project: smart electricity meter with historic energy use

fit a sensor to the electricity meter for a minute by minute readout

The UK electricity meter shows electricity use in kWatt hours, and a human sometimes pops by to read the meter and send you a bill. The meter has a flashing red LED, every flash of red corresponds to 1 Watt, after a thousand flashes you’ve used 1kW or about £0.40 of disposable income. A light sensor can respond to those flashes, so we can measure how much electricity we are using.

credit is due to the helpful folk at OpenEnergyMonitor

what this project achieves:

• measures electricity in any unit – Watts, kW or money
• display readings as a gauge or history graph on Home Assistant.
• uses Home Assistant’s ability to email readings; phone notifications or flash a smart bulb (eg when energy use is unusual)
• uses a photodiode with built-in amplifier TSL257-LF (~ £3.50 ebay) and an ESP32 wifi chip (~ £5).
• monitor a gas meter simultaneously so you get two sensors and double the value
• create an entity to use in the Home Assistant Energy Dashboard

how to connect up a light sensor and ESP32

The TSL257 photodiode has three pins, GND, 5v and signal. Connect a 10K ohm load resistor between signal and ground. (Luckily my RPi kit had a pack of 10K ohm resistors). CARE – this sensor is PLASTIC: I soldered the connections, using a large metal clip as a heatsink. Finally the sensor wires were glue-gunned, covered in heat-shrink and mounted in a 3D printed cover (below).

Thus far you’ve three wires from the sensor joining an ESP32. You now need a lead from a USB charger to bring 5v power and GND to the ESP32. I used an ESP32-CAM to provide me with an almost free extra camera. Here’s the box that I hope soon to make more attractive:

program (or install code) on the ESP32

For this step you need an already working ESPHome installation in Home Assistant as shown in HERE. It’s a project in four steps starting with installing Home Assistant on a Raspberry Pi – see HERE.

In Home Assistant go to the Esphome plugin and add a ‘new device’. A wizard walks you through. Eventually you see an editor where you can add the code below. Change the items in bold to suit your board, your wifi, your network and the GPIO pin you connected to. For more elaborate code see the Home Assistant Glow project.

substitutions:
   devicename: meter

esphome:
   name: ${devicename}
   platform: ESP32
   board: esp32dev
 
logger:
 
# Enable Home Assistant API
api:
   password: "YOUR HA password"
ota:
   password: "Optional password"
 
wifi:
 networks:
 ssid: "YOUR_SSID"
 password: "YOUR WIFI PASSWORD"
 manual_ip:
   # Set this to the IP you want on the ESP
   static_ip: 192.168.1.17
   # Set this to the IP address of the router. Often ends with .1
   gateway: 192.168.1.1
   # The subnet of the network. 255.255.255.0 works for most home networks
   subnet: 255.255.255.0
   
# The LED output and light sections are optional
 output:
   platform: gpio
   pin: GPIO4
   id: gpio_4 
 
light:
  platform: binary
  output: gpio_4
  name: ${devicename} light 
 
# THIS IS THE IMPORTANT PART - remember you connected the sensor to pin 12
sensor:
  platform: pulse_counter 
  pin: GPIO12 
  unit_of_measurement: 'Watts' 
  update_interval: 20s 
  accuracy_decimals: 0 
  name: 'power meter' 
  filters: 
    - multiply: 60 

# If you have a camera add the esp32_camera code section here (see here)

• Note the last bit of code above. The filter will convert pulses per minute to Watts. If you prefer a reading in kWatts, change ’60’ to ‘0.06’ and change the unit to’ kW’
• While you’re in the ESPhome section of Home Assistant look for options to VALIDATE your code and then to INSTALL (or COMPILE) it. The compile operation takes a few minutes as ESPhome assembles code libraries and creates a ‘firmware.bin’ file. Download this bin file. Flash the bin file using the ESPHome-flasher tool (see here).
• When the installing is done, power-up the ESP32. Home Assistant may ‘discover’ your new meter when it restarts. Otherwise go to Setting > Devices > Integrations > + > ESPHome. You might have to enter the IP address shown in the code above. You’ll then see the ‘meter’ entity that has been added to Home Assistant.
• Finally, go to the Home Assistant overview – the frontend view that displays your stuff – and choose Edit Dashboard (top corner). Add a ‘new card’ – use a history graph and a gauge to display your electricity power as follows.

