  The Internet of Things (IoT) comprises an increasing number of low-power and
low-cost devices that autonomously interact with the surrounding environment.
As a consequence of their popularity, future IoT deployments will be massive,
which demands energy-efficient systems to extend their lifetime and improve the
user experience. Radio frequency wireless energy transfer has the potential of
powering massive IoT networks, thus eliminating the need for frequent battery
replacement by using the so-called power beacons (PBs). In this paper, we
provide a framework for minimizing the sum transmit power of the PBs using
devices' positions information and their current battery state. Our strategy
aims to reduce the PBs' power consumption and to mitigate the possible impact
of the electromagnetic radiation on human health. We also present analytical
insights for the case of very distant clusters and evaluate their
applicability. Numerical results show that our proposed framework reduces the
outage probability as the number of PBs and/or the energy demands increase.
