Cross-layer error protection strategies couple systematic error correction at the physical layer with adaptive data retransmission at the data-link layer, to mitigate the effects of fast-fading propagation channels. This work, in the form of a case-study, aims to introduce useful tools for the assessment and analysis of the energetic performance of such strategies. In particular, it is shown that in energy-constrained applications, such as wireless sensor networks, the design guidelines should be based on a precise definition of the energy per bit. This definition also allows to evaluate the minimum energy that is necessary to transmit data in a given setting.