Electrolysis systems must be constructed from multiple stack units. This modular system of stack units (modular electrolysis system) requires systematic optimization of its composition. This optimization depends on numerous, and sometimes conflicting, criteria. While such criteria have already been evaluated for modular plants in the process industry, they must be re-assessed in the context of modular electrolysis systems. To address this challenge, an expert survey was conducted within two research networks (H2Giga and DECHEMA e.V. Research Network) and the VDMA P2X4A.The evaluation followed the two-stage process: After ranking the categories costs, flexibility, process engineering and time-to-process according to their overall importance a ranking of individual criteria within each category was conducted. The survey reveals a clear prioritisation: costs are in first place with 35.9%, followed by flexibility (25.4%), process technology (23.2%) time to process (15.4%). This ranking provides a structured overview of the most important factors guiding the optimisation of modular electrolysis systems. Another finding of the survey is how electrolysis systems should be operated in the future. Here, the focus was primarily on electrolysis systems related to fluctuating renewable energies, with a top-2-box result of 81% in each case. Based on this prioritisation, weightings for possible application in multi-criteria optimisation can now be derived. This makes it possible to determine suitable compositions of electrolysis stack unit systems when scaling systems and to refine optimisation efforts. The expert survey thus contributes to the optimised design of electrolysis systems and thereby supports the ramp-up of hydrogen production.