As its name implies, a solar collector collects the sun's heat and transfers it to the transport "medium," usually water or an anti-freeze solution. Collectors come in a variety of sizes and shapes. Some are made of black-extruded plastic material, while others consist of an enclosed metal box with clear glazing on top and a black absorber plate inside that has tubes attached to it. The tubes flare out in an array across the absorber plate and converge at the top and bottom. Fluid enters through a pipe at the bottom of the collector, flows into the collector piping where it is quickly heated by the absorber, and leaves through another pipe at the top of the collector. Circulating fluid can reach 120°F to 160°F. Most collectors heat one gallon of water a day for every square foot of glazing surface.
If one person uses from 10 to 20 gallons per day - a typical amount in a home with a clothes washer and dishwasher - then a typical household of four people requires 40 to 80 gallons per day. Two 4 foot by 10 foot panels could satisfy most of their annual hot water needs. Each panel creates 40 sq.ft. of collector area, and since there are two, that makes 80 sq.ft. of area. If each square foot can produce one gallon, that's 80 gallons per day.