We present an ultra-small wireless sensor node that can be effortlessly attached onto existing electric circuits for real-time energy monitoring systems. Called Granular Radio EnErgy-sensing Node (GREEN), this platform contains an Atmel 2564RFR2 microcontroller with an internal Zigbee-compatible radio transceiver, a battery unit, and an integrated a GMR magnetic sensor to non-intrusively monitor loads on circuits such as e.g. residential circuit breaker panels. The GREEN device is smaller, and can be installed at a lower cost than previously proposed solutions, and can be placed in a configuration that enables self-calibration. The device may also be used in a wide variety of other wireless sensing applications where small size and low power consumption are important.