We consider the design of the MAC layer for low power, low data-rate, impulse-radio ultra-wide band (IR-UWB) networks. In such networks, the primary concern is energy consumption rather than rate efficiency. We explore several dimensions such as power control, rate adaptation, mutual exclusion, slotted versus non-slotted operation, power saving modes and interference mitigation. We analyze the effect of these design choices on the energy consumption and rate efficiency. We use a method of energy quanta for computing the energy consumption. We find that for both cases, the optimal operation is non-coordinated and with no power control. Sources should send at their maximum power and not pay attention to neighboring nodes. However, sources should constantly adapt their transmission rate to the level of interference.