solar h/w rinnai /gas

thinking of getting a rinnai sunmaster hot water unit is solar panels on roof and storage tank on ground with a gas booster has a pump that crculates the water from the roof units down to the storage tank but cant see anything about how often the tank pumps are running ie dose the tank heat up (pump moving water down to heat storage tank ) from the solar panel ie to 50deg and then the pump stops or dose the pump keep going all the time
so when i draw off water the tank temp drops and the pump starts pumping heating water again from roof panels ?
i think this is what it dose but not sure as if pump is running all the time then there would be no electric savings as cost of pump would be as much as running h/water unit wouldnt it ??
hope i have explained this ok but if anyone has info on these units would be interested to hear from them