In this study, the authors analyse the average bit-error rate (BER) of a wireless powered three-node decode-and-forward (DF) relay system in a Rayleigh fading environment. The direct link between the source and the destination is present and the received signals are combined using equal gain combining at the destination. Data is assumed to be binary phase-shift keying modulated. The relay node of the system is powered by the source node, i.e. the relay node harvests energy using radio frequency transmitted at the source node and forwards the signal using the harvested energy. The source data is communicated in three time slots; energy harvesting in the first time slot and selective DF relaying in the second and third time slots. The asymptotic expression of the average BER is analysed and used for the suboptimal energy allocation in different time slots. Since energy consumed in a slot depends on the transmitted power and the slot duration, the relation between them is used for the suboptimal allocation of the transmit power and time among different slots.