Help for simple calculation * -1

[nubbler]

I need to change a item that was read out over MQTT to a negative value.

Background: The power of a solar inverter was read out with SBFspot and sent over MQTT to openhab. Now I need to sent it to openWB, but openWB needs a negative value, not a positive.

What is the easiest way to transfer thepositive value to a negative value and store in another item?

I tried it blockly but content of the target item is always null:

Idea was to just * -1 the value. But I’m not sure how to create this easily in Blockly…

[scha (Andreas Schmid)]

Try to use the transformation-feature to do this, no rule necessary.
Have a look here, that’s how I got the issue figured out:

Negative value in a chart

I’ve another silly question, also related to the chart-pages in OH3/4: Is there a possibility to use the value of an item in a chart NEGATIVE. I have an item showing my EVU-consumption/upload. Generally it has the correct algebraic sign, means a “-” (negative) when I buy power and no sign (positive) when I upload. For various reasons I do not want to change that at the item. But for some charts it would be more intuitive to understand if the value would be negative instead of positive and vice…

BTW: I´m running also an OpenWB, and I have the openWB directly read via Modbus-tcp from the Converter (in my case a fronius) and do most “the other way round”, because the fronius-binding do not get that much info from the converter than the OpenWB-Fronius-Interface. You should consider this maybe also, depending on your environment.

[nubbler]

Thank you. That helped a lot and now it is working.
I need to take the way over openhab because my inverter is without modbus/LAN-Interface. The only way is to get the values with SBFSpot over Bluetooth.