display your meter in Home Assistant

 # THIS IS THE CODE FOR THE GAUGE  
entity: sensor.power_meter
 min: 0
 name: electricity use
 severity:
   green: 1000
   red: 3000
   yellow: 2000
 theme: default
 type: gauge
 max: 5000 

 # THIS IS THE CODE FOR THE SMOOTH LINE GRAPH 
type: sensor
 entity: sensor.power_meter
 graph: line
 hours_to_show: 24
 detail: 2

# THIS IS THE CODE FOR THE DETAILED HISTORY GRAPH 
type: history-graph
 entities:
 entity: sensor.power_meter
 hours_to_show: 24
 refresh_interval: 0 

now it’s working set up a notification

I find it’s well worth being able to see a time graph of electricity use: for example, a few years ago the time graph on my electricity monitor helped me discover unusually high usage – I tracked it down to an outbuilding where an immersion heater had been left switched on for two years. So I may to create a routine (or ‘automation’) to alert of unusual electricity use. I’m still thinking about which alert might be useful. Here’s one automation project.

update: Home Assistant Energy Dashboard kW hours

A newer feature of Home Assistant is an energy dashboard to bring together home energy use as well as solar panel energy production. As of 2022 the dashboard needs a measurement entity in kW hours (a unit of energy) whereas above my ‘smart meter’ produces an in-the-moment measure of kW (a unit of the ‘power’ I’m using). To convert kW to kWh we need to calculate the power use over time – in other words ‘do an integration over time’. Fortunately ESPhome has a ready-made calculation feature called total_daily_energy. See below for the code. It replaces the sensor section above and adds the energy calculator. The time component is needed for the time based calculation.

Compile and install the code on your ESP32. You’ll now have an entity, sensor.meter_energy_today to add to the Home Assistant Energy Dashboard.

# THIS IS THE IMPORTANT PART - remember you connected the sensor to pin 12
sensor:
  - platform: pulse_counter
    pin: GPIO12
    unit_of_measurement: 'W'
    device_class: "power"
    state_class: "measurement"
    update_interval: 20s
    accuracy_decimals: 0
    name: 'power_meter'
    id: myelectricity
    filters:
    - multiply: 60
    - sliding_window_moving_average:
        window_size: 5
        send_every: 5
      
  - platform: total_daily_energy
    name: meter_energy_today
    power_id: myelectricity
    device_class: "energy"
    state_class: "total_increasing"
    unit_of_measurement: kWh
    accuracy_decimals: 1
    filters:
        # Multiplication factor from W to kW is 0.001
      - multiply: 0.001

time:
  - platform: homeassistant
    id: homeassistant_time

alternative approach: how to turn kW into kWh using an integration

Instead of (or as well as) using the ESPhome total_daily_energy integration feature to calculate energy (kWh) from power (K) you can get Home Assistant to calculate. Add the code below to configuration.yaml. After a Home Assistant restart you’ll now have an entity, sensor.electric_spent to add to the Home Assistant Energy Dashboard. It should be numerically the same as meter_energy_today.

# PASTE THE FOLLOWING CODE INTO configuration.yaml with the sensor section

sensor:

  - platform: integration
    source: sensor.power_meter
    name: electric_spent
    unit_prefix: k
    round: 1

alternative approach: how to turn kW into kWh using a utility_meter

TBA: as of 2022, my energy dashboard is still in development because I want to know the balance of my individual peak and off-peak energy use. The following code adds some entities to Home Assistant including

• sensor.electric_daily_day
• sensor.electric_daily_night

# PASTE THE FOLLOWING CODE INTO configuration.yaml
utility_meter:

  electric_daily:
    source: sensor.power_meter
    unique_id: electric_daily
    cycle: daily
    tariffs:
      - day
      - night