This paper presents the architecture and functionalities of a grid simulation tool called NetzKapa. NetzKapa is a tool used for assessing distribution grids performance for different electrical vehicle configurations (e.g. distribution of charging infrastructure, charging profiles) by means of Monte-Carlo simulations. The Monte-Carlo method allows assessing significant amount of grid variations and obtains statistical information on different performance indicators. One of the contributions of this paper is the implementation of a concept call region of interest (ROI) in the Monte-Carlos simulations, which allows reducing the number of simulated configurations during the estimation of the grid capacity.