This paper addresses the dual questions: What is the appropriate storage size and its related properties for matching very large photovoltaic plants to the grid; and what are the available technologies for achieving this end. For this purpose a "Usefulness Index" is defined, which, for any grid flexibility, leads to a PV-storage combination that allows high grid-penetration without storage being wastefully large. The paper then examines the sensitivity of this "appropriate storage size" to variations in our assumptions. The specific case of the Israeli electricity grid is employed for numerical discussion, but the formalism should be useful for wider application. In particular, the "appropriate storage size" deduced in this manner is argued to be a valuable point of departure for optimizations of a more sophisticated nature. Regarding available storage technologies, none is found to have all of the required properties for massive PV-grid penetration, but hybrid combinations should be capable of achieving this end. (C) 2011 Elsevier Ltd. All rights reserved.