Being able to estimate the energy consumption of sensor nodes is essential both for evaluating existing sensor network mechanisms and for constructing new energy-aware mechanisms. We present a software-based mechanism for estimating the energy consumption of sensor node at run-time. Unlike previous energy estimation mechanisms, our mechanism does not require any additional hardware components or add-ons. Our demonstration shows the energy estimation in practice on a small network of Tmote Sky motes running the Contiki operating system. A PC connected to one of the motes shows the real-time energy estimation of the network nodes and where the energy is spent: CPU active, CPU sleeping, radio transmitting, radio listening, and LEDs