Abstract Storage of electrical energy produced from an ocean thermal energy conversion (OTEC) system is considered to be extremely essential, since the conversion process could take place in a remote offshore area and distant from the actual utilization sites. Energy conversion from an OTEC system into hydrogen energy, which is used for power generation through fuel cells, is an important approach of storing such energy for further utilizations. In this paper, a technical analysis of hydrogen production through an OTEC system coupled with a polymer electrolyte membrane electrolyser (PEM), which is developed by the Japanese international clean energy network using hydrogen conversion (WE-NET), is performed. The analysis is conducted at an optimum temperature drop between the working fluid and seawater, δ T op. Furthermore, the analysis is carried out at various temperature differences between the surface and deep sea water, Δ T. The calculated results demonstrated the significance of temperature drop and temperature difference on the electrical power output and conversion efficiency. Moreover, the actual rate of hydrogen production varied from 2.5 N m 3/h to 60 N m 3/h as Δ T raised from 5 °C to 25 °C, respectively.