Both daily and seasonal fluctuations of renewable power sources will require large-scale energy storage technologies. A recently developed integrated battery and electrolyzer system, called battolyser, fulfills both time-scale requirements. Here, we develop a macroscopic COMSOL Multiphysics model to quantify the energetic efficiency of the battolyser prototype that, for the first time, integrates the functionality of a nickel-iron battery and an alkaline electrolyzer. The current prototype has a rated capacity of 5 Ah, and to develop a larger, enhanced system, it is necessary to characterize the processes occurring within the battolyser and to optimize the individual components of the battolyser. Therefore, there is a need for a model that can provide a fast screening on how the properties of individual components influence the overall energy efficiency of the battolyser prototype. The model is validated using experimental results, and new configurations are compared, and the energy efficiency is optimized for the scale-up of this lab-scale device. Based on the modeling work, we find an optimum electrode thickness for the nickel electrode of 3 and 2.25 mm for the iron electrode with optimal electrode porosities in the range of void fraction of 0.15-0.35. Additionally, electrolyte conductivity and the gap thickness are found to have a small effect on the overall efficiency of the device.