Our homes are filling with energy hungry devices. Some of them are constantly active like fridge, routers, lighting, some switched off or on stand-by. Each of them takes some portion of energy that reflects on the end month bill. Some things you can control, but some not. So in order to hunt inefficient nodes you need some sort of energy usage monitoring.
You can find lots of implementations where some are simple indicators, other are advanced and web enabled.
TSalwach @ github.com have built his own version of energy monitoring system which looks really promising. His implementation monitors all three phases where he extracts several parameters:
RMS Voltages and currents on each phase
Apparent power (product of RMS voltage and RMS current)
Real (active) power
Reactive power with inductance/capacitance indication (time delay method *)
Frequency (zero crossing detection with linear interpolation)
Costs of energy and alternative cost (on single tariff)
He used two microcontroller boards to for this task. First one is Raspberry Pi which takes care of web accessibility, data storage and interpretation. The other is Nucleo board with STM32F072RB ARM microcontroller. This is a front end which collects data from AC sensing transformers.