When a solar power plant produces less energy than expected, the cause is not always easy to identify.
Weather conditions, equipment behaviour, module temperature, degradation and site-specific environmental factors can all influence output. Among these variables, soiling is one of the most important to understand because it directly affects how much light reaches the solar modules.
Soiling is often described simply as dust on solar panels. In practice, it can include dust, snow, pollen and other contaminants that accumulate on module surfaces and reduce power production. It is recognised as a major loss factor in PV power plants.
The challenge is not only knowing that soiling exists. The real challenge is understanding how much performance is being lost, whether cleaning is justified, and how to separate soiling losses from other causes of underperformance.
That is where accurate PV soiling measurement becomes essential.
Understanding soiling requires more than observing dust on a module surface. Explore the principles of PV soiling measurement, the impact of non-uniform soiling, and the different approaches available for quantifying soiling losses.
PV soiling measurement is the process of quantifying the effect of contaminants on solar panel performance.
The purpose is to understand how much energy or power is being lost because material on the module surface is blocking light transmission. This can include dust, pollen, snow or other environmental deposits.
A good soiling measurement strategy helps replace assumptions with site-specific data. Instead of using a fixed estimate for soiling loss, operators can measure how conditions are changing over time and understand how those changes affect plant performance.
Soiling is not just a maintenance issue. It is a performance monitoring issue.
According to the source material, soiling measurement supports three main objectives: resource assessment, performance assessment and cleaning schedule optimisation. It can be used before construction to estimate future losses, during commissioning or operations to isolate soiling from other underperformance factors, and during maintenance planning to determine when cleaning produces a positive return on investment.
Soiling measurement is also required under IEC 61724-1, the PV performance monitoring standard referenced in the materials. The presentation notes that the standard requires soiling measurement and that weather station requirements can vary depending on plant capacity, terrain and site-specific conditions.
This matters because performance losses are rarely caused by one factor alone. Without soiling data, it becomes harder to understand whether a plant is underperforming because of environmental contamination or because of another operational or technical issue.
Not all soiling patterns affect solar modules in the same way.
Uniform soiling occurs when contamination is distributed more evenly across the module surface. Non-uniform soiling occurs when material accumulates more heavily in specific areas, often along module edges. The presentation identifies precipitation, gravity and wind as causes of non-uniform soiling.
This distinction matters because non-uniform soiling can result in greater power loss than evenly distributed soiling. The source material states that non-uniform soiling is measured only by IV curves using Pmax on actual PV modules.
For solar performance monitoring, this means the measurement method matters. A technology that captures general soiling trends may be suitable for one site, while another site may require a method that measures the power impact on real modules.
There is no single soiling measurement method that is suitable for every solar project.
The presentation groups the available approaches along a practical spectrum: lower-maintenance solutions for low or uniform soiling, and more direct measurement methods for high or non-uniform soiling. Optical sensors sit toward the lower-maintenance end, reference cell pair systems provide a more traditional measurement approach, and in-situ module IV systems provide a more direct measurement of module power loss.
The right approach depends on what the site needs to understand.
Reference cell pair systems are one of the traditional methods used for solar soiling measurement.
The principle is straightforward: one reference cell is kept clean, while another is allowed to soil naturally. By comparing the two, the system calculates a soiling ratio. This helps quantify the difference between clean and soiled conditions.
The presentation also describes automated washing systems that can calculate soiling ratio values and provide real-time, 10-minute and 24-hour rolling averages. These systems can reduce the need for manual washing and provide pre-calibrated irradiance measurements.
Reference cell pair systems can be a practical choice when the goal is to measure soiling using PV reference devices while reducing manual maintenance through automatic washing.
Optical soiling sensors offer a lower-maintenance approach to measuring soiling.
Instead of comparing clean and soiled PV reference devices, optical sensors assess contamination on a sensing surface. The presentation highlights two optical technologies: MARS and DustIQ. Both are optical sensors, but they use different measurement principles.
MARS uses an internal camera focused on the top surface of a glass window. The camera captures shadows from dust or other particles, and image analysis is used to calculate soiling loss. The presentation states that MARS takes one soiling measurement per day at sunset.
MARS is factory calibrated, is not dependent on dust colour, and is typically mounted in the plane of array with a clear view of the sky.
DustIQ uses a different optical principle. An internal photodiode measures reflected blue light from soiling deposited on a glass window. The sensor has two measurement points and provides continuous measurement data.
