As multi-mode is becoming standard in current mobile terminals, the necessity to reduce power consumption increases. In this paper, we propose to utilize multi-mode to cut the energy needs, by relaying on the least power consuming communication mode. The most convenient way to implement multi-mode is undoubtfully provided by Software Defined Radio (SDR). The idea is to replace the ever-increasing embedded radio chips by a single, general purpose, processor. We show that a multi-mode relay, composed of Software Defined Radios, has an impact on power reduction. We elaborate different scenarios to verify this proposition, considering that this multi-mode ability is based on an SDR. Firstly, we propose an 802.11g-to-UMTS relay scheme, where a mobile terminal relays other users' signal. We enounce different rules to minimize the global power consumption through multi-mode relaying. Then, we find different results for an 802.15.4-to-802.11g relay. After further inquiries, we explain how the most intuitive solution is not always the best one. Following a more realistic approach, we use network simulations to determine the important parameters having an impact on power consumption.