Photovoltaics and electrolyser coupling is one of the most promising options for obtaining hydrogen from a renewable energy source. Both are well known technologies and direct coupling is possible; however, due to high variability of the solar radiation, an efficient relative sizing still presents some challenges. In fact, relative sizing is always a key issue when coupling renewable electric sources to water electrolysers. Few previous works addressed the relative sizing and an easy and efficient method is still missing. This work presents a new method for relative sizing between both components based on simple modelling of both polarisation curves. Modelling and simulation is used for extracting a cloud of maximum power points at all the radiation and temperature conditions for a normalised PV generator. Then, the ideal ratio between the size of components is obtained by fitting a normalised polarisation curve for the electrolyser to this cloud of maximum power points. PV generator and PEM electrolyser models are proposed and the method is applied, as example, to two different PEM water electrolysers. The method helps the relative sizing issue for designing solar hydrogen production systems based on water electrolysis, because it is derived from manufacturer parameters and the used of uncomplicated numerical methods. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.