Blog: Calibrating an ESPHome flashed power plug: https://frenck.dev/calibrating-an-esphome-flashed-power-plug/

calibrate_current.yaml

# Current sensor
current:
  name: current
  unit_of_measurement: A
  accuracy_decimals: 3
  filters:
    # Map from sensor -> measured value
    - calibrate_linear:
        - 0.0 -> 0.013
        - 0.08208 -> 0.071
        - 1.34223 -> 1.066
        - 5.57170 -> 4.408
        - 6.69184 -> 5.259
        - 6.97187 -> 5.540
    # Make everything below 0.01A appear as just 0A.
    # Furthermore it corrects 0.013A for the power usage of the plug.
    - lambda: if (x < (0.01 - 0.013)) return 0; else return (x - 0.013)

calibrate_power.yaml

# Power sensor
power:
  id: power
  name: esphome_esp08_power
  unit_of_measurement: W
  accuracy_decimals: 0
  filters:
    # Map from sensor -> measured value
    - calibrate_linear:
        - 0.0 -> 1.14
        - 62.06167 -> 10.93
        - 1503.27161 -> 247.6
        - 1599.81213 -> 263.7
        - 3923.67700 -> 631.4
        - 7109.50928 -> 1148.0
        - 7237.0857 -> 1193.0
        - 7426.71338 -> 1217.0
    # Make everything below 2W appear as just 0W.
    # Furthermore it corrects 1.14W for the power usage of the plug.
    - lambda: if (x < (2 + 1.14)) return 0; else return (x - 1.14);

calibrate_voltage.yaml

# Voltage sensor
voltage:
  name: esphome_esp08_voltage
  unit_of_measurement: V
  accuracy_decimals: 1
  filters:
    # Map from sensor -> measured value
    - calibrate_linear:
        - 0.0 -> 0.0
        - 602.87506 -> 229.9
        - 609.8 -> 232.8

calibrated_firmware.yaml

---
esphome:
  name: powerplug
  platform: ESP8266
  board: esp8285

# WiFi connection
wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

# Enable logging
logger:

# Enable Home Assistant API
api:

# Enable over-the-air updates
ota:

# Enable Web server
web_server:
  port: 80

sensor:
  # Power sensor
  - platform: hlw8012
    sel_pin:
      number: GPIO12
      inverted: true
    cf_pin: GPIO04
    cf1_pin: GPIO05
    change_mode_every: 3
    update_interval: 3s
    # Current sensor
    current:
      name: current
      unit_of_measurement: A
      accuracy_decimals: 3
      filters:
        # Map from sensor -> measured value
        - calibrate_linear:
            - 0.0 -> 0.013
            - 0.08208 -> 0.071
            - 1.34223 -> 1.066
            - 5.57170 -> 4.408
            - 6.69184 -> 5.259
            - 6.97187 -> 5.540
        # Make everything below 0.01A appear as just 0A.
        # Furthermore it corrects 0.013A for the power usage of the plug.
        - lambda: if (x < (0.01 - 0.013)) return 0; else return (x - 0.013);
    # Voltage sensor
    voltage:
      name: voltage
      unit_of_measurement: V
      accuracy_decimals: 1
      filters:
        # Map from sensor -> measured value
        - calibrate_linear:
            - 0.0 -> 0.0
            - 602.87506 -> 229.9
            - 609.8 -> 232.8
    # Power sensor
    power:
      id: power
      name: power
      unit_of_measurement: W
      accuracy_decimals: 0
      filters:
        # Map from sensor -> measured value
        - calibrate_linear:
            - 0.0 -> 1.14
            - 62.06167 -> 10.93
            - 1503.27161 -> 247.6
            - 1599.81213 -> 263.7
            - 3923.67700 -> 631.4
            - 7109.50928 -> 1148.0
            - 7237.0857 -> 1193.0
            - 7426.71338 -> 1217.0
        # Make everything below 2W appear as just 0W.
        # Furthermore it corrects 1.14W for the power usage of the plug.
        - lambda: if (x < (2 + 1.14)) return 0; else return (x - 1.14);

binary_sensor:
  # Binary sensor for the button press
  - platform: gpio
    name: button
    pin:
      number: GPIO3
      inverted: true
    on_press:
      - switch.toggle: relay

switch:
  # Switch to toggle the relay
  - platform: gpio
    id: relay
    name: switch
    pin: GPIO14
    on_turn_on:
      - light.turn_on: led
    on_turn_off:
      - light.turn_off: led

output:
  # Relay state led
  - platform: esp8266_pwm
    id: state_led
    pin:
      number: GPIO1
      inverted: true

light:
  # Relay state light
  - platform: monochromatic
    output: state_led
    id: led

