Heating water with the sun can be done passively or actively. A passive system, which is mainly used in sunny equatorial regions, consists of a simple metal water container that's painted black and placed in a sunny spot on the roof.
During the hot afternoon, the water absorbs the solar heat and retains its heat well into the evening. Gravity is used to distribute it throughout the house for washing and bathing. At the end of the day, the tank is refilled to make it ready for the next day.
With an active system, a circulation pump (which requires electricity) pumps water through the system. The solar collector absorbs the sunlight and transfer the heat to the circulating water. The amount of the temperature increase depends upon the size of the installed solar collector and the speed at which the water flows through it.
Active solar power hot water heaters are common for heating swimming pools, radiant under-floor heating systems, and small room heaters that use a radiator. In some climates, it can provide most of the domestic hot water for a household.
Electricity generated from solar panels is very rarely used to heat water in an electric hot water tank. These systems have circuits and controllers that are too costly when compared with the direct heating of the water in a solar collector.
Simple systems have been used for thousands of years, before electricity was discovered.
Some countries require their citizens to save gas & electricity by heating water with the use of the sun. Countries like Israel, Zimbabwe and other sunny nations are using less imported fuel by requiring roof top water heating systems.
The systems are designed to meet a household's requirement with reasonably priced units. For North Americans, these units may not provide instant hot water but those nations have become accustomed to the lower convenience but higher energy savings.
To help save energy, these systems may be used together with an existing hot water system to preheat the incoming water. Less energy will be used to increase the warmed water to higher temperatures.
A solar power hot water heater may be more prevalent in a southern location but, if used with existing heaters, may help reduce our carbon footprint with minimal changes to our lifestyles.