@Severum,
Stupid question maybe, but why didn't you use black gardenhose, or polyethelene pipe for heating? This heats up much better than your iron tubes(as black plastic absorbs the heat better) propably cheaper and you would get a lot more heat? Due to the larger surface area you will create?
I lived a couple of years in Namibia in the middle of nowhere without power and we coiled about 100 meters of blabk 2" polyethelene pipe in a spiral which lay in the sun on a flat surface. The water was always that scalding hot that if you showered under it you need to add cold water. In the winter not that hot but when the sun shines still warm enough to shower.