Photovoltaically operated reverse osmosis plants in off-grid operation constitute a promising system technology for meeting a part of the water requirements in regions without dependable water supply and electric grid system power supply. This paper presents a new procedure for optimum system design configuration. The goal is to provide the cheapest possible water supply while fulfilling all regional and technical boundary conditions. The starting point of the procedure is a rough design based on a load duration curve. Subsequent time sequence simulations which image the system behaviour completely, permit checking of various plant variants for compliance with the boundary conditions. Objective mutual comparisons of the plant variants are possible, also taking the system costs into consideration. The possibilities of the developed procedure are demonstrated taking a village supply in Northeast Brazil as example.