Report temperature difference

Does anyone know a neat (or advanced) way to report difference between two temperature devices?
I have a FGBS-001 to control the solar panels in my swimming pool heating system.

I have 4x DS18B20 temperature sensors connected to the FGBS-001.
One is by the solar panel telling the pump to start (via PLEG) when the temperature is at 30 degrees.
Then there is one on inlet and one on outlet water, measuring the difference telling the pump not to stop until the temperature difference is less than 0.4 degrees (or if the inlet drops below pool temperature).
Then I have one in the pool for good measure of stirred water, which is the actual temperature of the pool.

I have everything monitored in dataMine2 (I attach a screenshot for the fun of it).

I would like to know the actual effect in watt that the solar panels add to it. It’s an easy equation to put into the dataMine2-multiplier if I can get to report the difference between the inlet and outlet temperature (and estimate the flow throught the pump which is about 6-10 cubic meters per hour).