Demand Response Resources Management is one of the most investigated solutions oriented to improve the efficiency in electricity markets. In this paper, the capability of customers to participate in short term markets is analyzed. An available methodology to analyze the daily and monthly energy consumptions of large customers is used to create energy offers and bids. This allows customers to participate in energy markets in order to buy, as first step, the usual electricity surplus for their consumption and, additionally, to offer demand reductions in the short term electricity markets. This paper shows the customer potential to participate in the Spanish Electricity Markets.