Along with the spread of mobile devices supporting multiple radio interfaces, users can connect to the mobile operator's network using different access technologies such as UMTS, WLAN and LTE. While heterogeneous network access technologies allow operators to offer next generation services to their customers at an affordable price, providing dynamic and seamless handover between these networks to minimize service interruption is a research problem. IEEE 802.21, which defines a set of standards for media independent handover, is capable of realizing both horizontal and vertical handovers. In this paper, a new algorithm is proposed to provide energy efficient handovers between UMTS and WLAN networks. The proposed algorithm is implemented within the IEEE 802.21 framework and its performance analysis is performed using the NS2.29 simulation tool. Simulation results show that the proposed algorithm significantly increases the service duration and hence decreases packet losses by taking the energy efficiency into account during its handover decisions.