The aim of this paper is to present and evaluate a proposal for designing an off-grid offshore electrolysis plant powered by wave energy. This plant includes PEM electrolyzers, a Reverse Osmosis system to produce water with adequate conductivity, a compression unit to store the hydrogen for transport, and batteries for temporary storage of electricity for short-time balances. First, the systems that compose the proposed plant are justified and described. Then a proposal for sizing these subsystems is given, based on using buoy-measured data at the expected location and simple mathematical models of the different sections of the plant. Finally the performance of the plant in a specific location is tested in detailed by using measured data, studying the influence of sizing on the expected performance. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.