Solar drying of biomass has gained prominence as a pretreatment step for biofuel production. The literature presents a wide diversity of definitions and terminologies for evaluating solar dryer performance, particularly in systems with supplementary energy inputs, limiting consistent comparisons between studies. This review provides a critical analysis of the main energy-based performance indicators, with emphasis on hybrid systems. Collector and drying chamber efficiencies are examined based on thermal and energy concepts, considering different dryer configurations. Standardized formulations are proposed to better represent the underlying processes and clarify the distinctions between PV and PVT-assisted systems. The analysis shows that collector performance in active systems is more consistently represented when based on absorbed solar energy and electrical power consumed by the fan. Accurate assessment of drying performance requires accounting for the heat of moisture desorption, sensible heating of the product, useful solar energy for air heating, fan power and supplementary energy inputs. Instantaneous efficiencies are more informative than cumulative ones, as they enable identification of less efficient drying periods and provide insights for optimizing operating conditions and reducing energy consumption. Continuous monitoring of key process variables including solar radiation intensity, air and product temperatures, and product mass is therefore required. Overall, this review establishes a consistent framework for the assessment of solar dryers, enhancing comparability and supporting more reliable analyses across studies. Future research should focus on the effects of intermittent and variable airflow rates on system performance.

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