The five-minute visual check
Stand back from your roof on a clear day and look at the panels. What you are checking for: visible soiling (a consistent grey or brown haze), bird droppings (dense white or dark spots), biological growth (green or black patches, particularly at edges and joints), cracked glass, and any panels that look significantly darker or discoloured than their neighbours.
Check the inverter display or app. On a clear day between 10am and 3pm, a healthy 4 kW system should be generating between 2,500 W and 4,000 W depending on the season and the angle of the sun. If the display reads significantly below this on a clear summer day, something is wrong.
Check that all isolator switches on the DC and AC sides are in the "on" position — it sounds obvious, but isolators that have been accidentally switched off are a surprisingly common cause of apparent underperformance.
Using your inverter monitoring to spot problems
Your inverter (or associated monitoring app) records historical output. The most revealing diagnostic is to compare your system's output on similar clear days across different months and years. A system that generated 25 kWh on a clear August day last year but is generating 18 kWh on a comparable day this August has lost 28% of output — that warrants investigation.
Most inverter platforms (SolarEdge, SMA, Enphase, Growatt, GivEnergy) allow you to view output graphs for any day or month in history. Some also show individual panel or string performance, which is particularly useful for identifying a single underperforming panel rather than a system-wide issue.
Persistent fault codes (typically displayed as E or F codes) mean the inverter has detected an electrical problem it cannot self-correct. Look up the code in your inverter manual or manufacturer's website — many are benign (brief grid fluctuations) but persistent codes require a qualified engineer.
Comparing against expected performance
The UK's Energy Saving Trust provides a postcode-based solar generation estimator, and most installers will provide a predicted annual kWh figure at the time of installation. Your actual generation should be within 10–15% of this figure for a well-functioning system. Consistent underperformance beyond this range — over a full year, not just a bad month — is a signal to investigate.
Natural panel degradation accounts for 0.5–0.7% output loss per year, so a system installed in 2015 should be producing around 90–95% of its original rated output. Significantly greater losses than this point to a specific problem rather than normal ageing.
What soiling looks like in the data
Soiling-related output loss is gradual and consistent — a slow decline across weeks and months rather than a sudden drop. If your monitoring shows a gradual output reduction over a period of 3–6 months on clear days, and the last professional clean was more than 12 months ago, soiling is the most likely cause.
A bird dropping causing a hotspot shows up differently: a sudden drop in output from one string, with other strings performing normally. On systems with panel-level monitoring, the affected panel is identifiable directly.
When to act immediately
Contact a qualified solar engineer without delay if: the inverter display is blank and the system has no output; you see or smell burning near the inverter or DC cables; a circuit breaker for the solar system has tripped and will not reset; water is dripping from the inverter unit; or the inverter is making unusual sounds.
Book a professional clean and review if: your monitoring shows output has declined steadily compared to the same period last year; you can see soiling or biological growth on the panels; or it has been more than 12 months since the last clean.