The normal setup would be to have a solar controller with at least 2 inputs for temps, 1 from the solar panel and 1 from the store so I would have expected your system to already have a sensor on the store. Is this what your electrician means by 'thermostat'?
Navitron sell the Sorel range of solar controllers. These come with two or three inputs for temp sensors, 1 for the panel, 1 or 2 for the thermal store. The controller operates by checking the difference in temp between the panel and the store and turns the pump on/off according to the temps (the actual values can be programmed by the owner).
Manual here -
https://sorel.de/en/?ddownload=13460