Electrochemical CO2 reduction (ECO2R) enables direct conversion of CO2 to various chemical products (e.g. CO, MeOH), but CO2 electrolysis consumes a large amount of electricity and thus the economic and environmental feasibility of ECO2R technology is heavily dependent on electricity price and global warming index. Modular and flexible operation of ECO2R system is one of the proposed solutions to cutting off the energy cost and carbon footprint while satisfying product quality and quantity. In this work, the hourly operation of a modular ECO2R process that produces synthesis gas was optimized. Also, the sustainability of flexibly operated ECO2R system in terms of degree of modularization, overcapacity variation and electricity price and GWI fluctuation scenarios is evaluated.