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.

Internet-connected home energy monitor

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’ll need the following major components:

The Particle Core (a.k.a. Spark Core) module

This comes in a nice little kit with a breadboard and USB lead. You can buy one direct from Particle at, from Adafruit, or in the UK the cheapest I’ve found is from CPC.

There are two versions of the Core – one with a built-in ‘chip’ antenna, and one with a u.FL socket for an external antenna. I’ve found the built-in antenna is fine anywhere in my house where there is normal (“two or three blobs on an iPod”) Wi-Fi reception. The external antenna would be better for poor signal areas (e.g. an outbuilding) but you’ll have to add the cost of a separate Wi-Fi antenna and a u.FL ‘pigtail’ lead.

AC Current Sensor

This is a small magnetic device which clamps over a current-carrying mains conductor, and produce an output voltage which is proportional to the current flowing in the wire. The one I used was for an Owl home energy monitor, but you can search Amazon, eBay, etc, for ‘AC Current Sensor’ for a variety of alternatives. Make sure you choose one with enough maximum current capability for the load you’re wanting to measure (e.g. 30A = 7kW approx). Please note – you don’t want to use a ‘current shunt’: these are not isolated from the mains itself and cannot be used in this design.

AC output mains adaptor

The circuit needs an AC power input of between 6V and 12V (RMS). I found a 9VAC adapter for an old modem in my junk box; power consumption is low (3W or less), so pretty much any adapter will work. Note that a DC adapter won’t work, because the monitor needs timing information from the mains to measure real power consumption accurately.

5V output DC-DC converter

My original plan was to use an old car USB charger to provide a step-down regulator to provide 5V to the Spark module. Sadly it proved faulty, so I designed a circuit based on the MC3063 IC. You can build this, or for simplicity you could use a ready-made module. A variety of modules based on TI’s LM2596 chip are readily available from Amazon, eBay, or elsewhere.

Other components

The passive components used in the circuit are:

  • C1 1000uF 25V electrolytic
  • C2 100uF 10V electrolytic
  • C3 100n ceramic
  • C4 220u 25V electrolytic
  • C5 470p ceramic
  • D1-D4 1N4001
  • Q1 2N2222A
  • R1 10K
  • R2,R7 1K
  • R3 22K
  • R4,R5 47K
  • R6 220R

All resistors can be 0.25W, 5% tolerance or better. Metal film is preferable to carbon types, as they have lower noise.

You will also need:

  • Connector for power input (and optionally the current sensor)
  • An enclosure (I used an ABS box approx 14cm x 8cm x 4cm)
  • Prototyping board (pad board or strip board)
  • 2 x 12-pin 0.1″ socket for Spark module
  • Nuts, bolts and spacers for mounting

Step 2: The Particle Core Module

The Particle Core Module Internet-connected home energy monitor

You’ll need to be familiar with the basic operation of the Particle Core in order to connect it to a network and load firmware onto it.

The best place to start is Particle’s own guide at You don’t need to have built anything at this point, as the Core can be powered via its USB connector.

Following the tutorial, make sure that:

  • You have set up an account (user name and password) to access the Particle developer site.
  • You have given the Core a name and connected it to a Wi-Fi network (using the Particle app on an iOS or Android device).
  • You’re familiar with the web IDE (, and are able to download simple programs (e.g the ‘Blink an LED’ example) onto the Core.

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