A solar system makes less power for one of two reasons. Either the conditions changed, or something on the roof or in the loft has stopped working properly.
Most worries about falling output turn out to be the first kind. The year has seasons, and the sun in December is not the sun in June. This guide helps you tell an ordinary change from a real fault, in the order worth checking.
Key Points
- Compare like with like. Compare this September with last September, never with July.
- Dirt costs less than most people expect in a wet climate. A European study of soiling put the average annual loss at 0.9% where rain is treated as a perfect cleaner, rising to 5.3% if rain is assumed to remove only a tenth of the dirt.
- Bird mess and lichen are different from dust. They sit in one place and do not wash off.
- Trees grow. A branch that cleared the roof three summers ago may not clear it now.
- New buildings, extensions, aerials and satellite dishes all cast shadows that were not there at the survey.
- Inverters log faults. The error code on the screen or in the app is the fastest route to a diagnosis.
- One dead string halves a two string system. That looks dramatic and is easy to spot in the numbers.
- Panels do lose a little output every year by design. It is small and gradual, never sudden.
| What you see | Likely cause | How to tell |
|---|---|---|
| Lower than last month, similar to this time last year | Seasonal change | Compare the same month across years |
| Gradual drop over two or three years | Panel degradation or growing shade | Check the trend and look at the roof in the afternoon |
| A sharp step down that stays down | String fault, isolator tripped, or inverter fault | Check the inverter display and app |
| Output good in the morning, poor from mid afternoon | New shading on one side | Watch the roof through the day |
| Nothing at all | Inverter fault or supply issue | Check the inverter and the consumer unit |
| Patchy output on bright days | Panel level fault or a failed optimiser | Needs an on site test |
Common Misconceptions
"My panels must need cleaning." Sometimes, but rarely by as much as people expect. In a wet climate rain removes most loose dirt. The measured average annual loss in Europe is under 1% where rain is doing its job.
"Panels wear out fast." They do not. Manufacturers publish a small annual figure. AIKO states a first year loss of 1% or less, then 0.35% a year or less. JA Solar publishes an annual degradation rate of 0.3%.
"A single shaded panel does not matter." It can matter a lot. Panels are wired in series, so one heavily shaded panel drags on the string it sits in.
"If the app shows something, the system is fine." Not always. Check the figures, not just the green tick.
"It stopped, so the panels have failed." Panels almost never fail all at once. A total stop is far more often the inverter, an isolator or the supply.
Real-World Advice
Our published 21 panel installation in Ninfield is a 9.66 kWp array with 20 kWh of Anker storage. See the Ninfield case study. The customer set up notifications about his charge levels afterwards, and Lee changed the settings for him remotely.
That is the point of monitoring. If you can see what the system does every day, a real change stands out quickly. If nobody is watching, a fault can sit there for a whole season.
UK-Specific Considerations
Winter output in East Sussex is a fraction of summer output. That is normal and it is built into the estimate you were given.
Along the coast at Eastbourne, Bexhill, Hastings and Seaford, salt air and gulls both leave marks. Gull mess is the type of soiling worth acting on, because it does not rinse off in the way road dust does. Light dust in a wet British winter usually does.
Inland at Hailsham, Uckfield, Heathfield and the Wealden villages, trees are the more common cause. Growth over three or four summers is easy to miss because you see the garden every day.
The other local factor is the sea fret and low cloud that sits over the coast on some summer mornings. It clears, and the day still produces. It is not a fault.
When This Isn't Right For You
Do not go on the roof yourself. Do not open the inverter. Do not use a pressure washer on panels or on the roof around them. Any of those can turn a small problem into a large one, and working at height is the most dangerous part of this job.
Before You Spend Money
Look at your own monitoring first. A year on year comparison of the same month costs nothing.
Read the inverter screen or the app and write down any code. That code tells an engineer more than a description does.
Check the isolators and the consumer unit. A switch that has tripped once can be reset. A device that trips again must not be reset repeatedly, so contact a qualified installer or electrician.
Walk outside in the middle of the afternoon and look at the roof. Take a photo. New shade is visible.
Only then book a visit. If the answer is a cleaning job, ask what the cleaning is for. Bird mess and lichen are worth removing. Light dust usually is not.
If your inverter is out of warranty and the fault is inside it, read repair or replace the inverter before you decide.
Frequently Asked Questions
Q: How much less should my system make in winter?
A: A lot less. The estimate you were given was for a full year, with the winter months already counted in.
Q: How often should panels be cleaned?
A: There is no single answer. It depends on birds, trees and how much the rain reaches the glass. Bird fouling is the case worth acting on.
Q: My inverter shows an error code. What now?
A: Note the code, then contact your installer. Do not clear it and hope. The code is the diagnosis.
Q: Can one faulty panel bring the whole system down?
A: It can pull down the string it sits in. Whether that is half your system or a smaller share depends on how the array is wired.
Q: Do panels stop working when they get old?
A: No. They lose a small percentage of output each year and keep going. The published warranties run for decades.
Summary
Start with the calendar, not the roof. Compare the same month across years before you assume anything is wrong. If the drop is real, look for a sharp step rather than a slow slide. A step means a fault. A slide usually means shade or age. Read the inverter code. Look for new shade in the afternoon. Treat bird mess as worth removing and light dust as normal. Panel degradation is real but small, and the manufacturers publish the number.