An experimental facility of a photovoltaic array mounted on a one-axis east-west tracker, coupled to a pumping system, was tested for several months, under two working conditions: fixed and tracking mode. Results show that the gain obtained in the daily water volume is higher than the one obtained for the collected irradiation. For values of hemispherical irradiation equal to 5000 and 6000 Wh/m(2), the irradiation collected by the tracker plane is 19 and 24% higher than the one collected by the fixed system. However, gains in water volume are equal to 37 and 41%. The reasons for the increase in pumped water volume being larger than the increase in collected irradiance are analyzed. To generalize the experimental results, a long term estimate of the water volume pumped by PV systems driven by tracking collectors was carried out via the utilizability method. As a result, we describe how the benefit ratio in pumped water volume is affected by the contributions due to collected irradiance, utilizability function and a factor 12, related to the non-linearity of the characteristic curve. Finally, the influence of the critical irradiance (I-C) (minimum value of irradiance, necessary to start the water pump operation) on the benefit obtained in pumped water volume, by using tracking systems, is discussed. Numerical simulations show that the increase in pumped water volume, annual average, varies between 1.29 and 1.53 for (I-C) within the interval from 275 to 575 W/m(2). (C) 2003 Elsevier Science Ltd. All rights reserved.