As the data traffic demand increases continuously,LTE Heterogeneous Networks (HetNets) with macro-cells and small-cells seems to be the best solution to enhance the quality of user experience by increasing the coverage and capacity of cellular networks. One of the important challenges in such networks is the user association problem. Several approaches have been proposed for this purpose: based for instance on spectral efficiency or load balancing. However, since many factors impact the power consumption of the eNodeBs and therefor the total energy consumption in the cell, defining new rules and approaches to reduce the power consumption in LTE HetNets is needed. In this paper we propose a power efficient user association scheme. Our solution aims to achieve a trade-off between user quality of experience and power efficiency in such heterogeneous cellular networks taking into account user mobility. For this to happen, we introduce a new metric: the Green Topological Potential approach (GTP). It combines energy efficiency (EE) and spectral efficiency (SE) metrics when selecting the target cell. Simulation results show that a power gain can be achieved compared to path-loss based and received power based algorithms while maintaining acceptable spectral efficiency