This paper develops a realistic battery model that is combined with transmitting signal energy and circuit power consumption, and applies it to wireless sensor networks (WSNs). Then, we calculate the critical values of distances of the source-destination node pairs without considering the relay path. Meanwhile, we intuitively present the relay area and the difference of energy consumption between the single-hop direct path and the multi-hop relay path. Several results give the criterion for the selection of single-hop direct path or the multi-hop relay path to save energy.