Short answer
First compare with the same month last year, because the weather varies a lot. If the difference stays large, look at shade, dirt, a failed string or an inverter switching off due to grid voltage. A string that drops out completely shows up as roughly half or a third less yield.
Who this article is for Anyone who feels their panels are doing less. Finding out starts with the app, and you can do that yourself.
Compare with last year first
Sunny years easily differ by ten per cent. So do not look at a week but at the same month last year, and preferably at the annual total. Almost every monitoring app can show that.
If the difference is less than ten per cent, there is probably nothing wrong.
The four most common causes
1. Shade that was not there before. A tree that has grown, a neighbour's dormer, a new chimney. Look at your roof on a sunny day around half past two.
2. Dirt. On a roof with a shallow pitch, dirt stays put. Moss along the bottom edge costs noticeable yield. With enough pitch, rain washes most of it away.
3. A string that drops out. In the app you see that as a graph stuck at a third or half of normal. Usually a connector, sometimes an optimiser.
4. The inverter switches off. If you see a flat top or gaps in the graph right around midday, that is almost always grid voltage. That is explained in inverter cuts out.
How to recognise it in the graph
| What you see | What it is usually down to |
|---|---|
| Smooth curve, but lower than before | dirt, ageing, shade at the start or end of the day |
| A bite out of midday | inverter switches off, grid voltage too high |
| Consistently a share less | a string or group of panels is not contributing |
| Irregular dips | clouds, or a connection that sometimes drops out |
The shape of the graph almost always tells you where to look.
What I measure when I come
- the voltage and current per string, to see whether all panels are contributing
- the insulation resistance on the DC side, to detect a damaged cable
- a thermal image of the connectors and the junction boxes
- the grid voltage at the inverter over a day, if switching off is suspected
With that it is usually clear within an hour. What it costs is on the 24/7 fault service page; an inspection can be scheduled and then falls under the regular rate of € 103 including VAT per hour.
From my own work
A customer reported a third less yield. Nothing was broken: the neighbours had a new dormer that cast shade on the bottom row from half past two. With optimisers on that row the yield came back up, because without optimisers one panel in the shade drags the whole string down.
When to call an electrician
- The yield is more than fifteen per cent lower than last year in the same month
- A string or part of the panels does nothing any more
- The inverter shows error messages
I test the strings and check the installation with a thermal imaging camera. See the 24/7 fault service.
Frequently asked questions
How much difference between years is normal?
Ten per cent difference between sunny years is normal. So compare whole months or an annual total, not a week.
Does it help to clean the panels?
On a roof with a shallow pitch it does, especially with moss along the bottom edge. With enough pitch, rain washes most of it away.
What does it cost to have the installation checked?
An inspection can be scheduled and falls under the regular hourly rate. It is usually clear within an hour where the problem is.
Further reading
A dynamic contract and a home battery: is it worth it
With a dynamic contract the battery can charge when electricity is cheap. That works, provided you let it be controlled automatically.
The net metering scheme is ending: what changes for my panels
Your panels keep generating just as much. What changes is the value of the electricity you feed back.
Feed-in charges: why you pay for your own electricity
More and more suppliers charge for feeding your solar power back into the grid. Your bill shows what that costs you.
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