Design of energy-efficient wireless sensor networks (WSNs) has become an important area of scientific research. In this regard, modeling and minimizing of energy consumption are the main objectives for designing WSNs. In this paper, an energy consumption model for WSNs based on physical layer parameters is proposed by calculating the total energy that is required for successfully received one bit over Rayleigh fading channels. The proposed energy consumption model is validated with real measurements. Results show a good agreement between proposed model and experimental measurements with a mean absolute percentage error less than 5.5%. The validated model is used to optimize transmitted power to achieve minimum energy consumption. Finally, a closed-form expression for optimum transmitted power is derived for M-QAM modulation scheme