Whether positioning solar collectors for passive or active solar water-heating systems, the collectors should be angled at no more than 45 degrees east or west of due south. If the angle away from due south is kept to less than 30 degrees, the collector will still capture about 90 percent of the maximum solar energy available. If your system is only going to be used during the summer, it should be tilted at an angle equal to the latitude of your site minus 15 degrees. For example, Jefferson City, Missouri, lies at 38.5 degrees above the equator. To optimize summertime collection of solar heat, a collector should be tilted at 23.5 degrees above horizontal to catch the most energy while the sun is at its high summer angle in the sky.
If your solar water heater is for year-round use, the collector should be tilted at latitude plus 15 degrees. This allows the capture of maximum sunlight during the cold winter months when the sun is low in the sky. If you are going to mount a solar water-heating collector on a pitched roof on your house, the roof angle is usually close enough to an optimal angle to simply mount the collector flush with the surface of the roof. Sometimes additional collector area is needed to capture sufficient solar energy when mounting collectors parallel to the existing roof.