Wireless Sensor Networks (WSN) are used in various fields with the purpose of establishing low cost network for data exchange. An important aspect in ZigBee WSN study is the reduction of node power consumption in order to reduce the overall power consumption of a network and to reduce the induced CO2 emissions. One approach in power consumption reduction is the Adaptive Transmission Power Control (ATPC), where the end nodes reduce their transmit power in certain conditions. This paper analyses the impact of adaptive power control on the power consumption of Digi's XBee PRO WSN modules. To determine the conditions for adaptive power control, this paper presents measurements analysis for indoor RF propagation in order to determine the minimum received signal strength for high data transfer reliability. Sequentially, the impact of radio irregularity on the received signal is shown against numerous factors, and the proposed method for adaptive power control is discussed. It is shown that by implementing ATPC the reduction in power consumption is evident while retaining the data transfer reliability.