Reduced levels of energy consumption is one of the major goals in Wireless Sensor Networks (WSNs), making the design of a sensor node a challenging task. On-field real-time measurement of the energy consumption of sensor nodes could have a major impact on developing wireless networks. It could be applied to predict the remaining battery charge or to asses the energy efficiency of communication protocols. However, designing such a system presents many challenges that will be discussed in this work. Moreover, we present a measurement method and circuit, named self-energy meter (SEM), that easily adds to a sensor node the capability of measuring its own energy consumption. SEM is a novel approach focused in solving the problem of covering a dynamic range of five decades in duty-cycle battery operated sensor nodes. Experimental results show that SEM has very low power consumption, ensuring an almost negligible impact in the battery lifetime, is highly linear, presents a very low temperature drift, and is almost independent of the power supply.