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New sensor properties for long-term statistics

· 2 min read

The sensor entity model has been updated with two new properties, state_class and last_reset. The driver for both the new properties is to enable automatic generation of long-term statistics.

state_class#

Sensor device classes such as DEVICE_CLASS_TEMPERATURE are used to represent wildly different types of data, for example:

  • A regularly updated temperature measurement
  • Historical or statistic data, for example daily average temperature
  • Future data, for example tomorrow's forecast

Differentiating between those sensors which represent a measurement and those which don't is needed in order to automatically make a reasonable selection of sensors to include in long-term statistics.

The state_class property classifies the type of state: The state could be a measurement in present time from a temperature sensor or an energy meter, a _historic value such as the average temperature during the last 24 hours or the amount of energy used last month, or a predicted value such as a weather forecast or the next garbage pickup schedule. If state_class="measurement", the state represents a current value, and not a historical aggregation or a prediction of the future. Otherwise, state_class=None. There is an architecture discussion with some additional background.

Note that measurement in present time above does not imply that the state has to be updated with a certain frequency, or that the sensor is not allowed to do indirect measurements such as integrating power to calculate energy. To put it in another way, if the sensor represents the latest observation or the newest data point in a time series it qualifies as state_class="measurement".

last_reset#

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.