Actually, on average only 1/3 of the circumference (1") is available for capturing the sun's energy. This is because you have to take the angle of the sun's incidence on to the surface of the pipe into account and when you do that, you find that the capture area is proportional to the pipe diameter and not it's circumference. Basically, the energy captured by a solar panel is proportional to the area of the shadow it casts.I make it 78 sqft. (3.142 x 1" for the circumference in inches = 3.142"; divide by 12 to give circumference in ft; multiply by 300' to get the surface area of the tubing in sq ft). Now, not all of that surface area may be available for solar heating (the underside).