As technology scales for increased circuit density and performance, the management of power consumption in embedded systems is becoming critical. Because the Operating System (OS) is a basic component of the embedded system, the reduction and characterization of its energy consumption is a main challenge for the designers. In this paper, a flow of low power OS energy characterization is introduced. The variation of the energy and power consumption of the embedded OS services is studied. The remainder of this paper, details the methods used to determine energy and power overheads of a set of basic services of the embedded OS: scheduling, context switch and inter-process communication.