Assuming we start out with a tank near mains temperature after everyone has showered in the morning, there is a certain amount of energy required to bring the tank to the maximum of 160 F in the afternoon. If this energy comes from SunEarth panels with an average clear day output of 1,000 BTU/sq. ft. then the quantity of water that each square foot would raise from mains temperature to 160 F could be expressed as.
Vgal=120/(160-Tmains)
For most multi-family applications, however, the ratio is good practice and results in a good tradeoff between cost on the longevity of the system. For more information on how to choose a solar water heater, visit our design resources page.