A good knowledge of power system dynamics is necessary for the implementation of efficient demand-side-management strategies. This article proposes a model of Unit-Commitment and Economic Dispatch of a power system that can be used to assess the marginal mix for different demand-response events. The model is based on two calibrated merit-orders to reproduce the daily and intra-daily dynamics, including the conservation of hydroelectric energy and electricity exchange, coupled with a simplified model of the European grid. This method is then applied to the French use case for year 2018. The validation protocol includes yearly and weekly timescales. Finally the French model is used to assess the impact of three demand-response strategies: peak clipping, valley filling and load shifting. These strategies turned out to increase the utilization rate of the French nuclear fleet and decrease the French generation electricity from fossil fuels. Moreover, they also impacts the interconnected power systems, in particular Germany, Portugal, Switzerland and Netherland. This model could be used to assess the environmental impact of demand-side-management, if completed with models evaluating the marginal emission factor of the electricity generation in the other European countries and the national power plants.