Help for simple calculation * -1

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…

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:

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.

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.