Helper using "sum" funktion not working in energy dashboard

I’ve been using home assistant for just a week and wanted to combine two sensors (two chargers that charge my solar storage 12V battery) to display them in the energy dashboard. However, the Settings - Devices & Services - Helpers entity (I configured it as “min_max” - “Sum” doesn’t seem to have the correct device class and last_reset is missing. The two values that I add up are both daily reset values which are displayed correctly in the energy dashboard:
Name: Solarspeicher Gesamt
Entity: Ladegerät 1
Entity: Ladegerät 2
Statistic characteristic: Sum
Here is the error message that comes up when i try to use the value in the energy dashboard:

I’m sorry if this topic or a similar one already exists, but this is the first entry I make in a forum.