The efficiency of Photovoltaic (PV) cells degrades, when the temperature increases more than a certain limit. To maintain the temperature with in the limit, we consider and analyze a back surface based water cooled PV system. This analysis shows that the cell temperatures are proportional to the negative exponent of the water flow rates, higher flow rates increase the power consumption. Keeping this in mind, we present interval based cooling in order to reduce the total consumed power. Moreover, the active pump duration and water flow rates are optimized to maximize the electrical efficiency of PV system. Due to non-convex nature of the objective function, the Genetic algorithm is employed to get the solutions. Simulation results show that the optimized water flow rate for a given interval duration minimizes the consumed power in pumping system, while maintaining the temperatures within the limit.