Large amounts of new wind power are currently under construction or planning in many countries. The constantly increasing percentage of wind power in the electricity generation mix has to be taken into consideration when planning power systems. This paper introduces a Monte Carlo simulation based methodology that can be used to assess the effects (e.g. need for new transmission lines, reserves, wind curtailment or demand side management) of large amounts of existing and planned wind power generation on the power system. The presented methodology is able to assess new wind power scenarios spread over a wide geographical area, comprising numerous existing and planned wind generation locations. The Monte Carlo simulation results are verified against measured aggregated wind power generation in Finland from 2008 to 2014. In addition, case studies of future scenarios with 232 individual wind generation locations are presented to show the applicability of the methodology as a tool in power system planning. (C) 2015 Elsevier Ltd. All rights reserved.