Sizing The Solar Thermal Array

Sizing The
Solar Thermal Array

Regardless of the system design you will choose, you must first determine the hot water load you will need to cover with your installation. Once you have calculated the BTU/day heating needs, you can determine how many solar collectors you will need to have to meet your needs.

The table below shows the average daily hot water loads (the figures are based on ASHRAE guidelines as well as SunMaxx installer/dealer experience).

Typical Hot Water Load Profiles

ApplicationAverage Daily Draw
20 units or less20 GPD/unit
60 units14 GPD/unit
100 units or more10 GPD/unit
1 Bedroom Unit20 GPD/unit
2 Bedroom Unit25 GPD/unit
3 Bedroom Unit45 GPD/unit
4 Bedroom Unit55 GPD/unit
Schools & Colleges
Dormitories13 GPD/unit
Elementary Schools0.6 GPD/unit
Junior & High Schools1.8 GPD/unit
Food Service
Full Meal Restaurant & Cafeteria2.4 GPD/meal served
Fast Food / Drive-Ins / Delis0.7 GPD/meal served

Determine Your BTU Requirement

To start, we need to covert the daily draw (from the table above) from gallons per day to BTU per day. A BTU (British Thermal Unit) is the industry standard for heating energy.

It requires 8.34 BTU’s to raise the temperature of 1 gallon of water 1 degree F.

When using the solar loop as a pre-heater for the existing boiler, we will need to heat every gallon of water drawn from mains/well from it’s incoming temperature (map above or on-site measurement) to the typical desired setpoint of 135F.

To determine the BTU required to heat a single gallon of water at your location, subtract the well water temperature from the setpoint temperature and then multiply the difference by 8.34.


Southern Florida: (135F – 77F) * 8.34 BTU = 484 BTU
Northern Maine: (135F – 42F) * 8.34 BTU = 776 BTU

Then multiply the BTU’s by the number of gallons of water you will need to heat each day (your daily draw) to find the total BTU your solar thermal system will need to generate each day.


Southern Florida (20g/day): 20g * 484 BTU/g = 9680 BTU/day
Northern Maine (20g/day): 20g * 776 BTU/g = 15,520 BTU/day

Determine The Collector Area Required

To get an overall solar fraction of 60-70% (optimal sizing) of your solar thermal system, we should match the load heating requirement to the output of the solar array on a clear summer day. The significant advantage of sizing your system this way (based on summer time output) is that you will design a system that operates at maximum performance without hitting stagnation temperature (which can damage components and cripple a system).

When you are determining collector output for these calculations, you should use the “Category C” from the SRCC OG-100 Certification report.

You can see the BTU/ft2 output of SunMaxx solar collectors below:

  • Flat Plate Collectors
    • TitanPower-Plus-SU2 | 1173 BTU/ft2
    • TitanPower-Plus-SU2.4 | 1147 BTU/ft2
    • TitanPower-AL2 | 987 BTU/ft2
    • TitanPower-AL2DH | 987 BTU/ft2
  • Evacuated Tube Collectors
    • ThermoPower-VHP10 | 1295 BTU/ft2
    • ThermoPower-VHP20 | 1325 BTU/ft2
    • ThermoPower-VHP25 | 1328 BTU/ft2
    • ThermoPower-VHP30 | 1333 BTU/ft2

Using these output measurements, we can determine how many square feet of collector we will need to heat a single gallon of water.

Use the formula below to do this calculation:

Sizing Ratio = 1.15 * 8.34 * (X – Y) / Z

  • X is your set point temperature (typically 135F)
  • Y is the well/mains water temperature
  • Z is the BTU/ft2 rating of the solar collector
  • Note: The 1.15 is used to oversize by 15% to account for efficiency losses in piping, the storage tank, the heat exchanger, etc


TitanPower-Plus-SU2 in Southern Florida

Ratio = 1.15 * 8.34 * (135 – 77) / 1173

Ratio = 556 / 1173

Ratio = 0.47 ft2/gallon

With your ratio, you can now determine the total ft2 of your solar collector array by multiplying the total gallons per day you need to heat by the ratio you just calculated.

Divide the total ft2 of your array by the aperture area of the solar collector to determine the number of solar collectors needed for your array and you have successfully sized your solar array.

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