I noticed that the temperature jumps all the time up and down (it is day all the time) so my question is if this is a bug or if this is intented because it decreases the power output if the temperture jumps all the time. Another question is how much energy can a heat storage can store?

When I set to always day it seems to only go up, albeit in small amounts at regular game speed. Can you screenshot your setup so I can try to duplicate?

These are the settings that the original author had and I have not modified, I am admittedly less familiar with the deeper workings of the heat system:

Tier 1:

max_temperature = 2000,
specific_heat = "5MJ",
max_transfer = "500MW",
min_temperature_gradient = 0.7,

Tier 2:

max_temperature = 2000,
specific_heat = "10MJ"
max_transfer = "1GW"
min_temperature_gradient = 0.4

Tier 3:

max_temperature = 2000,
specific_heat = "20MJ"
max_transfer = "2GW"
min_temperature_gradient = 0.1

I only placed some thermal solar panels and in the back I placed a liquid salt battery and it seems that the temperature rises slower but then it jumps arround for example from 15.82 to 15.93 and down again to 15.87. But if you want I can make a screenshot of the the setup.

Yea, it would still be helpful, just in case there's a situation that isn't accounted for.

Based on experiments, with only day or vanilla-ish day/night cycle, the batteries are unnecessary. Just get rid of them. You need LOTS of panels and a VERY long night to make the batteries necessary and possible.