We are a Velux Solar Hot Water dealer and installer. On one of the installations that we completed this year we accidentally heated the water with the sun for 4 months!  The intent was to heat the water with the sun with an electrical backup, however this was not the case for that period of time.  There was a disconnect attached to the water heater that was not working, however we did not realize this until we got several days of overcast and the sun could not heat up the water. The whole time they did not have to pay a dime to heat up their water.  Here is post from my homeowner about their experience- Lake Claire LEED

On average- in Atlanta you will save around 85% on water heating using a Velux system because the sun can preheat the water so the secondary heat source rarely has to kick on!  According the DOE the average payback is between 3-7 years.   Solar thermal is 5-10 times as efficient as tankless water heater in most cases and the payback is faster as a result.

To see how Solar Thermal works visit Fresh World.