Since their introduction, Wireless Sensor Networks (WSN) have been proposed as a powerful support for environment monitoring, ranging from monitoring of remote or hard-to-reach locations to fine-grained control of cultivations. Development of a WSN-based application is a complex task and challenging issues must be tackled starting from the first phases of the design cycle. We present here a tool supporting the DSE phase to perform architectural choices for the nodes and network topology, taking into account target performance goals and estimated costs. When designing applications based on WSN, the most challenging problem is energy shortage. Nodes are normally supplied through batteries, hence a limited amount of energy is available and no breakthroughs are foreseen in a near future. In our design cycle we approach this issue through a methodology that allows analysing and optimising the power performances in a hierarchical fashion, encompassing various abstraction levels.