How-to · 4 min read · Updated 6 November 2026
How to tell if your solar panels are underperforming: UK step-by-step
Step 1: Find your expected output
Your installer should have given you an estimated annual generation figure, usually on the MCS certificate or the quote. It is based on the system size, roof direction, pitch, shading and location. If you cannot find it, a rough guide is that each kWp of panels in the UK produces about 800 to 1,000 kWh a year, with southern England at the top of that range and northern Scotland at the bottom.
| System size | Typical yearly generation (UK guide) |
|---|---|
| 2 kWp | 1,600 to 2,000 kWh |
| 3 kWp | 2,400 to 3,000 kWh |
| 4 kWp | 3,200 to 4,000 kWh |
| 6 kWp | 4,800 to 6,000 kWh |
East or west facing roofs, shallow pitches and partial shading usually produce less than these figures. The free PVGIS tool from the European Commission can give a more tailored estimate for your exact location and roof.
Step 2: Read your actual generation
There are three common places to find out what your panels are producing.
- The generation meter, a separate meter usually near the inverter or consumer unit that shows total kWh generated
- The inverter display, which often shows today's, this month's and lifetime generation
- A monitoring app or web portal linked to the inverter or battery
Take a reading on the same date each month and write it down. Comparing the same month across different years is more useful than comparing one month with the next, because UK generation swings enormously with the seasons. June often produces four or five times as much as December.
Step 3: Rule out the weather
A dull spring or a wet summer can easily cut generation by 10 to 20 percent compared with a sunny year. Before assuming there is a fault, ask a neighbour with panels how their output compares, or look up whether that month was sunnier or duller than usual in your region. If everyone locally is down by a similar amount, the weather is the likely cause.
Step 4: Look for warning signs
- Zero or almost zero output during a bright day
- A red, orange or flashing light on the inverter, or an error code on its screen
- A tripped switch or isolator labelled solar or PV on the consumer unit
- The monitoring app showing offline for days
- A sudden step down in output rather than a gradual change
- Visible damage such as cracked glass, slipped panels or hanging cables
- Pigeons, droppings or heavy debris on or under the panels
- New shading from trees, extensions or neighbouring buildings
Sudden drops and error codes usually point to an inverter, wiring or connection problem. Gradual declines usually point to soiling, shading or normal ageing, which is often around half a percent a year for modern panels.
Step 5: Check the simple things safely
If the inverter shows an error, check the manual for what the code means. Many inverters can be restarted following the manufacturer's shutdown and start-up sequence, which is often printed on a label near the inverter. Check whether a solar isolator or circuit breaker has tripped. If it trips again after resetting, leave it off and call an installer or electrician.
Never open the inverter, touch damaged cables or disconnect connectors on the roof. Solar panels generate voltage whenever light falls on them, even when the inverter is switched off.
Step 6: Decide what to do next
| What you found | Likely cause | Next step |
|---|---|---|
| Error code or zero output | Inverter, isolator or wiring fault | Call your installer or an MCS-certified installer |
| Visible dirt, droppings or lichen | Soiling | Book a pure water clean |
| Pigeons under the panels | Nesting and fouling | Clear nests and fit bird proofing |
| Output down in line with neighbours | Weather | Keep monitoring |
| New shadow across part of the array | Shading | Trim trees if possible or ask an installer about optimisers |
| Slow decline over many years | Normal ageing | Compare with the performance warranty |
What a professional check involves
If the simple checks do not explain the drop, the next step depends on what you suspect. A cleaner can remove soiling and carry out a visual check from the ground, noting cracked glass, slipped panels, nesting and loose cables. That rules out the most common physical causes quickly.
For electrical faults, a qualified solar installer or electrician will usually test each string of panels, compare voltages and currents with what they should be, inspect isolators and connectors, and read the inverter's fault history. Some use thermal imaging cameras, which can show hot spots caused by damaged cells or bad connections. A fault-finding visit by an installer often costs around £100 to £250, depending on the area and the system.
- Have your generation records and installer estimate ready
- Note the inverter make, model and any error codes
- Take clear photos of the panels from the ground
- Mention when the drop started and whether anything changed, such as roof work or tree growth
Good records make a visit quicker and cheaper, because the person diagnosing the problem can see straight away whether output fell suddenly or slowly.
Frequently asked questions
How much less do solar panels produce as they age?
Modern panels commonly lose around half a percent of output a year, so a ten-year-old panel might produce roughly 95 percent of what it did when new. Performance warranties usually guarantee around 80 to 90 percent of rated output after 25 to 30 years. A much faster decline than this suggests a fault, damage or heavy soiling.
Can dirty panels cause a big drop in output?
Light dust on well-pitched UK panels usually makes only a small difference because rain helps keep them clean. Heavy bird droppings, lichen and dirt lines along the bottom edge can cause larger losses, because shading part of one cell can reduce the output of a whole string. If you can see obvious fouling from the ground, a clean is worth considering.
How long do solar inverters last?
String inverters in UK homes often last around 10 to 15 years, which is usually much shorter than the panels. If your system is over ten years old and output has dropped sharply or stopped, the inverter is a common culprit. A qualified installer can test it and advise whether a repair or replacement makes sense.
solar output inverter monitoring troubleshooting
