We formulated a nonlinear scheduling problem to minimize the cumulative electricity costs of an electrochemical system while optimizing the operation of each sub-process. The case study demonstrated our operation idea on co-electrolysis of CO 2 -H 2 O for the production of syngas. The entire system is made of four sub-processes, each of which produces syngas and separates unreacted CO 2 independently. We used the wavelet-based grid adaptation algorithm to obtain near-optimal solutions in a reasonable computation time. The simulation results suggest that the flexible operation of the modular process can reduce the electricity costs while maintaining the syngas ratio. Moreover, the maximum ramping speed of the sub-process, which is likely to be determined by the CO 2 separation unit, is found to be a key factor that has a considerable impact on cost savings.