Soiling
Dirt on the glass — dust films, bird droppings, ash or lichen. Mostly an energy-yield issue, but hard localised soiling behaves like shading and can drive real cell heating.
Thermal signature
Two distinct looks. Uniform dust films show as subtle, low-contrast temperature variation following deposition patterns (bands along the lower edge, drainage streaks). Hard soiling — droppings, lichen — shows as small warm points that mimic hotspots but sit on the glass surface and match marks visible in the RGB pair.
Typical ΔT
2–10 °C at hard-soiled points; uniform films often show little ΔT while still costing yield (typical industry interpretation aligned with IEC TS 62446-3).
What causes it
- Wind-blown dust and agricultural activity
- Bird droppings, especially near perch points and row ends
- Pollen, ash or industrial fallout
- Low-tilt layouts that rain does not rinse effectively
Power impact
Uniform soiling typically costs 1–5% of site yield between cleans, more in dusty regions or low-rainfall periods. Hard soiling concentrates the loss: a single covered cell can push its substring into bypass and heat like a genuine hotspot until cleaned.
Recommended action
Fix: next maintenance window
Clean at the next maintenance window, prioritising hard-soiled modules flagged with elevated ΔT. Weigh cleaning cost against the measured loss for uniform films, and re-inspect after cleaning to confirm that hotspot-like findings were soiling rather than cell damage.
Frequently asked questions
How does the analysis tell soiling from a real hotspot?
Context and the RGB pair. Soiling marks are visible on the glass in the paired visible-light frame and often sit in characteristic deposition patterns; a true hotspot has clean glass above it. Where doubt remains, the finding is verified by cleaning: soiling signatures disappear, cell faults persist.
Is it worth cleaning a solar farm for a few percent?
It is arithmetic, not doctrine. A 5 MW site losing 3% to soiling forgoes roughly 150 kW of capacity — cleaning pays if its cost is below the value of the recovered energy before re-soiling. An inspection quantifies the loss so the decision can be made on numbers rather than habit.
Related anomalies
Suspect soiling on your site?
Fly the site, upload the thermal frames, and get every module graded against this taxonomy — severity-ranked, with flagged imagery and per-module locations. Quote your site size in seconds.
ΔT figures and abnormality classes on this page describe the typical industry interpretation aligned with IEC TS 62446-3 — indicative engineering guidance, not normative text from the specification. Temperature differentials are only meaningful when captured at ≥600 W/m² irradiance (per the TS) and verified on site before repair work.