Researchers in Spain have found that the effective bifaciality of solar modules in utility-scale PV plants can be up to 15% lower than their nominal rating. Based on two years of operational data from a 50 MWp solar plant in Spain, with findings validated across four additional installations in Spain and the United Kingdom, the study highlights the influence of irradiance, temperature, mounting geometry and operating conditions on rear-side power generation.
A research team from the Universitat Politècnica de València and Spanish engineering company Grupotec has investigated the gap between nominal module bifaciality and actual performance under operating conditions in utility-scale PV plants.
The study The bifacial gain gap: Nominal and regulatory vs. effective performance across five utility-scale PV sites was published in Renewable Energy.
The researchers analyzed two years of operational data from a 50 MW bifacial PV plant equipped with single-axis trackers in Spain. They subsequently validated their findings using data from four additional solar installations in Spain and the United Kingdom.
Their analysis revealed significant differences between nominal module bifaciality and effective bifacial performance at plant level, with mounting configurations, irradiance, temperature and operating conditions all influencing the contribution of rear-side irradiance to power generation.
Effective bifaciality
At the reference plant, the modules have a nominal bifaciality factor of 0.70. However, the researchers calculated an average effective bifaciality of 0.610 on a time-weighted basis and 0.599 on an irradiance-weighted basis, representing a reduction of approximately 13% to 15% from the nominal value.
The discrepancy was particularly pronounced at medium and high irradiance levels, which account for a substantial share of annual electricity generation. At irradiance levels above 600 W/m², effective bifaciality dropped below 0.60, falling to approximately 0.55 to 0.57 at irradiance levels exceeding 1,000 W/m².
The researchers used 15-minute supervisory control and data acquisition (SCADA) records collected in 2024 and 2025, including meteorological measurements and string-level electrical data.
Rear-side irradiance was measured using albedometers equipped with pyranometers, while module temperatures were recorded using PT100 sensors installed on the rear surface of the modules. The dataset was filtered to remove nighttime measurements, anomalous readings and sensor failures, while retaining fluctuations associated with variable cloud cover.
During the middle of the day, available rear-side irradiance at the Spanish plant amounted to approximately 20% to 25% of average front-side irradiance. Although substantially lower than front-side irradiance, this additional contribution had a measurable effect on string electrical operating conditions and module temperatures.
Higher rear-side gains
The researchers calculated a time-weighted average bifacial power gain (BPG) of 0.495 at the reference plant, with a median of 0.511 and a 90th-percentile value of 0.804. Their analysis identified an inverse relationship between BPG and effective bifaciality.
In the second quarter, BPG reached its quarterly maximum of 0.579, while effective bifaciality declined to 0.590. In the first quarter, BPG fell to 0.352, while effective bifaciality increased to 0.641.
According to the researchers, higher irradiance levels in spring and summer increase the absolute contribution of rear-side irradiance to power generation. However, the associated rise in module temperature reduces the efficiency with which that additional irradiance is converted into electricity.
The effect was also reflected in the modules’ electrical characteristics. As rear-side irradiance increased, string current rose from approximately 3.9 A to 8.2 A, representing an increase of up to 110%. Meanwhile, string voltage declined from around 1,275 V to approximately 1,165 V, a reduction of 8% to 9%. Module temperatures increased by as much as 3.5 C, resulting in an estimated BPG reduction of 0.013.
Mounting geometry
To assess the broader applicability of their findings, the scientists compared results from five utility-scale PV installations featuring the same module technology – bifacial monocrystalline silicon half-cell modules – but different mounting configurations and climatic conditions.
The installations comprised three Spanish plants equipped with single-axis trackers and two fixed-tilt PV plants in the United Kingdom. Across the five sites, irradiance-weighted effective bifaciality ranged from 0.464 to 0.722, equivalent to approximately 66% to 103% of the nominal bifaciality factor of 0.70. Median BPG values ranged from 0.229 to 0.535.
The researchers observed a common relationship between effective bifaciality and irradiance across all installations. Effective bifaciality initially increased with irradiance, reaching a peak at approximately 400 W/m² to 500 W/m², before declining at higher irradiance levels. However, the shape of this relationship varied according to the mounting configuration. Single-row trackers exhibited the most pronounced response, followed by dual-row trackers, while fixed-tilt systems displayed a flatter relationship.
The findings suggest that mounting geometry and the rear-side view factor play important roles in determining effective bifacial performance, alongside ground albedo, row spacing, shading, soiling and electrical losses.
Regulatory bifacial factor
The study also examined the relationship between measured bifacial performance and the bifacial factor of 1.15 established under Spain’s Royal Decree 997/2025. The researchers argued that the regulatory factor should be interpreted as an administrative capacity-conversion coefficient rather than an indication of the power gain that can be expected during actual plant operation.
At the 50 MW reference installation, none of the observed BPG values approached 1.15. The mean BPG was 0.495, while the 90th-percentile value reached 0.804.
The authors also cautioned that BPG should not be interpreted directly as the energy yield gain relative to an equivalent monofacial PV plant. Instead, they described it as a metric for characterizing the bifacial contribution under actual operating conditions.
Implications for PV system design
The researchers said their findings have implications for energy yield modeling, DC-side system design and PV performance assessment. In particular, relying on a single nominal bifaciality factor could lead to an overestimation of rear-side contributions under the irradiance conditions responsible for a substantial proportion of annual electricity generation.
They recommended incorporating the irradiance and temperature dependence of effective bifaciality into PV performance models, rather than assuming a constant nominal value across all operating conditions.
The researchers also proposed using field-data-based performance metrics to enable more meaningful comparisons between PV installations with different mounting configurations and climatic conditions.
Their findings highlight the importance of distinguishing between nominal module specifications, regulatory conversion factors and the actual contribution of rear-side irradiance to electricity generation in utility-scale bifacial PV systems.
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