calculating the heatflux within paraview is somehow a bit tricky and hard. I would suggest you (if possible) to use the openfoam inhouse programm wallHeatFlux. This will return exactly the heat flux in [W/m^2] or in [W] (do not remember). The problem calculating the flux in paraview is as follow:
we need to calculate the temperature gradient T normal to the surface. As far as I know there is no function available that is calculating this one (we need the distance from face center to cell center and the magnitude of the face normal vector). Nothing else is done in the wallHeatFlux tool within Openfoam. How you could calculate this within simscale environment, I don’t know even with paraview.
I always use the tool wallHeatFlux or build my own field that calculates the heat flux within the whole internal field (but normally it is sufficient to know the heat flux only on the walls).