Properly sized storage tanks are essential to a smooth running solar system. An undersized tank will often lead to over-temperature conditions in the tank that can cut out the pump and bring the array into stagnation. On the other hand, an oversized tank yields little benefit in performance at a significant financial cost.
The purpose of the solar tank is to store the thermal energy from the array that is gathered during the day until it is used in the typical evening and early morning draws. Typical tank temperatures start out near mains water in the morning and rise to around 140-160 in the late afternoon. Afternoon temperatures significantly above this level lead to accelerated corrosion (corrosion rates double with every 20 F rise in temperature!).
The Simple Math of Sizing Solar Tanks
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.
For somewhere like Florida with a mains temperature of 77 F, each square foot of collector could raise 1.45 gallons (120/[160-77]) of water up to 150. We would therefore want 1.45 gallons of storage for each square foot of collector area in that region. At the opposite end of the spectrum we have places like Boston where each square foot of collector can raise only 1.05 gallons of water to 150 F, and we would therefore only need 1.05 gallons storage for each square foot of collector area.
This general rule applies in most circumstances and is a conservative estimate because it assumes that there is no hot water draw during the day. In certain situations there may be considerable draw during the day that allows for the above ratio to be lowered. For most multi-family applications however the ratio is good practice and results in a good tradeoff between cost on longevity of the system.