On average, one square foot of area on a good quality solar panel is able to raise one pound of water from ground water temperature , about 50F, to 120F in one hour. Thus in an 8-hour solar day, the one square foot of solar panel will raise the temperature of one gallon of water.
This is the least understood feature of solar water heating. Wishful thinking or magical thoughts lead to the belief that the sun is able to heat a lot of water with only the surface of one side of a tank or a few square feet of radiator. The tank-in-a-box was one idea. The radiator in a glass-covered wall was one of the first sold.
Experimentation is not to be discouraged; but it is possible to research solar design. This was well-documented folowing the energy crisis in 1974. There were all sorts of schemes to put one roll of copper or plastic tubing on the roof and save a fortune. An internet search for solar influx and design will bring a wealth of information gleaned from 25 years of research.
They can give a quantum leap in the opinion of how much work is already done in this research.