The main problem in using photovoltaic (PV) systems is the low energy conversion efficiency of PV cells. The efficiency of PV cells will decrease significantly as the temperature of the cells exceed to a certain limit. In order to increase the efficiency, it is necessary to reduce the operating temperature of array. One of the ways for improving the system operation is cooling PV cells with a thin film of water. The aim of this research is to study the effects of nominal power of array and system head on the operation of system by using this method. For this purpose, a photovoltaic water pumping system is installed in Kerman city (Latitude: 30° 17′ and longitude: 57° 50′) and different methods examined to reduce PV cells temperature. The most effective way was chosen and used in set. This method is based on providing water for cooling cells by the pump itself. Experiments show that with decreasing of array nominal power and increasing in system head, the power generated by the array increases significantly. This increases the panel and total efficiency and therefore the pump flow rate. This method is ineffective as the array nominal power increases significantly.