Spanish study finds 15% gap in bifacial PV performance

Researchers from Universitat Politècnica de València and Grupotec have published a study in the journal Renewable Energy detailing significant discrepancies in bifacial solar performance. The analysis relies on two years of operational data from a 50 MW utility-scale plant in Spain. This extensive dataset provides a robust baseline for evaluating real-world gains against theoretical expectations.

Measured vs. Nominal Performance

The distinction between a module’s nominal bifaciality factor and its effective bifaciality represents a critical gap in energy yield modeling. The nominal factor is a laboratory-derived metric that quantifies the ratio of rear-side to front-side power output under standard test conditions. It assumes idealized, uniform irradiance and specific spectral properties that rarely persist across an entire day or season. In contrast, effective bifaciality accounts for real-world variables, including ground albedo, module mounting height, and local shading. For developers, this difference determines whether projected energy production matches bankable financial models. A discrepancy here directly impacts the levelized cost of energy, as overestimating rear-side gains leads to underestimating the number of modules required to meet contractual power obligations.

The study reveals that the effective bifaciality measured in the field ranged from 0.599 to 0.610, while the nominal module bifaciality factor was 0.70. This indicates that the actual rear-side contribution was 13% to 15% below the theoretical expectation. Such a gap is not uncommon in bifacial deployments, as real-world conditions often fail to replicate the pristine environment of laboratory testing. Ground reflectivity, for instance, varies with soil type, vegetation cover, and moisture levels, none of which are captured in a static nominal rating. The angle of incidence and the specific spectral distribution of sunlight change throughout the day, further deviating from standard test conditions.

Metric	Value
Nominal Bifaciality Factor	0.70
Effective Bifaciality Range	0.599 – 0.610
Performance Gap	13% – 15% below nominal

This table highlights the quantitative divergence between design assumptions and operational reality. The consistent shortfall suggests that relying solely on nominal data for financial modeling can result in significant yield errors. Investors and project financiers must account for this variance when assessing risk, as the effective value is the only figure that translates directly into grid injection and revenue. Understanding this margin of error allows for more accurate sizing of storage systems and grid connections, ensuring that the plant operates within its permitted parameters without unexpected curtailment or revenue loss.

Environmental and Electrical Drivers

Solar performance is rarely static, and the gap between nominal and effective bifaciality widens under specific environmental conditions. The study highlights that high irradiance levels significantly depress rear-side efficiency. When front-side irradiance exceeds 1,000 W/m², the effective bifaciality factor dips to a range of 0.55 to 0.57. This phenomenon occurs because intense direct sunlight casts sharper shadows and reduces the relative contribution of diffuse light, which is the primary driver of rear-side generation. For system designers, this implies that peak production hours may not deliver the proportional bifacial gains assumed in steady-state models.

Mid-day conditions further illustrate this imbalance. The data shows that rear-side irradiance typically constitutes only 20% to 25% of the front-side irradiance during these peak periods. This ratio reflects the physical reality that the ground reflects a limited portion of incident light, and much of that reflected energy is lost to atmospheric absorption or geometric inefficiencies before reaching the module’s back surface. Developers must account for this asymmetry when calculating hourly energy yields, as the rear side acts more as a supplemental contributor rather than a co-equal power source during the brightest part of the day.

Beyond optical factors, thermal dynamics also play a role in module behavior. The study notes that module temperatures increased by up to 3.5 C due to rear-side gains. While this specific thermal shift was observed in the field, it underscores how bifacial modules interact with their environment differently than traditional single-sided designs. In general, higher operating temperatures can affect module voltage output and long-term degradation rates. Understanding these thermal interactions is essential for accurate performance prediction, as the additional heat absorbed from the rear side can influence the overall electrical characteristics of the string, particularly in high-temperature climates where thermal losses are a primary concern for grid output and system efficiency.

System-Level Electrical Responses

The electrical behavior of bifacial arrays diverges significantly from single-sided designs, fundamentally altering how inverters and grid connections must be configured. The core electrical shifts observed in the study include:

• String current increased up to 110% with higher rear-side irradiance.
• String voltage declined 8% to 9% with higher rear-side irradiance.

These figures illustrate the direct trade-off inherent in bifacial technology. Current is an additive parameter; when light strikes the rear side of a dual-glass module, it generates additional electron flow that sums directly with the front-side current. Consequently, the total ampere output rises proportionally to the aggregate light exposure. This increase in current is the primary driver of the energy yield gains that justify the adoption of bifacial modules. However, this gain comes with a distinct penalty in voltage. As the current density within the module increases, internal resistive losses and the specific diode characteristics of the solar cells cause the operating voltage to drop.

For system designers, this voltage depression is a critical engineering constraint. Inverters have fixed input voltage windows, and excessive voltage drops can push the system away from its maximum power point, reducing overall efficiency. Lower string voltages can affect the sizing of combiner boxes and the number of modules that can be safely connected in series. While the current boost ensures higher energy production, the voltage decline requires careful modeling to ensure the plant remains within the safe operating limits of its power electronics. Investors should note that these electrical responses are not static; they fluctuate with environmental conditions, meaning that system performance models must account for dynamic voltage behavior rather than assuming a constant module output.

What to watch

The immediate next milestone involves verifying these findings across broader geographic and climatic conditions. The research team has already validated the core observations using data from four additional sites in Spain and the United Kingdom. This expansion helps confirm that the performance gap is not an anomaly of a single location but a consistent characteristic of bifacial technology in real-world environments. Readers should look for how these new datasets refine the understanding of local albedo effects and shading patterns, which can vary significantly between regions.

A key open question centers on the discrepancy between regulatory assumptions and measured reality. The study highlights that the regulatory bifacial factor of 1.15 used in Spain does not reflect actual operation. This mismatch raises important issues for project finance and grid compliance. Developers and investors need to monitor how regulators and grid operators respond to this data. Will standards be updated to reflect the time-weighted average bifacial power gain of 0.495 observed in the study? Such adjustments would directly impact revenue models and permitting processes.

The analysis relied on 15-minute SCADA records from 2024 and 2025, providing a high-resolution view of system behavior. Future studies should continue to leverage this granular data to track long-term degradation and seasonal variations. As the industry moves toward more precise performance guarantees, the focus will shift from nominal specifications to effective, field-validated metrics. Stakeholders should watch for updates in financial modeling tools that incorporate these dynamic electrical responses, ensuring that grid connections and storage sizing align with actual yield rather than theoretical maximums.