---
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

:::tip
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.
| device_class | string | `None` | Type of sensor.
| last_reset | `datetime.datetime` | `None` | The time when an accumulating sensor such as an electricity usage meter, gas meter, water meter etc. was initialized. If the time of initialization is unknown and the meter will never reset, set to UNIX epoch 0: `homeassistant.util.dt.utc_from_timestamp(0)`. Note that the `datetime.datetime` returned by the `last_reset` property will be converted to an ISO 8601-formatted string when the entity's state attributes are updated. When changing `last_reset`, the `state` must be a valid number.
| state_class | string | `None` | Type of state.
| unit_of_measurement | string | `None` | The unit of measurement that the sensor is expressed in.

### 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.
| carbon_dioxide | ppm | parts per million of carbon dioxide concentration
| carbon_monoxide | ppm | parts per million of carbon monoxide concentration
| humidity | % | % of humidity in the air.
| illuminance | lx/lm | Light level.
| signal_strength | dB/dBm | Signal strength.
| temperature | °C/°F | Temperature.
| timestamp | ISO8601 | Timestamp.
| power | W,kW | Power.
| pressure | hPa,mbar | Pressure.
| current | A | Current.
| energy | Wh,kWh | Energy.
| power_factor | % | Power Factor.
| voltage | V | Voltage.
| monetary | ISO 4217 | Monetary value with a currency

### Available state classes

| Type | Description
| ---- | -----------
| measurement | The state represents _a measurement in present time_, not a historical aggregation such as statistics or a prediction of the future. Examples of what should be classified `measurement` are: current temperature, accumulated energy consumption, accumulated cost. Exampled of what should not be classified as `measurement`: Forecasted temperature for tomorrow, yesterday's energy consumption or anything else that doesn't include the _current_ measurement.

## Long-term Statistics

Home Assistant has support for storing sensors as long-term statistics if the entity has the right properties. A requirement to opt-in for statistics is that the sensor has `state_class` set to `measurement`. That means that the current value represents the current value.

### Metered entities

Metered entities have a value that keeps increasing until reset, like energy consumption or production. To have Home Assistant track this entity, you need to include a [`last_reset` property](#properties).

Home Assistant will track the growth over the statistics period for metered entities.

### Value entities

Home Assistant tracks the min, max and mean value during the statistics period.

All sensors with a unit of measurement of `%` are automatically tracked. Other entities opt-in based on their `device_class`.