This paper presents a method for estimating the loss-of-load probability (LLP) of a photovoltaic water pumping system. The study has been carried out for a constant profile, using a tank with a two day autonomy capacity and two pumping heads applied to a centrifugal pump. The method developed in this paper gives a simple tool for LLP calculation. The procedure can also be used to draw LLP maps with normalised parameters using long term observed or generated sequences of meteorological data. This technique gives a generalised and practical graphical tool for systems sizing. Some examples from Algeria are used. But the method can be applied to any geographical area. (C) 2004 Elsevier Ltd. All rights reserved.