The aim of this paper is to propose an additional approach for market monitoring in power exchanges, based on the cluster analysis of supply offers. Clustering procedures, in particular, were implemented to properly aggregate offers submitted in the Italian electricity Day-Ahead Market in 2006, so as to identify groups of bids characterized by a «similar» strategic behaviour adopted by market players. On the basis of obtained results, the paper highlights the main implications for competition of the shape of bid curves submitted by different operators in the Italian market.