A DC-DC switching converter, originally included to reduce the power consumption of a Wireless Sensor Networks (WSN) node, has been proposed as the core of an on-field self-energy meter. In this paper we present a method and circuit that improves the electronics proposed by previous work by conditioning the signal from the switching converter that is connected to the microcontroller's counter. A software module that allows a WSN node to measure its own charge and current consumption was also implemented. The proposed method allows to measure the current consumption in a wide range, from 0 to 30mA, is highly linear and is ultra-low-power (the maximum current consumption is 8µA). Finally, we present a case study in which the proposed method is used to power profile a WSN node. Results show that a time-based estimation (Energest) overestimates the Clear Channel Assessment consumption for more than 10%.