The Hidden Variable: Why Diffuse Irradiance is a Multi-Million Dollar Blind Spot for Bifacial PV

As utility-scale PV plants face mounting pressure to maximize energy yield and meet performance guarantees, the precision of irradiance measurements has become more critical than ever. While operators diligently monitor Global Horizontal Irradiance (GHI), a less-explored variable presents a critical oversight: Diffuse Horizontal Irradiance (DHI).

For plant operators, EPCs, and asset managers, relying on estimated DHI is no longer just a technical shortcut; its a growing operational and financial liability.

Why DHI Deserves Your Full Attention

Diffuse irradiance is the portion of sunlight scattered by the atmosphere that reaches the surface indirectly. This distinction is vital because modern PV systems, particularly those with bifacial modules and single-axis trackers, rely heavily on this scattered light.

Consider these facts:

• Under cloudy skies, high-latitude climates, or winter conditions, diffuse irradiance can account for 60% to 90% of total irradiance.
• Unlike direct sunlight which mainly hits the front side, diffuse irradiance reaches both sides of a bifacial panel due to its multi-directional nature.
• For bifacial systems on trackers, diffuse and ground-reflected light are essential contributors to rear-side energy capture, as the tracker itself often shades parts of the rear side from direct sun.

Despite its importance, many systems use algorithms to estimate DHI from GHI, introducing significant modeling errors. In utility-scale PV, even a 1% deviation in expected yield can translate into financial impacts of tens of thousands of euros per megawatt each year. Accurate, on-site DHI measurements allow operators to fine-tune tracking strategies, sometimes finding that a flat or fixed-tilt position outperforms sun-tracking during diffuse conditions.

Closing the Measurement Gap: The Challenge of Accuracy

While measuring rear-side irradiance is crucial for bifacial systems, covering every array on a utility-scale site with sensors is cost-prohibitive. This leads many operators to rely on modeling, but these models are only as accurate as the data behind them, making high-quality DHI measurements essential for bankability.

The industry has faced a trade-off:

• Sun tracker-based systems deliver the most accurate, bankable data but are often too expensive and complex for many sites.
• Satellite-based estimates have broad coverage but lack precision, especially under fast-changing skies.
• Simplified ground-based sensors are more affordable, but their accuracy, consistency, and traceability can vary widely.

A Compliant, Cost-Effective Solution: The MS-80SH Plus

The EKO MS-80SH Plus solar monitoring station was designed to bridge this gap, delivering high-precision DHI measurements that are fully compliant with ISO 9060 Class A and IEC 61724-1 Class A standards.

It works by pairing a Class A pyranometer with a motorized rotating shadow band (RSB), allowing the system to dynamically measure GHI, DHI, and Direct Normal Irradiance (DNI). The system also supports a continuous Tracking Shading Band (TSB) mode for high-frequency DHI data, which is ideal for analyzing bifacial energy gain under variable conditions.

Built for utility-scale reliability, it features a durable direct-drive motor, built-in GPS for precise positioning, and seamless integration with SCADA systems via Modbus RTU.

Navigating Hidden Costs: Why Spectral Accuracy and Traceability Matter

Not all simplified DHI solutions are created equal. Many come with hidden costs and performance trade-offs.

• Spectral Accuracy: Some instruments use silicon sensors described as spectrally matched to PV modules. While this works reasonably well in clear skies, these sensors can experience significant spectral mismatch under cloudy conditions, leading to measurement errors of up to 5%. Spectrally flat sensors, like the thermopile-based pyranometer in the MS-80SH Plus, maintain a consistent response across a broad wavelength range, ensuring accurate, technology-independent data regardless of sky conditions.
• Calibration Traceability: Many alternative systems require proprietary indoor calibration and do not comply with international standards. In contrast, the Class A pyranometer in the MS-80SH Plus can be calibrated using widely accepted ISO 9847 procedures at certified labs worldwide, simplifying long-term maintenance and lowering operating costs.

The Bottom Line

In todays evolving solar landscape, accurately measuring diffuse irradiance is indispensable. From performance modeling and forecasting to bifacial optimization, DHI plays a central role in a plants performance, profitability, and operational risk.

While simplified sensors may seem appealing, they often fall short in meeting the stringent requirements for compliance, traceability, and accuracy that are essential for validating the performance of todays utility-scale PV systems. For bankable, traceable, and truly accurate DHI data, a fully compliant solution is no longer a luxury its a necessity.