Instructables' TheSpodShed brings you the 'Internet-Connected Home Energy Monitor'
This is a magic box which will let you monitor the power consumption of your house from anywhere on the Internet! It measures both true power (Watts) and apparent (VA) power, it keeps a running total of electricity units used, and measures mains frequency as a bonus.
The project uses a Particle Core(*) module - a little board with an ARM microprocessor and a Wi-Fi interface - to do all the hard work. To build it, you'll need to be able to solder and follow a simple circuit diagram. At UK prices, total parts cost should be no more than £50.
The project was designed for 230V 50Hz (European) mains circuits, but with suitable calibration ought to work on US or other systems.
* - until a few days ago, Particle were known as Spark. You'll find 'Spark' sprinkled throughout the supporting files for this project - please be assured these are the same thing!
You can find the 'all steps' tutorial at; http://www.instructables.com/id/Internet-connected-home-energy-monitor/?ALLSTEPS