Energy-aware algorithms have proven to be a crucial part of sensor network applications, especially if they are required to operate for extended periods of time. Among these, efficient routing algorithms are of utter importance since their effect can be experienced by all other layers. Thus, the optimization and accurate prediction of the lifetime of the system can only be performed in the presence of accurate execution models that take energy consumption into account. In this paper, we propose a generic routing metric and associated policies that encompass most other existing metrics in the literature and use this model for the optimal construction of a routing tree to the sink. We also provide experimental results that show the benefits of using our novel metric.