For homes that use a large amount of hot water and receive substantial sunlight year-round, a solar water heater may be economical. After all, water heating is the second largest energy user in most homes, right after space conditioning. Solar water heaters work as pre-heat systems assisting a conventional water heater. Cold water enters a conventional water heater at 50°F to 70°F. Then energy is added to heat the water to the desired temperature of about 120°F.
With a solar water heater, the cold water is piped to the solar collector first, where it is warmed by the sun. Then it is sent to the water heater tank. If the water from the collectors is hot enough, the conventional water heater does not add more heat. If the solar collector delivers cooler water than is needed, then the conventional water heater turns on. So, the temperature or availability of hot water is not affected.
Households that use large amounts of hot water and can adapt the time when hot water is used to match when it is available will benefit most. Savings will be greatest if laundry, dishes, and bathing are done in the afternoon and early evening after the sun has heated the water stored in the tank.