Reverse osmosis equipment (RO), driven by photovoltaic systems, can be a reliable way to provide drinking water for the population of remote areas. The specific energy consumption (SEC) of an RO equipment is an important parameter to evaluate the equipment quality and its operating conditions. In this paper, we describe a theoretical study of the SEC of an RO equipment. It is briefly analyzed the SEC for ideal systems. Then, we obtain analytic expressions for real systems, operating under variable conditions. Finally, a limiting case, when the applied pressure is equal to the transmembrane pressure at the output of the equipment, is analyzed. An experimental study of a PV-RO equipment conducted on a commercial multistage desalination unit over a broad range of operating conditions is described. A variable speed drive couples the PV generator to the RO system. Experimental results obtained for the system present a minimum (SEC) lying in a range of (R) between 0.2 and 0.4, smaller than the value obtained for the limiting case (approximately 0.5). The minimum values obtained for hydraulic (SEC) are around 0.32 kWh m-3 for 2,000 mg L-1 and 0.65 kWh m-3 for 5,000 mg·L-1. For the limiting case the minimum hydraulic SEC are 0.18 kWh m-3 and 0.47 kWh m-3, respectively (R = 0.5). A good agreement is found between predicted and measured results. The proposed procedure for calculating (SEC) is simple, precise and general, yielding a valuable tool to evaluate whether the region of operation of a PV-RO system is adequate, according to the main aim of reducing water costs. Copyright © 2012 John Wiley & Sons, Ltd.