The main bottleneck to achieve energy autonomy in body area networks (BAN) is the design of an ultra low power yet reliable wireless system. In this paper, we first demonstrate the feasibility of an ultra low power receiver by presenting our implemented receiver chip that could operate on a total power of 479.5 uW, which is more than one order of magnitude lower than commercially available low power transceivers working at 2.4 GHz. We then show the reliability of this chip, which can achieve a receiver sensitivity of -72 dBm for a data rate of 1 Mbps. We further demonstrate that this receiver sensitivity is sufficient to guarantee reliability by evaluating this chip in different to-body communication in BAN environments. By using typical BAN channels, simulation results show that our system can provide a reliable link in both the standing and walking situations. With the measured data, we show that a transmit power of -15 dBm is sufficient for our receiver to achieve reliable communication link in different BAN environments. This transmit power requirement is 15 dB lower than the widely known low power Zigbee system. It could thus significantly facilitate the ultra low power transmitter design, and minimize the human exposure to radio frequency electromagnetic fields. The design and evaluation of our receiver presented in this paper therefore provides a way to move towards the energy autonomy of BAN, and opens access to many new applications in BAN.