Hello I need to design a storage for DHW where the source side are going to be the Solar Thermal collectors with a fuel for a back up option

in the photos below for the Heater1 the capacity is 0 because this is the solar source, for Heater2 is the fuel capacity

The results were showing this, fuel spent for DHW should have been lower than 25000kwh because im using solar collectors, but in this simulation went up to 80000kwh (it supposed to decrease the amount of fuel spent for DHW, not to increase) And also i am using 6 solar thermal objects collectors and there is a minus (why is there a minus)

Should i not use this Storage Heater? Thanks