Here's a simple experiment. I hooked a 4'x10' solar hot water panel up to just a small (3 watt) pump and a 60 foot coil of 3/8th" copper pipe set down in a tub of water.
I wanted to see how effective the solar panel would be at transferring heat through that copper coil into the water in the tub.
Since it was below freezing the night before, the tub of water started off with a half inch of ice on it. Three hours later, the water temperature was up to 140 degrees!
This tub holds between 10 and 15 gallons. When I finish experimenting. I plan on using an insulated 50 gallon tank in my house with TWO coils of copper pipe in it - one for anti-freeze circulating through the solar panel, and another one for fresh water from the well to warm up before going to the traditional water heater.
I think I will also use 3/4 inch diameter copper pipe for the domestic water loop, as it will hold more volume, the main water line is 3/4 inch, and it will give less resistance than smaller diameter pipe.