown no data since the router change, while everything is producing flawlessly outside. Only in third place are the genuine defects, and even they are mostly unspectacular: an inverter that the monitoring announces in good time. Our conclusion after many such calls is unspectacular but reliable: it is not the technology that decides how much a system delivers – but whether someone looks at the numbers before a small decline becomes a big one.
Frequently asked questions
How much reduced yield is still normal?
From year to year, the yield fluctuates noticeably due to the weather – individual years are significantly above or below the average without anything being defective. Less suspicious is a single weak year than a trend over several years or a sudden drop that the weather does not explain. The monthly comparison with the previous year separates the noise from the real problem.
My app shows zero – is the system broken?
Mostly not. The most common reason is an interrupted data connection, for example after a router or Wi-Fi change – the system continues to produce outside, only the message is missing. Check whether the grid operator's last bill or the meter shows production before suspecting a defect.
Can heat in summer lower the yield?
Yes, and that is normal. Solar modules lose some efficiency as the temperature rises, which is why a very hot day is not automatically the one with the highest yield. This is a physical effect, not a defect – and one of the reasons why the cooler, sunny spring days often deliver surprisingly strongly.
How do I recognise that the inverter is defective?
Typical are a fault message in the portal, an error code on the device or a system that is completely at a standstill instead of just delivering more weakly. The inverter is the component that is most likely to need replacing at some point. Read the message and provide the code to the specialist company – this shortens the diagnosis considerably.
When should I call a specialist company?
If weather, season and data connection are ruled out as an explanation and a genuine decline remains – or if a fault message does not disappear on its own. A permanently conspicuous string in the comparison is also a good reason. Bring your observations with you; they are more valuable for narrowing things down than many people think.
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Sources: General operation and monitoring practice for PV systems; empirical values from the service and support practice of ecoEn GmbH, Zurich region.
Last updated: 9 July 2026 · Author: ecoEn editorial team
