One of the greatest challenges in stand-alone photovoltaic irrigation systems (PVIS) without batteries is the tuning of PID controller. Once the system is tuned, it is necessary to evaluate its performance in the most unfavorable circumstances, which occur when the passage of a cloud causes a sudden drop in available power. But there are several kinds of clouds with different effects on the system operation. In this paper, the impact of a considerable number of clouds on the operation of a carefully tuned system has been analyzed. The clouds have been characterized based on the decrease in irradiance, the duration and the slope, and its influence on the voltage of the PV generator has been analyzed. Subsequently, a method has been proposed to simulate the response of the system to the passage of clouds. This method can be applied on clear days, under conditions of constant irradiance, which greatly simplifies its implementation. This also greatly reduces the time required for commissioning the system. The proposed method consists of applying a signal to the "feedforwdard" input of the PID controller that is proportional to the irradiance profile of the cloud to be simulated. Results show that the performance of the system, characterized by the voltage of the PV generator, with simulated clouds is similar to the performance with real clouds.