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Soiling Measurement

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Yield Loss & Monitoring

Soiling

Soiling — the accumulation of dust, pollen, bird droppings, and other contaminants on module surfaces — is one of the most significant and underestimated sources of yield loss in operating PV systems. Losses of 1–5% are common in temperate climates; in arid and desert environments they can exceed 30% between cleaning events. Accurately quantifying soiling loss requires dedicated instrumentation and a clear methodology, distinct from the irradiance and temperature correction applied to standard performance analysis. This section covers soiling sensor technologies, monitoring system design, data interpretation, and the use of soiling measurements to optimise cleaning schedules and validate cleaning effectiveness.

Emazys
Tempirra
Wireless irradiance and temperature sensor — Apogee SP-110-SS pyranometer (0–2,000 W/m²), dual PT1000 RTD temperature channels, Bluetooth cloud-connected. Works alongside Z300 PVT string testers.
Articles
Entec Solar
E-Sens
Wireless irradiance and temperature sensor for the E-1500 IV curve tracer. 2 km wireless range — place once and walk the full array. Connects to a calibrated reference cell; transmits plane-of-array irradiance and module temperature to the tracer for automatic IEC 60891 STC correction. Supports up to three Pt1000 probes.
Entec Solar
E-Dust
Soiling sensor that quantifies PV plant dirt losses at Pmax — comparing clean vs. soiled reference module maximum power. Captures non-uniform soiling and angular incidence effects that Isc-only methods miss. Sub-minute sampling, IEC 61724-1, online platform.
Entec Solar
E-Ref
DIN-rail datalogger for reference PV modules — measures Isc and Voc to derive irradiance and cell temperature, outputs directly to SCADA via Modbus. E-Ref/01 monofacial; E-Ref/03 bifacial with three channels for front irradiance, rear irradiance, and temperature.
Sonel
IRM-1
Compact solar irradiance and temperature meter — plane-of-array irradiance (W/m²), PV module temperature, ambient temperature, inclination sensor, compass. LoRa wireless sync with PVM-1530, reSYNC gap recovery, 5000-record standalone datalogger. IP65.
Field Testing Guide
Reading IV Curves: A Field Technician's Guide
What a healthy IV curve looks like, how six common fault types deform it, and when alternative test methods can identify the same issue faster — or catch what the IV curve misses.
Field Testing Concept
Understanding the Geff Calculation
A plain-language explanation of effective irradiance — what it is, why front-side irradiance alone falls short for bifacial modules, and how to calculate Geff correctly in the field.
Articles Coming Soon
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Frequently Asked Questions

In temperate climates with regular rainfall, soiling losses typically range from 1–5% annually. In arid environments — desert southwest US, Middle East, North Africa — losses of 10–30% or more between cleaning cycles are common. The rate depends on particle type (fine dust settles and adheres more than coarse particles), local pollution sources, and module tilt angle. Horizontal modules accumulate soiling fastest; steeper tilts allow more natural rain-washing.

Cross-string Isc comparison is the fastest electrical method. Under stable irradiance, strings with higher soiling produce proportionally lower Isc than clean strings. Comparing corrected Isc values across all strings of the same sub-array identifies outliers within minutes. Visual inspection confirms whether the anomaly is soiling. This makes Isc screening the standard first step before any cleaning decision or O&M intervention.

Soiling reduces Isc uniformly and is reversible — Isc returns to baseline values after cleaning. Degradation also reduces Isc but affects the full IV curve shape over time and does not reverse after cleaning. Compare corrected Isc against the commissioning baseline before and after cleaning: if Isc recovers to within 2–3% of the commissioning value after washing, the loss was predominantly soiling. A persistent gap after cleaning indicates degradation requiring further investigation.

Yes. A soiling ratio measurement uses two irradiance sensors or reference cells — one cleaned regularly to represent true available irradiance, and one left unattended to accumulate soiling like the array. The ratio of dirty-to-clean sensor output quantifies soiling loss in real time without electrical string testing. This approach, described in IEC 61724-1, is used for continuous soiling monitoring in performance ratio calculations on utility-scale sites.

Not equally. Module surface texture, glass coating, and tilt angle all affect soiling accumulation and rain-washing behaviour. Textured anti-reflective coatings may trap fine particles more readily than smooth glass. Bifacial modules mounted close to the ground accumulate rear-surface soiling from dust and vegetation that front-surface soiling assessments miss entirely. HJT modules with thinner glass may show different soiling adhesion characteristics than standard tempered glass modules.