you have to schedule around it. The water won't be warm until the afternoon and cools off as soon as the sun is off it.
Is the tank not insulated then?
My first thought was that the tank couldn't be insulated. If it was, the sun wouldn't be able to heat it up to 80. That was the only concern I would have had with the system. If you have it feeding into a propane heater, I can't see any drawback (as long as you don't mind it being on your roof I guess).
The sun does not heat the water tank directly, in that system. From the photos, it looks like an array of glass solar collectors, which are vacuum insulated, with the upper ends of the collectors transferring the heat into the water inside the tank.
Inside the glass tubes are small heat transfer tubes, which do the actual solar absorption and transfer the heat upwards into the water medium. There is no water inside the actual solar tubes.
The water tank at the top could very well be insulated.
A variation of this system has the collector tank located elsewhere, perhaps adjacent to the 'normal' water heater inside the building. A small circulation pump continually moves the water through the solar array and back into the storage tank, which would be insulated. This allows the heat storage tank to be as large as you need it to be to accumulate hot water during the day.
Some systems circulate propylene glycol 'coolant' through the solar manifold, then use a compact heat exchanger to transfer the heat from the coolant into the domestic hot water system, and then into the solar storage tank. More pumps and equipment involved, but the domestic water is then not exposed to the solar collector and all the piping running up to the roof and back down.
There is no upper limit to how hot the water can get. My neighbour has such a system, and when the water circulation pump failed, the water inside the collector manifold boiled, venting through a pressure release valve which is installed for just that situation.