# Uses the red LED as a status indicator
status_led:
  pin:
    number: GPIO13
    inverted: true

final_firmware.yaml

---
esphome:
  name: powerplug
  platform: ESP8266
  board: esp8285

# WiFi connection
wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

# Enable logging
logger:

# Enable Home Assistant API
api:
  password: !secret esphome_api_password

# Enable over-the-air updates
ota:
  password: !secret esphome_ota_password

# Enable Web server
web_server:
  port: 80

# Sync time with Home Assistant
time:
  - platform: homeassistant
    id: homeassistant_time

# Text sensors with general information
text_sensor:
  - platform: version
    name: version
  - platform: wifi_info
    ip_address:
      name: ip
    ssid:
      name: ssid
    bssid:
      name: bssid

sensor:
  # Uptime sensor
  - platform: uptime
    name: uptime
  # WiFi Signal sensor
  - platform: wifi_signal
    name: wifi_signal
    update_interval: 10s
  # Power sensor
  - platform: hlw8012
    sel_pin:
      number: GPIO12
      inverted: true
    cf_pin: GPIO04
    cf1_pin: GPIO05
    change_mode_every: 3
    update_interval: 3s
    # Current sensor
    current:
      name: current
      unit_of_measurement: A
      accuracy_decimals: 3
      filters:
        # Map from sensor -> measured value
        - calibrate_linear:
            - 0.0 -> 0.013
            - 0.08208 -> 0.071
            - 1.34223 -> 1.066
            - 5.57170 -> 4.408
            - 6.69184 -> 5.259
            - 6.97187 -> 5.540
        # Make everything below 0.01A appear as just 0A.
        # Furthermore it corrects 0.013A for the power usage of the plug.
        - lambda: if (x < (0.01 - 0.013)) return 0; else return (x - 0.013);
    # Voltage sensor
    voltage:
      name: voltage
      unit_of_measurement: V
      accuracy_decimals: 1
      filters:
        # Map from sensor -> measured value
        - calibrate_linear:
            - 0.0 -> 0.0
            - 602.87506 -> 229.9
            - 609.8 -> 232.8
    # Power sensor
    power:
      id: power
      name: power
      unit_of_measurement: W
      accuracy_decimals: 0
      filters:
        # Map from sensor -> measured value
        - calibrate_linear:
            - 0.0 -> 1.14
            - 62.06167 -> 10.93
            - 1503.27161 -> 247.6
            - 1599.81213 -> 263.7
            - 3923.67700 -> 631.4
            - 7109.50928 -> 1148.0
            - 7237.0857 -> 1193.0
            - 7426.71338 -> 1217.0
        # Make everything below 2W appear as just 0W.
        # Furthermore it corrects 1.14W for the power usage of the plug.
        - lambda: if (x < (2 + 1.14)) return 0; else return (x - 1.14);
  # Total daily energy sensor
  - platform: total_daily_energy
    name: daily_energy
    power_id: power
    filters:
      # Multiplication factor from W to kW is 0.001
      - multiply: 0.001
    unit_of_measurement: kWh

binary_sensor:
  # Binary sensor for the button press
  - platform: gpio
    name: button
    pin:
      number: GPIO3
      inverted: true
    on_press:
      - switch.toggle: relay

switch:
  # Switch to restart the plug
  - platform: restart
    name: restart
  # Switch to toggle the relay
  - platform: gpio
    id: relay
    name: switch
    pin: GPIO14
    on_turn_on:
      - light.turn_on: led
    on_turn_off:
      - light.turn_off: led

output:
  # Relay state led
  - platform: esp8266_pwm
    id: state_led
    pin:
      number: GPIO1
      inverted: true

light:
  # Relay state light
  - platform: monochromatic
    output: state_led
    id: led

# Uses the red LED as a status indicator
status_led:
  pin:
    number: GPIO13
    inverted: true

initial_firmware.yaml

---
esphome:
  name: powerplug
  platform: ESP8266
  board: esp8285

# WiFi connection
wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

# Enable logging
logger:

# Enable Home Assistant API
api:

# Enable over-the-air updates
ota:

# Enable Web server
web_server:
  port: 80

sensor:
  # Power sensor
  - platform: hlw8012
    sel_pin:
      number: GPIO12
      inverted: true
    cf_pin: GPIO04
    cf1_pin: GPIO05
    change_mode_every: 3
    update_interval: 3s
    # Current sensor
    current:
      name: current
      unit_of_measurement: A
      accuracy_decimals: 3
    # Voltage sensor
    voltage:
      name: voltage
      unit_of_measurement: V
      accuracy_decimals: 1
    # Power sensor
    power:
      id: power
      name: power
      unit_of_measurement: W
      accuracy_decimals: 0

binary_sensor:
  # Binary sensor for the button press
  - platform: gpio
    name: button
    pin:
      number: GPIO3
      inverted: true
    on_press:
      - switch.toggle: relay

switch:
  # Switch to toggle the relay
  - platform: gpio
    id: relay
    name: switch
    pin: GPIO14
    on_turn_on:
      - light.turn_on: led
    on_turn_off:
      - light.turn_off: led

output:
  # Relay state led
  - platform: esp8266_pwm
    id: state_led
    pin:
      number: GPIO1
      inverted: true

light:
  # Relay state light
  - platform: monochromatic
    output: state_led
    id: led

# Uses the red LED as a status indicator
status_led:
  pin:
    number: GPIO13
    inverted: true

Line 77 and 108 of the combined firmware lambda functions are different -- one adds the offset the other subtracts the offset. Shouldn't they both be addition in the first half of the function?

This calibration looks useful, I have some caveats.
Power measurement is highly dependent on the power factor of the load. Power factor is the cosine of the angle between applied Voltage and load current. I expect that the power measurement of these sockets does not apply a power factor correction. If that is true, power measurements for loads with motors or switch mode power supplies will not be accurate. Calibrating with the intended load will come closer but, even then, accuracy will vary with load.
Current measurements will be distorted for switch mode power supplies, because their current draw is not continuous. Most modern line operated power supplies are switch mode.