A study of five utility scale solar photovoltaic plants in Spain and the UK has found that the real world performance of bifacial solar modules can differ significantly from their nominal ratings, raising questions about the assumptions used in project design and regulatory assessments.
The study is titled ‘The bifacial gain gap: Nominal and regulatory vs. effective performance across five utility-scale PV sites.’
The research, published in the journal Renewable Energy, analyses two years of operational data from a 50 MWp solar plant in Spain, with results validated against four additional plants covering different mounting configurations and climatic conditions.
Bifacial photovoltaic modules generate electricity from both their front and rear surfaces. Their ability to capture additional sunlight can increase energy production compared with conventional monofacial modules, making them a widely adopted technology for large scale solar installations.
However, the study found that the amount of additional electricity generated depends on site conditions, mounting structures, irradiance levels and the electrical and thermal behaviour of the modules in operation.
At the reference plant in Spain, the time weighted bifacial power gain was 49.5%, with the 90th percentile reaching 80.4%. These figures describe the study’s calculated power gain relative to temperature and irradiance corrected front side nominal power and should not be interpreted as a direct percentage increase in total plant energy production.
Effective bifaciality, which measures the contribution of rear side irradiance relative to front side irradiance, averaged 0.610 on a time weighted basis and 0.599 when weighted by irradiance. Both results were below the modules’ nominal bifaciality rating of 0.70.
The researchers found that the difference was greatest under irradiance conditions that contribute most to energy production, highlighting the limitations of relying on manufacturer ratings alone when estimating operational performance.
Across all five plants, irradiance weighted effective bifaciality ranged from 0.464 to 0.722, equivalent to 66% to 103% of the nominal rating. Median bifacial power gain varied from 22.9% to 53.5%, reflecting differences in site conditions and system configurations.
The study also identified a seasonal relationship between bifacial power gain and effective bifaciality, with the two indicators showing an inverse pattern. This suggests that the factors determining additional power generation do not remain constant throughout the year.
Rear side irradiance also affected the electrical and thermal operating conditions of the systems. Direct current string current increased by up to 110%, while string voltage fell by 8% to 9%. Module temperatures increased by as much as 3.5°C.
These effects have implications for direct current system design, including string sizing, cable selection and protective equipment. They also reinforce the importance of incorporating measured operating conditions into energy yield assessments and long term performance evaluations.
The findings are particularly relevant to Spain’s Royal Decree 997/2025, which introduces a factor of 1.15 for calculating the installed capacity of bifacial photovoltaic systems based on their front side standard test condition power.
The regulation effectively treats installed capacity as 115% of the front side rated capacity for administrative purposes. However, the researchers emphasise that the factor is an administrative capacity conversion rule rather than a guarantee of a 15% operational performance increase.
The distinction matters for project developers, engineering companies and system operators because capacity calculations can influence permitting, grid connection agreements, equipment sizing and the technical and financial assumptions underpinning a project.
The study’s findings suggest that a single fixed factor cannot fully represent the operating behaviour of bifacial solar plants across different sites and mounting configurations. While the regulatory approach simplifies administrative procedures, project level assessments still require realistic performance assumptions based on local conditions.
The research compared three Spanish plants with relatively high direct irradiance resources against two UK plants with more diffuse irradiance and average irradiance levels between 35% and 45% lower. All five installations used monocrystalline silicon half cell bifacial modules, allowing the researchers to examine the effects of mounting geometry and climate on performance.
The authors conclude that field derived effective bifaciality and bifacial power gain provide useful indicators for solar yield modelling, direct current design and performance assessment. Applying these metrics across different sites could help developers better account for the contribution of rear side generation and reduce the risk of discrepancies between expected and measured performance.
As bifacial technology continues to play a major role in utility scale solar development, the study underscores the importance of using operational evidence alongside nominal module specifications to improve project design and performance forecasting.
Link to the full paper HERE
Author: Bryan Groenendaal






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