You can see that the area under the curve for the "Potenza OUT" ( output power) is greater than the area under the "Potenza Resistore" ( power resistor, input power to run the heater) Or input power vs output power. Time shift it toward the right and the whole curve will fit under the output curve.
This video shows the apparatus in detail. It is now on YouTube. I saw this on Ny Teknik already, but it is also handy on YouTube now.
Basically, you can see where and how the delta t's were obtained for the graph above. Delta t, or change in temperature, is one of the data points needed to calculate the power. The other is the water flow, also shown as 600 liters per hr.