The present paper proposes to analyze the consumption of Energy or/and Water (E/W) in the conceptual design (CD) stage of a product, in order to optimize this consumption. The product normally spends more resources in the use stage. The paper also discusses issues in the worldwide and in Europe, and particularly in Catalonia, a Mediterranean country which has small fossil fuel deposits and has recently had periods of drought, making it necessary to watch over water consumption. This article proposes a brief methodological guide including five questions for the designer, or design team, to consider more environmental-friendly consumption alternatives. Ideally, these should be applied in the early conceptual design stage of any product for the entire world. A case study shows the work by a group of students of Creativity, Ecodesign and Patents subject (academic year 2012-13) entitled "Rainwater storage system for blocks of flats". While thinking about the questions in the methodological guide for the use of E/W, appeared a solution that eliminates the energy consumption in the operation of the installation.