Soiling is a well‑established cause of energy loss in photovoltaic (PV) systems. Less widely recognized is its impact during capacity and performance testing, where even small losses can have contractual and financial consequences.
Field research presented at the IEEE Photovoltaic Specialists Conference shows that soiling losses of just 1–2% can influence capacity testing outcomes in utility‑scale solar plants. The research also highlights a recurring issue: soiling is often measured too late to be useful during commissioning.
This page explains why early, accurate soiling measurement is essential for reducing performance risk, supporting defensible capacity testing, and enabling confident decision‑making across the solar project lifecycle.
Soiling refers to the accumulation of dust, dirt, pollen, agricultural residue, and other airborne particles on the surface of PV modules. These particles reduce light transmission through the module glass, resulting in measurable power losses.
The magnitude of soiling losses varies depending on:
While soiling is commonly addressed during operation, its effect during commissioning and capacity testing is often underestimated.
Utility‑scale PV projects must demonstrate performance compliance before reaching milestones such as substantial completion. Capacity testing is commonly used to validate system output, often following standards such as ASTM E2848 or evolving international methods.
Capacity tests allow only limited total losses. Under these conditions, even modest unexplained losses can cause a test to fail.
Field data shows that soiling losses of 1–2%, when combined with normal measurement uncertainty, can push a technically sound system outside acceptable performance limits, leading to:
When capacity tests fail due to unexplained losses, the consequences extend beyond technical troubleshooting. Delays at this stage can trigger contractual disputes between developers, EPCs, asset owners, and third‑party engineers—particularly around responsibility for cleaning, loss compensation, and test acceptance criteria.
In many projects, the treatment of soiling during capacity testing is not fully defined in contracts prior to construction. This can lead to last‑minute negotiations, unplanned cleaning activities, or repeat testing campaigns, all of which increase cost and schedule risk. By contrast, projects that incorporate defensible soiling measurement strategies early in the construction timeline are better positioned to manage these risks transparently and efficiently.
Accurate soiling data does not eliminate commercial discussions—but it provides a shared, objective basis for them.
Capacity testing in utility‑scale solar is typically performed within tightly defined frameworks, such as ASTM E2848 in the United States and evolving international methods including IEC 61724‑1. These methodologies are designed to validate system performance against modeled expectations within narrow uncertainty bands.
Under these standards, total allowable losses are limited. When soiling losses are not explicitly measured—or cannot be defensibly quantified—they are effectively treated as unexplained performance degradation. Even small unaccounted losses in the range of 1–2% can therefore cause a capacity test to fall outside acceptable limits, despite the system being fundamentally sound.
This is why soiling measurement for capacity testing must be approached differently than long‑term operational monitoring. Measurement strategies that are sufficient for O&M reporting may not provide data of adequate timing, resolution, or representativeness for commissioning and performance validation. Early, standards‑aligned measurement planning is essential to ensure that environmental losses can be confidently distinguished from system‑related issues during formal testing.
Most utility‑scale solar plants include soiling sensors as part of the meteorological station. These sensors are designed primarily for long‑term operational monitoring, not commissioning.
As a result, they are often installed after PV modules have already been exposed to environmental conditions for weeks or months.
This creates a mismatch:
Because the sensors do not share the same exposure history as the modules, their data cannot reliably quantify soiling losses during capacity testing.
When representative soiling data is unavailable, project teams often rely on:
These approaches introduce subjectivity, cost, and delay. Research indicates that many of these challenges can be avoided through earlier measurement planning.
Field research consistently points to a simple recommendation:
Soiling sensors should be deployed concurrently with PV module installation.
Installing sensors at the same time as modules ensures identical environmental exposure throughout construction and commissioning. This enables:
Importantly, this approach does not require early SCADA integration. Sensors can be installed and left unpowered during construction because it does not require any calibration, allowing natural soiling accumulation.
A frequent barrier to early soiling sensor deployment is the assumption that sensors must be powered, logged, and integrated into SCADA systems from the moment they are installed. Field research shows this is not the case.
For capacity testing, the most critical requirement is that soiling sensors share the same exposure history as PV modules. Sensors can be installed concurrently with module deployment and left unpowered during construction. During this time, they naturally accumulate dust and are cleaned by rainfall in the same way as the modules.
Once commissioning approaches, the sensors can be activated and immediately provide representative soiling loss data—without requiring months of prior logging. This simple shift in deployment strategy removes a major practical obstacle to early soiling measurement and enables more reliable compensation of soiling losses during testing.
Research identifies four requirements for sensors used during construction and commissioning:
These principles align with Kipp & Zonen’s long‑standing focus on measurement accuracy, robustness, and traceability.
With Atonometrics now part of the Kipp & Zonen portfolio, optical soiling measurement is integrated into a broader, bankable solar measurement framework.
Field data from a ~100 MW utility‑scale PV plant shows that soiling is rarely uniform across a site. Measurements revealed:
These results demonstrate that assuming uniform soiling across a utility‑scale site can materially misrepresent actual performance during capacity testing, reinforcing the need for distributed soiling measurement, rather than relying on a single sensor to represent an entire plant.
Across multiple capacity testing campaigns, field measurements showed a clear relationship between:
This demonstrates the value of soiling data in distinguishing environmental effects from system‑related issues during performance validation.
While optical soiling sensors accurately measure transmission losses, they do not capture all electrical loss mechanisms. Non‑uniform soiling patterns can lead to additional power losses that require complementary diagnostics.
Soiling measurement should therefore be part of a comprehensive performance assessment strategy, not a standalone metric.
Soiling has traditionally been treated as an operational issue. Research now shows it should also be considered a commissioning and project‑delivery risk.
Early soiling measurement strategies complement broader solar resource and irradiance measurement practices, helping ensure consistency between commissioning data, long‑term performance monitoring, and bankability assessments across the project lifecycle.
At Kipp & Zonen, accurate measurement is not just about data — it is about enabling confident, defensible decisions across the full lifecycle of a solar asset.
Contact us to learn how early, accurate soiling measurement can reduce performance risk and support confident capacity testing in your utility‑scale solar projects.