The energy efficiency of a wireless sensor depends of three criteria: the transmission power (transmission range), the radio links quality (BER) and finally the message length. In this paper, we propose to reduce the overall consumption of a sensor by adapting the transmission power according to the last two criteria. The originality of our approach is to improve an energy model by the use of a realistic propagation model and a realistic BER estimation. Thus, from this complete model we show the propagation channel influence on the adjustment of the transmission power. The consequences on the energy consumption are illustrated in terms of lifetime on a forwarding sensor. Simulation results demonstrate clearly that significant gains on the lifetime of a sensor are obtained by adjusting the transmission power in function of the frame length.