I struggled following a great guide to get Home assistant's energy to work as I already have an SDR at 915M and rtl_433 running to capture weather station data without Cloud API delay. The solution there is great and purpose built. I'm just difficult. These are my notes.
Below is my rtl_433.conf:
device :00000857 #Could be different for you(000001) frequency 915M output mqtt://homeassistant.local:1883,user=USERNAME,pass=PASSWORD convert si protocol 149 # ERT Standard Consumption Message (SCM)
Then in homeassistant I had:
  - platform: mqtt
    name: "Power Meter"
    state_topic: rtl_433/HOSTNAME/devices/ERT-SCM/METERID/consumption_data
    last_reset_topic: rtl_433/HOSTNAME/devices/ERT-SCM/METERID/consumption_data
    unit_of_measurement: kWh
    last_reset_value_template: 1970-01-01T00:00:00+00:00
    value_template: >
      {% set myval = value | float %}
      {{ (myval/100) }}
    state_class: total_increasing
    device_class: energy
This required me to upgrade to 2021.9.3 as 2021.8.8 didn't support the state_class: total_increasing
I found the topic via “Configuration” tab, click “Integrations” and click the “Configure” option under the “MQTT” integration. There I used the # wildcard to find the topic rtl_433/#.