The need for sustainable architecture and urban design and planning has long been acknowledged, along with the necessity for adequate, early-phase guiding instruments. This paper aims at exploring the effectiveness and usability of a novel decision-support workflow for neighbourhood-scale projects, developed to provide practitioners with early-stage design alternatives in an interactive and iterative sequence. The prototype includes a performance assessment engine, which quickly computes an estimate of the daylight and passive and active solar potential for each design alternative. To assess the added value for design and the educational features offered by the workflow, workshops were organized with architects and urban planners. Participants were asked to work on a realistic micro-urban design project by means of two different approaches: making use of their conventional tools and methods, and then using the prototype. In addition to these design phases, the workshop included ranking design alternatives with respect to their performance before and after using the prototype, and filling pre- and post-workshop questionnaires to gather the participants’ level of experience and their feedback. The main outcomes from these tasks show that the prototype yields a strong potential in terms of design guidance, despite mixed results in the level of success in the before and after ranking phases. Results also highlight the necessity to pursue the development and adoption of energy-oriented early-stage design instruments.