The unit “watt” gets lost.
And “magically” the unit of “e_Wallbox_CurrentP_Sum” changes from Watts to kW, because kW is set as Diyplay Unit for the item.

Unfortunately I don’t have power channels that when created with number:power will result into a number with the unit, so I went forward and created an example with temperatures like so

[org.openhab.rule.temperatureSumRule ] - 12.78 °C12.59 °C
[org.openhab.rule.temperatureSumRule ] - 25.369999999999997
[org.openhab.rule.temperatureSumRule ] - 25.369999999999997 °C

which, to me, looks good in terms of temperatures.

The unit “watt” gets lost.

This is because the way javascript handles values with postfixes when doing math like adding numbers - it just strips them away when adding them (which honestly is kind of neat as you don’ t have to strip the unit first). Of course JavaScript has no clue about units at all.

If It jumps into the else Path, everything works as assumed:

In this case not addition is done in Javascript but the string containing the unit is just passed forwared

As you know W(att) and KW(att) are a tricky thing because strictly spoken >K< is not a unit but a magnitude (* 1000) and I doubt that this is taken into account when displaying the item.

As far as I know the display unit is just a label that is put behind the value.

In General:

When doing Math with units the unit prefixes get stripped of and the values are pure numbers

Unit Prefixes are powers of 10th which are not taken into account in formulas.