Solution found - Calculate variable and save in item

A few hours after my posting I found some Information that helped me

=> https://zukunftathome.de/stromzaehler-und-aktuellen-stromverbrauch-in-openhab-darstellen/

In German, but now I have the values that I want.

Hint: This post is for OpenHab 2.x, in OpenHab 3.x the timestamps are different

=> Migrate JodaTime to Java LocalDateTime in OH3 (withTimeAtStartOfDay) - #5 by MrSebi

Thank you