Home Energy Meter

To satisfy one's curiosity about energy usage in the home (and to get some practice with small embedded system design), here's a simple and inexpensive meter. It measures current in two phases of a US style split-phase 120/240 volt home electrical system, with an option to measure current in a third circuit. It could also measure current in two or three branches of any old AC circuit.

This page is currently under construction and incomplete. Hopefully the information here will be somewhat helpful anyway.

The system as implemented uses an ESP32 microcontroller on a typical inexpensive breakout board; a simple custom printed circuit board; two or three current transformers; and a Raspberry Pi in the back of a closet.

• The ESP32 microcontroller runs a MicroPython program which measures AC current usage by taking bursts of data quickly and calculating the root-mean-square value with respect to an average reading. The ADC in an ESP32 isn't nearly as accurate as those in most microcontrollers, but using the new machine.ADC.read_uv() function, a reading may be made which is sufficiently good to help one optimize the energy usage in a home.

  The microcontroller uses its WiFi module to hop onto your home network.

• The custom circuit board is hosted on OSHPark and a link will be made available when some problems with OSHPark's project sharing have been solved. The project is named "Home Energy Board." The board provides:

  • Current to voltage converters in the form of precision(ish) resistors; these are large(ish) through-hole resistors

  • Simple passive level shifters which shift the zero-current reading to about half the ADC's maximum reading

  • Passive RC filtering to get some really bad noise out of the signals and prevent aliasing, while letting ugly but real harmonics generated by modern circuitry be measured

  • Labeled screw terminals and mounting holes for convenient connections

Rather than the typical BOM and part numbers, I've just put values on the board because the SMD parts are all size 0805 and fancy stuff like voltage and power ratings aren't too critical for these parts. The through-hole current to voltage resistors are 10Ω 1% which happened to be lying around. Different values could be used with minor changes to the software.

• The current transformers which are used to measure the usage are 100 amp to 50 mA split-core transformers and were ordered from that big web store at: https://www.amazon.com/gp/product/B09YRBVNZW/

  I chose to cut off the audio plugs and use screw terminals to connect the transformers to the circuit board. Audio plugs are unreliable IMHO.

  Transformers which do not have built-in current to voltage resistors should be used with this system, so avoid the ones which specify a voltage output...although it might be OK to use those and omit the current-to-voltage resistors on the circuit board; I haven't tried that.

• The Raspberry Pi runs two programs of interest:

  • The Mosquitto MQTT broker runs on the Pi to gather and distribute readings from the energy meter and other items.

  • Node-RED is used to produce the plots and serve them on a web page.

• The MicroPython program is hosted here. It began as a test of Firebase™ for home IoT use, but it was found that connecting to an existing Raspberry Pi in a closet was a more efficient solution.

  The networking functions web_up() and web_down() are declared in boot.py which is a little unusual but it works. The user must create a file mycerts.py which defines two strings, ssid containing the name of the local WiFi network and password containing the password. Other constants such as the pin numbers, MQTT topics (yes, my home is referred to as a Travisty[sic]), and the address of the MQTT server are in main.py.

Dependencies

• The software needs the micropython-mqtt library to work: https://github.com/peterhinch/micropython-mqtt.git

Schematic

Node-RED flow

The flow diagram code will be added to the software in this repository.

Screenshot

This wouldn't be much of a README without a screenshot, would it?

The image shows an EV being charged as well as smaller loads from cooking breakfast and lunch.

Warning

Installing the current transformers usually requires opening circuit breaker panels to access the main wires. Please do not attempt this unless you have the proper training and experience with high voltage electrical circuitry. Being shocked hurts like heck at best (don't ask how I know) and can be a real inconvenience to your next of kin at worst. Burning down one's residence is another possible unpleasant consequence. The author has no control over how you use the information in this repository and cannot take any responsibility for the consequences. All software and other information in this repository is supplied as-is and without any warranty of any kind; I cannot and do not guarantee the accuracy, safety, or merchantability of any of this content, nor has the author evaluated the accuracy, safety, or merchantability of any of the products used in the project. By using any of this information, you assume any and all risks and all responsibility caused by such use, and you agree to indemnify and hold harmless the author(s) of all information in this repository.