The presentation notes that DustIQ clamps to the module frame and can be mounted alongside automatic cleaning robots. It also identifies DustIQ as bifacial compatible.
Optical sensors are especially relevant where lower maintenance is a priority, because the presentation identifies both MARS and DustIQ as no-maintenance options that are cleaned only when the modules are cleaned to represent module cleanliness.
For sites where a more direct measurement is needed, in-situ module IV measurement provides a different level of insight.
In-situ means the measurement is connected to string-connected modules and can be installed on modules within the site. The presentation states that measuring real modules provides a more representative and direct measurement.
The RDE30i is an in-situ module IV curve system that measures Pmax soiling loss, supports degradation monitoring and can provide effective irradiance measurement.
The key distinction is between Pmax and Isc. The presentation states that Pmax soiling measurements are more accurate than Isc soiling measurements when non-uniform soiling is present. It also states that Pmax measurements reveal losses due to non-uniform soiling.
This makes in-situ IV measurement particularly relevant when the objective is to understand actual power loss on PV modules rather than only track soiling trends near the array.
Cleaning solar modules has a cost. Not cleaning them can also have a cost if soiling losses continue to build.
The source material frames the cleaning decision around a practical question: clean now if the cost of soiling losses until expected rain is greater than the cost of cleaning.
This is where measured soiling data becomes valuable. It helps move cleaning decisions away from fixed schedules or assumptions and toward observed site behaviour.
For O&M decision-making, the value of soiling measurement is not simply that it identifies dirt on panels. Its value is that it helps clarify when intervention may be justified.
The best soiling measurement technology is not the one with the longest feature list. It is the one that matches the site objective.
If maintenance access is limited and soiling is expected to be low or relatively uniform, an optical sensor may provide the simplest path to ongoing visibility. If the goal is a traditional soiling ratio measurement using clean and soiled PV reference devices, a reference cell pair system may be appropriate. If the site needs a more direct view of power loss, especially where non-uniform soiling is a concern, in-situ IV measurement may provide the most representative data.
In other words, the selection should begin with the question the site needs to answer:
Do we need low-maintenance soiling trend monitoring?
Do we need reference cell-based soiling ratio data?
Do we need to measure power loss directly on PV modules?
Do we need to understand non-uniform soiling?
Do we need data to support cleaning decisions, performance testing or operational analysis?
A well-chosen system helps make the resulting data more useful, not just more available.
PV soiling measurement helps quantify how dust, snow, pollen and other contaminants affect solar module performance.
Soiling measurement supports resource assessment, performance assessment and cleaning schedule optimisation.
Non-uniform soiling can create greater power loss when contaminants accumulate along module edges, and the source material states that it is measured using IV curves based on Pmax on actual PV modules.
Reference cell pair systems compare clean and soiled reference cells to calculate a soiling ratio.
Optical sensors such as MARS and DustIQ reduce maintenance requirements but use different measurement principles.
In-situ IV systems measure soiling-related power loss directly on PV modules and are especially relevant where non-uniform soiling needs to be understood.
This article provides an overview of the principles, technologies and considerations involved in PV soiling measurement. For a deeper technical discussion, including technology comparisons and practical applications, watch the full on-demand webinar.
What is PV soiling measurement?
PV soiling measurement is the process of measuring how contaminants such as dust, snow, pollen or other materials on solar panel surfaces affect PV module performance.
Why is soiling measurement important?
Soiling measurement helps support resource assessment, performance assessment and cleaning schedule optimisation. It also helps separate soiling-related losses from other causes of underperformance during commissioning or operations
What is non-uniform soiling?
Non-uniform soiling occurs when contamination accumulates more heavily in specific areas of a module, often along the module edges. The presentation identifies precipitation, gravity and wind as contributing causes.
How do reference cell pair systems measure soiling?
Reference cell pair systems compare a clean reference cell with a soiled reference cell. The difference between the two is used to calculate a soiling ratio.
How do optical soiling sensors work?
Optical sensors measure contamination on a sensing surface. MARS uses a camera-based method, while DustIQ uses reflected blue light measured by an internal photodiode.
Why are IV curve measurements useful for soiling measurement?
IV curve measurements based on Pmax can measure non-uniform soiling on actual PV modules. The source material states that Pmax measurements are more accurate than Isc measurements when non-uniform soiling is present.