Home Assistant can integrate the energy usage of individual devices into Home Assistant. That way you can see the impact of individual devices on your total enery consumption.
Smart plugs sit between the device and the outlet and measure the energy flowing through the device.
Depending on what protocols you use at home, you can use Zigbee, Z-Wave or Wi-Fi based plugs.
Smart relays sit behind your “normal” switches and make them smart. It allows you to control the devices via Home Assistant and via the connected buttons/switches.
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