Real-time measurement of the energy consumption of wireless sensor network nodes in the field has many interesting applications that range from predicting the remaining battery charge to the energy efficiency assessment of communication protocols. However, designing such a system presents many challenges. In this work 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. The SEM developed has very low power consumption, ensuring an almost negligible impact in the battery lifetime. It also solves the problem of handling a dynamic range of five decades. Results from simulations show that the SEM is highly linear, with a coefficient of determination of 0.996, presenting a very low temperature drift, and is almost independent of the power supply.