Electricity monitoring application note

This page accompanies the main openenergymonitor guide. This page details application specific notes for electricity monitoring

As detailed in the main guide an openenergymonitor system comprises of wireless sensor nodes that send data at periodic intervals to a web-connected base-station. The wireless sensor node that is used for the electricity monitoring is the multi purpose board called the emontx.

There are two main configurations for electricity monitoring:

Current only (Firmware: emonTx_CT123)

Measuring current only is comparable to many if not most of the home electricity monitors that are available. The only sensor needed is a sensor that clips around the mains wire coming into the house. This sensor is called a current transformer or CT sensor. For an explanation for how they work see CT sensors - Introduction.

Power is calculated by multiplying the current measurement (RMS current) with a fixed voltage value (i.e 230V). This particular type of power measurement is called Apparent Power and is actually different from the power that utitilty companies bill for which is called Real Power. AC Power is a complex subject, to learn more start with the An introduction to AC Power.

Voltage and current (Firmware: emonTx_CT123_Voltage)

By measuring mains voltage as well as current its possible to calculate Real Power in addition to other quantities (Apparent Power, RMS Voltage, RMS Current). Measuring voltage as well as current is better both in terms of the measurement being what is billed for and due to hardware intricacies the measurement is more accurate at lower power's.

To measure mains voltage safely we use a standard AC to AC power adapter that steps down and isolates the AC Voltage waveform. See building blocks section: Measuring AC Voltage with an AC to AC power adapter

The decision whether to monitor voltage or not often comes down to if there is a AC plug socket in close proximity to the electricity meter cabinet.

At this point it may also be useful to review the CT and AC power adapter installation and calibration theory page.

Battery operation

An other decision to make is whether to power the emontx from 2x AA batteries or whether to power it via its phone charger style USB mini port. This again probably depends on whether there is a AC plug socket in close proximity. If you have an emonTx V2 and you're measuring voltage you will now need two spare AC Plug sockets as the ACAC Adapter provides only and AC waveform for measurement and the USB mini port needs 5V DC.


Mains operation

If you have an emonTx V3, you can also power it using the AC-AC adapter, so you will need only one mains plug socket.


The display

You may like to add an energy display to your setup and there is electricity monitor specific firmware that can be used for this. Once you have installed the Arduino IDE, libraries and firmware following the Setting up the arduino environment guide. The Home Electricity monitor firmware for the display can be selected by going to:

File > Sketchbook > OpenEnergyMonitor > EmonGLCD > EmonGLCD_HEM

This video gives a demo of what the display looks like running the home energy monitor code:

The display code is configured by default to display only one power value power1 (channel one on the emontx).