Offshore production of macroalgae biomass, which was recently given the name seagriculture, is one of the important but least explored alternative energy resources. Unlike microalgae, macroalgae cultivation can be done offshore and therefore brings real news to the biofuel food land agriculture conflict. A wide variety of small-scale laboratory experiments are done lately in order to deepen the knowledge and develop expertise in macroalgae cultivation and its downstream processing. For energy applications, it is common to evaluate the performance of an energy source or system in exergy efficiency terms. Another important parameter that is evaluated to determine the system's environmental impact is it's volumetric and areal footprint. The current work examines two exergy efficiency indexes, the Exergy Efficiency (EE), which takes into account all exergy inputs, and the Exergy Return On Investment (ExROI), that includes only fossil fuel exergy inputs, both on a green macroalgae Ulva grown in the macroalgae photobioreactor system (MPBR) incorporated into a building. Cultivation of macroalgae in the building embedded MPBR achieved maximal values of 0.012 and 0.22 for EE and ExROI, compared to a range of 0.05-8.34 and 0.013-0.327 found in published papers of microalgae systems. In addition, a modelled optimization of the initial biomass density leads to maximal values of about 0.035 for EE and 0.433 for ExROI, while further improvement may be achieved by optimization of nutrient addition and mixing methodology. This work demonstrates a tool to measure the performance of laboratory scale macroalgae biomass cultivation systems, followed by preliminary efficiency and environmental impact values, important for future upscaling.