We provide a detailed analytical model for estimating the total energy consumed to exchange a packet over a wireless link. Our model improves many of the current models by considering details such as consumed energy by processing elements of transceivers, packet retransmission, reliability of links, size of data packets and acknowledgments, and also the data rate of wireless links. To develop the model, we use experimental results based on IEEE 802.15.4 devices to show that consumed energy for receiving erroneous packets is comparable to the consumed energy for receiving error-free packets.