This paper elaborates an accurate energy model for Wireless Sensor Device (WSD) based on power consumption measurement. A deep understanding of WSD's energy cost depends on the WSD's functional components contributing to the wireless communication process. The model includes different stages of transmission and reception communication modes. The practical energy model (EM) is developed with data packet lengths, data rate, operation status, pre-designed stages versus appropriate communication bus and power mode including mutual data transfers between the microcontroller and the radio module. The validity of the EM is verified through an experimental setup having a TelosB device. As expected, the modelled and measured energy consumed by the WSD operating from 1-to-4-MHz CPU frequency and 2-to-100 Bytes payload are in very good agreement. It was found that compared to the energy consumption in processing, transmission, listening and reception phases, the data transfer consumes up to 40% of total WSD energy whereas this aspect is often neglected in usual models. Overall, the proposed model provides significant accuracy and might be helpful in improving the energy efficiency of future wireless sensor networks.