To check the functioning of the Solesta solar water heater, i have used a arduino Wemos D1 with 4 Dallas ds18 temp sensors. Using the blink app and the blink cloud interface a temp logger is created within a couples of hours.

Here you see the all the temperatures over a day in action.

The solesta is switching of the pump to the collector if the water in the boiler is hot enough, so this explains the wave patern.