We’re going to learn how to calculate the solar water heating system capacity in a building. Here we have one example.

First, we have to determine the quantity of the fixture units in the facility and also the use of the facility. Then when we have the fixture units quantity, we will check how much it will be in total using the ashtray table, as given below. Let's say for example, a dormitory for a private laboratory equals 0.75. The public laboratory is equal to 1 and so on. From this example we can see the total fixture units is 230 almost.

Then we will use the center curve from Ashray given below.

We put the fixture units and using this curve, which is for dormitory we will find that the total hot water needed is almost 49 gallons. So the fixture unit, we need 40 centigrade as per the NFPA given below. The blended water, the hot water and the cold water from this equation at the end the factor is 0.6 from the hot water so here we apply this percentage to water needed which is 49, so we will need 29.4 gallons per minute.

Next we will multiply it by 60 then it will be 1764 gallons per hour. The cumulative consumption chain is considered here two hours for this building. So if we multiply the 2 hours by the gallon per hour needed, then we will have how much gallon per day we need as total, which is 3,528. That demand factor usually is 0.75, which is 75 % so if we apply this demand factor, we will see here that the needed gallons per day is 2646. And then we will convert it into liters, which will be almost 10,000 liters per day. Here in our example we are using ATC Collector from Africas Company and its biggest size, which is 30 evacuated per collector as per TUV Test report, it can give 350 liters per day of hot water. So when dividing the total amount of water needed per day by this 350 liters, we will find that we will need 29 collectors in total.

Now we will calculate the storage capacity needed for this collector, which is 30-tube and where each collector area is 30.57 square feet. For this collector, we will find that the total area is 29, multiplied by 30.57. The total collector's area is 886.53 square feet.

And for evacuated tube type, which has high thermal output, we will need 2 gallons of storage per square foot collector area. So by multiplying 2 by this number, we will find that we will need 1773 gallons as storage. Converting it into a liter multiplied by 3.785, we will need almost 6711 liters. Here, as we are calculating the solar water heating system, usually the solar fraction is 75 % and the other source fraction is 25 %. So when we divide according to this demand to this fraction, we will find that from solar we will need almost 5,000 liters and from the electric we will need 1677 liters. So in this case, we will need 22 collectors and 5,000 liters of storage.