In this paper, we study power consumption in a multi-hop spectrum sharing system (MHSSS) in the presence of co-channel interference and under constraint of the outage probability (OP). Considering imperfect channel state information (CSI), we propose an algorithm which jointly performs relay selection and power allocation. In this regard, convex optimization framework is used to determine the optimal power allocation of the secondary transmitters. Furthermore, we derive an upper band and a lower band for the power allocation which can be used to reduce the calculation complexity. Numerical simulations are made and results verify theoretical analysis and demonstrate power efficiency of the proposed algorithm.