To determine the proper model and quantity of collectors for a solar thermal system the water heating load must first be determined. Once the heating load is determined the solar collector array can be sized to meet the entire or a fraction of it. For residential systems on single-family homes the figure below can be used to determine system size.

Solene Residential System Sizing Map

Residential System Sizing Guide
Region# of PeopleSystemCollector AreaSystem Capacity
A & B 440 / 8040 sq. ft.80 gal.
A & B4 – 564 / 8064 sq. ft.80 gal.
A & B664 / 12064 sq. ft.120 gal.
A & B780 / 12080 sq. ft.120 gal.
C, D, & E 440 / 8040 sq. ft.80 gal.
C, D, & E4 – 564 / 8064 sq. ft.80 gal.
C, D, & E664 / 12064 sq. ft.120 gal.
C, D, & E780 / 12080 sq. ft.120 gal.
F 4 – 780 / 12080 sq. ft.120 gal.

For solar thermal systems used in a domestic hot water application the hot water usage in gallons/day (GPD) needs to be determined. This can be done using site-specific data or using maximums and/or averages published by ASHRAE for various type of facilities.

Facility TypeAverage Daily DrawMaximum Daily Draw
Hotels / Motels
20 units or less20 GPD / unit35 GPD / unit
60 units14 GPD / unit25 GPD / unit
100 units or more10 GPD / unit15 GPD / unit
Nursing Homes
18 GPD / bed30 GPD / bed
Men13 GPD / student22 GPD / student
Women12 GPD / student26 GPD / student
Apartment Buildings
20 units or less42 GPD / unit80 GPD / unit
50 units40 GPD / unit73 GPD / unit
75 units38 GPD / unit66 GPD / unit
100 units37 GPD / unit60 GPD / unit
200 units or more35 GPD / unit52 GPD / unit
Office Buildings
1 GPD / person2 GPD / person
Full Service3 GPD / meal11 GPD / meal
Fast food, Sandwich shops, etc1 GPD / meal6 GPD / meal
Elementary1 GPD / student2 GPD / student
Middle & High2 GPD / student4 GPD / student

Converting Domestic Hot Water Usage to Water Heating Load

Once the daily hot water usage has been determined the temperature difference must then be calculated. This is done by taking the desired hot water setpoint temperature (usually 130-140 F) and subtracting the mains water temperature from that. The mains water temperature is the temperature at which water enters the building from the utility and can be determined by using the average groundwater temperature for the area in question. To heat one gallon of water up by 1°F requires 8.33 BTUs of energy so the total heating load can be found by multiplying the gallons/day usage found by the temperature difference and then multiplying the product of that by 8.33. For example, if we needed to determine the daily water heating load for 20 unit apartment building in Orlando, FL using the average daily hot water usage we would calculate the following:
8.33 BTU/gal-F * [(42 GPD/unit * 20 units) * (140°F – 72°F)] = 475,809.6 BTU/day.

Solene groundwater temperature map

Determining Collector Area Required based on Water Heating Demands

To safely and optimally determine the size of solar thermal collector array the calculated water heating load should be matched to the expected output of the solar array. For initial sizing purposes we can use the rating published by SRCC for Category C, High-Radiation conditions. The Solene AR Series is rated at 1095 BTU/ft2-day and the Solene SG Series is rated at 1070 BTU/ft2-day. In reality the power output of the solar thermal collectors varies constantly with solar insolation and weather but using the figures as noted helps ensure that the system is sized so that it will not stagnate during the warmest, brightest months and possibly cause issues due to extreme temperatures and pressures. A multiplier of 1.20 should factored into the sizing result to make up for real-world energy losses from piping, tanks, etc. Using the previous example of the apartment building, and assuming the Solene SG Series collector will be used, the system would be sized as follows: 1.20*[(475,809.6 BTU/day) / (1070 BTU/ft2-day)] = 533.6 ft2. The calculation yields a result of 533.6 ft2 which would equate to (14) SLSG-40 collectors. It should be noted that the collectors will not perform at this level 100% of the time but, conservatively, should yield a solar fractions of 50-75% over the course of a year.