Tank Quick Sizing
A properly sized storage tank is extremely important to a properly functioning and cost-effective solar thermal system. There are a couple of important factors that make the sizing of the storage tank important:
- If the storage tank is undersized, it can overheat, turn off the pump and the solar collectors can stagnate
- If the storage tank is too big, you get little (if any) performance benefit at a high cost
What Is The Storage Tank For?
The storage tank is meant to store up the thermal energy that was generated by the solar collectors during the day for use in the evening and following morning. Typically, the tank temperature will start out around the temperature from the mains water supply in the morning and rise to 140-160F late in afternoon (however, if the temperatures rise too far above this, corrosion can become a problem – corrosion doubles every 20F in temperature rise)
Sizing Storage Tanks: The Math
After everyone has showered in the morning, your storage tank temperature will likely be down to right around the mains water temperature, and we will need a certain amount of energy to bring the temperature back up to 140-160F.
You can use the following formula to calculate the size of your storage tank:
V = 120/(X – Y)
- V is the storage tank volume per ft2 of solar collector
- X is the setpoint temperature of your system
- Y is the mains water temperature at your location
This formula is a pretty safe rule of thumb that will serve you well in most cases (and does a good job balancing cost and longevity of the system). If you have a considerable draw during the day, then a lower volume can be used.
Our engineers have put together the following list of recommended sizing ratios for storage tanks with SunMaxx collectors:
- 1 gallon per ft2 (for high-temp loads in northern climates with flat plates)
- 1.5 gallons per ft2 (for medium-temp loads and typical loads with flat plates)
- 2 gallons per ft2 (high-temp loads with evacuated tubes and low-temp loads with flat plates)
- 2.5 gallons per ft2 (for medium loads for evacuated tubes)
- 3 gallons per ft2 (for low-temp loads)
- 4+ gallons per ft2 (sometimes this ratio makes sense, but must be carefully analyzed for proper payback calculation)