This is documentation for an energy monitoring device built around the nRF52832 system-on-chip from Nordic. It includes a 2.4 GHz BLE-enabled radio transceiver, a Cortex M4F microcontroller, and a host of hardware peripherals. The device is capable of extracting current and voltage waveforms from a standard wall power plug without contacting the mains power. The current is measured inductively, as a mild steel flux guide channels magnetic flux through a sensitive hall effect device. The voltage is measured capacitively, as the flux guides couple with the conductors in the plug. Measuring both quantities allows us to separate active from apparent power, measuring the real power used by an appliance plugged through our device.
This device produces around 0.1 volts/amp out of the sensor (or ~1 mV per Watt through the plug). The nrf adc has a +-0.6V reference, 4x gain, and 12-bit conversion, which might resolve 100mW (maybe finer with oversampling). Certainly not precision measurement, but good enough for monitoring appliances.
The nRF52 radio+microcontroller draws 5mA in transmit and receive, so it could operate well within the specs of a coin cell, or harvest from the power line (say, using http://www.linear.com/product/LTC3588-1) to charge a capacitor for power. Below, we see the flux guides registered into pockets on the pcb. <imgsrc='img/bc832-watt-0.jpg'width=40%>