Seawater electrolysis has attracted much attention in marine and offshore industries over the past few years. However, there is still not much information available to guide the design of undivided cells, especially around the energy consumption performance. In this study, four theoretical models of the seawater electrolysis in an undivided cell were developed based on physical and chemical principles. Experiments using both artificial seawater and natural seawater were conducted to obtain model parameters and verify the accuracies of these models, respectively. The total residual oxidant (TRO) concentration and cell voltage increased as the current density increased, while the current efficiency was independent of current densities at seawater salinity of 30.0 psu. The seawater salinity had significant impacts on cell performance, especially in the range of 4.0-15.8 psu. When the seawater salinity increased, the TRO concentration and current efficiency increased accordingly, while the cell voltage decreased. These phenomena were closely related to the kinetics of electrode reactions, and conductivity of the electrolyte. Additionally, prediction values of theoretical models suggested that on-site generation of relatively low concentrations of TRO from concentrated seawater discharging from desalination facilities would reduce the overall energy consumption, as well as eliminate the environmental footprints, particularly for marine and offshore applications. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.