---
title: Sensor Entity
sidebar_label: Sensor
---

A sensor is a read-only entity that provides some information. Information has a value and optionally, a unit of measurement.

## Properties

> Properties should always only return information from memory and not do I/O (like network requests). Implement `update()` or `async_update()` to fetch data.

| Name | Type | Default | Description
| ---- | ---- | ------- | -----------
| state | string | **Required** | The value of the sensor.
| unit_of_measurement | string | `None` | The unit of measurement that the sensor is expressed in.
| device_class | string | `None` | Type of sensor.

### Available device classes

If specifying a device class, your sensor entity will need to also return the correct unit of measurement.

| Type | Unit | Description
| ---- | ---- | -----------
| battery | % | % of battery that is left.
| humidity | % | % of humidity in the air.
| illuminance | lx/lm | Light level.
| temperature | C/F | Temperature.