See https://github.com/brianhanifin/esphome-config for current configuration + more.

sonoff_s31_01

# Sonoff S31 Power Monitoring Plug
#
substitutions:
  devicename: sonoff_s31_01
  upper_devicename: Sonoff S31 01

esphome:
  name: ${devicename}
  platform: ESP8266
  board: esp01_1m
  
  # Turn on the LED on boot
  on_boot:
    priority: -10
    then:
      - light.turn_off: ${devicename}_green_led

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_pass

# Disable logging over USB
logger:
  baud_rate: 0

# Configuration for power monitor
uart:
  rx_pin: RX
  baud_rate: 4800

# Enable Home Assistant API
api:

ota:
  safe_mode: True


binary_sensor:
  - platform: status
    name: ${upper_devicename} Status
    id: ${devicename}_status
    
  # - platform: gpio
  #   pin:
  #     number: GPIO0
  #     mode: INPUT_PULLUP
  #     inverted: True
  #   name: ${upper_devicename} Button
  #   id: ${devicename}_button
  #   on_press:
  #     - switch.toggle: relay
  #   filters:
  #     - delayed_on: 10ms

light:
  # Expose the "light" to Home Assistant
  - platform: monochromatic
    name: ${upper_devicename} Green LED
    id: ${devicename}_green_led
    internal: true # hide from Home Assistant
    output: green_led

output:
  # Register the green LED as a dimmable output
  - platform: esp8266_pwm
    id: green_led
    pin:
      number: GPIO13
      inverted: True
    
sensor:
  - platform: wifi_signal
    name: ${devicename}_wifi_signal
    update_interval: 30s

  - platform: uptime
    name: ${devicename}_uptime
    
  - platform: cse7766
    current:
      name: ${upper_devicename} Current
    voltage:
      name: ${upper_devicename} Voltage
    power:
      name: ${upper_devicename} Power

switch:
  - platform: gpio
    name: ${upper_devicename}
    id: relay
    internal: true # hide from Home Assistant
    pin: GPIO12
    restore_mode: ALWAYS_ON

text_sensor:
  - platform: version
    name: ${devicename}_esphome_version

I use this plug as a power monitor ONLY. I have chosen to leave the code as is, but I am confident that making the two changes I mention below will give you the standard full functionality.

Line #93 restore_mode: ALWAYS_ON ensures the relay is turned on when started (remark this line out if this is not desired). And the disabled code starting at line #43 disables the physical button (restore these lines if you want the physical button to work).

Bonus: if you want the green LED to turn on at startup, change line #16 to - light.turn_on: ${devicename}_green_led.

You may wish to visit my new https://github.com/brianhanifin/esphome-config for more samples.