There are different types of solarwaterheaters. The differences are mostly dependent on the climate you are in. For most people in most places the collectors are typically 4' x 8' x 4" aluminum boxes that have a glass front and are mounted on a roof or on a rack. This style has proven itself over a long time to be most efficient and cost effective. You can do a web search and see lots of examples. I have a whole chapter of my book dedicated to this subject.