Why Measurement Strategy Matters More Than Ever
Executive Summary
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.
What Is Soiling in Solar PV Systems?
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:
- Local climate and precipitation patterns
- Land use and nearby activities such as farming or construction
- Wind direction and seasonal effects
- Cleaning frequency and site accessibility
While soiling is commonly addressed during operation, its effect during commissioning and capacity testing is often underestimated.
Why Soiling Measurement for Capacity Testing Requires a Different Strategy
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:
- Delays in project completion
- Disputes between developers, EPCs, and asset owners
- Unplanned cleaning or retesting costs
- Increased uncertainty for financiers and investors
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.
Soiling in the Context of Capacity Testing Standards
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.
The Hidden Risk: Soiling Is Often Measured Too Late
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:
- Modules have accumulated soiling
- Sensors begin from a clean state
Because the sensors do not share the same exposure history as the modules, their data cannot reliably quantify soiling losses during capacity testing.
Common Workarounds and Their Limitations
When representative soiling data is unavailable, project teams often rely on:
- Visual inspection or photographs
- Pre‑ and post‑cleaning electrical measurements
- Negotiated assumptions about acceptable losses
These approaches introduce subjectivity, cost, and delay. Research indicates that many of these challenges can be avoided through earlier measurement planning.
Measure Soiling from Day One: A Proactive Strategy
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:
- Reliable compensation of soiling losses during capacity testing
- Early identification of excessive soiling
- Data‑driven cleaning decisions
- Reduced contractual ambiguity
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 Common Misconception: SCADA Integration Is Not Required Early
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.
What Makes a Soiling Sensor Suitable for Capacity Testing?
Research identifies four requirements for sensors used during construction and commissioning:
- Calibration‑free operation – Allows immediate installation without delaying project schedules.
- High sensitivity at low loss levels – Capacity testing requires resolution below 1%, with sensitivity to changes under 0.5%.
- Ease of deployment – Sensors must be compact, robust, and suitable for construction environments.
- Cost‑effective scalability – Soiling varies across large sites; distributed measurement improves confidence.
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 Evidence: Soiling Is Spatially Variable
Field data from a ~100 MW utility‑scale PV plant shows that soiling is rarely uniform across a site. Measurements revealed:
- Rapid accumulation during nearby agricultural activity
- Losses reaching several percent within weeks
- Clear spatial variation depending on location and wind exposure
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.
Linking Soiling Data to Capacity Test Outcomes
Across multiple capacity testing campaigns, field measurements showed a clear relationship between:
- Reductions in soiling losses (from rainfall or cleaning)
- Improvements in measured capacity test ratios
This demonstrates the value of soiling data in distinguishing environmental effects from system‑related issues during performance validation.
Understanding the Limits of Soiling Measurement
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.
From Reactive Testing to Proactive Performance Assurance
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.
