WSN nodes are usually powered by batteries. Energy consumption during node operation determines battery life. Power consumption depends on the different hardware and software components in a WSN node and their various activities. In order to determine the life of the battery, we must measure the energy consumption of a node that is active in a network. That is, we must know the power consumption and time duration for node activities including computations, and RF transmission and reception. In this paper, we introduce an easy and accurate method for dynamic energy measurements without disturbing the node or network. The method consists of an oscilloscope, which can stream digitized voltages into a PC, which calculates the profile of energy consumption. We employed this capability to determine energy consumption for different security algorithms in CrossBow MICA2 nodes.