UWB impulse radios show interesting features for the RF transmission link. The more interesting characteristics of such RF front-end for low power sensor networks are (i) very low energy per bit consumption, (ii) very simple architecture leading in low cost deployment, (iii) capability to achieve high data rates. In addition, the use of the sub-GHz UWB channel allows to reduce the path loss in comparison with traditional UWB channels using high frequency bands and also to benefit from better hardware performances because active components are used far away theirs cutoff frequencies. In this paper we investigate the sub-GHz UWB RF link capabilities regarding energy consumption in low power sensor networks. The sub-GHz UWB RF link is used in a Radio triggered asynchronous protocol and the energy consumption of the corresponding network is modeled as a function of the number of network nodes and of the network activity.