This work describes a design experience focused on the theme of eco-feedback, a technology complementary to traditional energy control systems and meant to obtain additional savings by increasing the awareness of the users about the factors that determine energy consumption. This experience, held during a multidisciplinary ideation workshop and involving students and researchers at the University of Toulouse, was focused on the conceptual design of an Energy-Box, conceived as an artefact (or a set of artefacts) for making people inhabiting the University campus aware of energy issues and leading them to more energy conscious lifestyles. Three different proposals stemmed from this experience, that were evaluated first by the participants to the workshop themselves and then, some months later, by 80 undergraduate students of the University of Venice working on the same themes. We extracted interesting lesson learned from this evaluation, that should be taken into account for the development of user's centred eco-feedback systems in public contexts.