openhab-core: Group SUM aggregation does not inherit the member's UoM

A Group containing members with a UoM setting that differs from the ‘default’ will not use the members’ UoM setting in the SUM aggregation. This probably also applies to the AVG aggregation since it also generates a new value (as opposed to returning a matching Item’s state when aggregating with MIN or MAX).

An example is provided below:

The Buienradar binding exposes rain intensity forecasts as Number:Speed expressed in mm/h. This is also ‘inherited’ by the Items linked to these binding channels: the relating Items are of subtype Number:Speed and the state is expressed in mm/h.

When I create a Group and assign all rain intensity Items from the Buienradar binding, and when I set the SUM aggregation, the Group uses a different UoM than the individual members, even if I set the member’s subtype to ‘Number:Speed`. The Group reports a state in km/h (1 million bigger unit than the individual members’ unit)

Where an Item can be configured to have an explicit UoM for the quantity represented in the state, this is apparently not possible with a Group.

For Main UI widgets you can of course use the displayState property which behaves similarly to what the label="..." construct allows in sitemaps.

See also the following discussion: https://community.openhab.org/t/oh3-group-item-number-speed-defaults-to-km-h-how-to-set-item-uom-e-g-to-mm-h/125087