Packet-size optimization plays an important role in energy efficiency, thus increasing the battery lifetime of battery-powered wireless body area networks (WBAN). However, for a fully batteryless WBAN, lifetime is not a concern due to endless harvestable energy from ambient sources such as body heat. Instead, network performance should be considered due to the very stringent power budget. In this chapter, energy efficiency of three common schemes, including one-hop, cooperative, and two-hop networks for a battery-powered network, are studied. A new method for packet-size optimization in a batteryless network is proposed. It uses the results of battery-powered packet-size optimization as an initial condition. Numerical results prove that the one-hop, on-body, LOS-transmission scenario has the minimum payload and thus the fastest network due to minimal processing and energy consumption.