The design of a node of a wireless sensor network is a challenging task, as it is expected to be small, reliable and low cost, using hardware that is characterized by limited resources, including energy, computation power, memory and communication bandwidth. In order to save the valuable energy it would be instrumental to measure the energy consumption by every deployed node on the field. Prospective applications include, energy-aware routing and hardware diagnosis for detecting subsystems' faults or unusual power consumption. In this work we propose a measurement method and circuit, named Smart Coulomb Counter (SCC), that easily adds to a sensor node the capability of measuring its own energy consumption. This hardware-based approach consists in duty-cycling the measurement circuit together with the sensor node, thus lowering its power consumption. Experimental results demonstrates that SCC has a very low power consumption, ensuring an almost negligible impact in the battery lifetime. Other advantages of SCC include high dynamic range, high linear range, very low temperature drift, independence of the power supply, easy to integrate and low resource requirements in terms of memory and processor usage, as well as in terms of component count (area and cost). A fully-functional prototype based on the sensor node TelosB running ContikiOS was developed. The main features have been tested on the field and the advantages of our system were demonstrated in real conditions. Finally, a comparison between our solution and a software-based approach (Energest) is presented.