From another thread
Diesel hot water and heat:
DickinsonMarine.com - Hot Water Coils
Solar hot water and heat (look through their library and manuals... LOTS of good info here)
Radiant solar heating systems from Radiantec
They have info sheets for all combinations of using solar water heaters with water tanks, heat exchangers, water heaters and hydronic heating. Both in closed/open or direct/indirect combinations.
Long story short- at the least, the solar HW heater boosts the temp of the water in your tank so you need less energy to bring the water up to temp with a water heater. Same concept with hydronic heating. Even if it doesn't fully heat your system, it takes a lot of the load off your electric load for heating.