A method for optimal sizing of an electrolyzer directly connected to a PV module or array is presented. By combining the electrolyzer cells in series and in parallel it is possible to closely match the electrolyzer polarization curve to the curve connecting PV system's maximum power points at different irradiation levels. The method presented here is based on linear approximation of both curves. With such a method it is possible to achieve the power transfer efficiency of up to 99%. The effect of PV temperature on optimum electrolyzer sizing is also investigated. The optimum electrolyzer size decreases with the increase of the PV temperature for the same PV system size. It was found that it is better (in terms of the system efficiency and the hydrogen generation rate) to size the EL system for a higher PV operating temperature. Copyright (c) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.