This chapter presents an energy balance model tailored for the evaluation of macroalgae cultivation in a Marine Photobioreactor (MPBR) system, with a focus on Ulva sp. We propose specific Exergetic Efficiency (EE) and Exergy Return on Investment (ExROI)Exergy Return on Investment (ExROI) formulas to assess the energy dynamics within a controlled MPBR environment. These indicators consider direct and indirect exergy inputs, including solar irradiance, electrical energy, and nutrient assimilation, alongside the exergy content of the produced biomass. The study develops an exergetic framework for evaluating the efficiency of MPBR systemsMPBR system, comparing them with other biomass productionBiomass productivity methodologies. The efficiency models incorporate cumulative exergy demand, emphasizing the impact of solar energy, often excluded in conventional exergy analyses. A sensitivity analysisSensitivity analysis is conducted to determine key optimization parameters, such as initial biomass density, for improving large-scale production efficiency. Findings highlight the potential for optimizing energy efficiencyEnergy efficiency through controlled parameter adjustments, offering insights into the scalability of MPBR systemsMPBR system for offshore and land-based macroalgae cultivation. The study provides a basis for further refinement of energy efficiencyEnergy efficiency metrics in biofuel and sustainable aquaculture applications, underscoring the significance of exergy-based performance assessment in renewable biomass productionBiomass productivity.