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Active Solar Water Heater Systems

System Sizing
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Sizing a Solar Water Heater System

The size of a solar water heating system can depend on many factors such as location, hot water consumption per day, desired average water temperature and time of year. For a general sizing, follow the guide in the table below. If you want hotter water, add reflectors ort additional tubes to the system. If you want more heat in the winter, angle the collector steeper towards the horizon.

General Tank Sizing

The first step to finding out how large of a system is needed is by calculating the amount of hot water used per day or per week and dividing it into a daily average. The chart below may be used for figuring out average hot water consumption per person and per device. The chart assumes the hot water tank contains 50°C / 122°F water.

Hot Water Usage per person/device
10 Minute shower @ standard 3.8 gpm with 38°C / 100°F
95 Liters
10 Minute shower @ low flow 1.6 gpm with 38°C / 100°F
47 Liters
1 Bath @ 42 Gallons
159 Liters
Washing Dishes by Hand with 38°C / 100°F @ 5 gallons per minute
12 Liters per minute
Dishwasher Load @ 15 gallons per load
57 Liters
Top Load Washing Machine (one load) @ 15-20 Gallons
57 - 76 Liters
Front Load Washing Machine (one load) @ 8-15 Gallons
30 - 57 Liters


Example of Calculating Hot Water Usage

As an example, let's say we have 2 people living in a home. Each day they both take shower, 1 at 5 minutes and the other at 10 minutes, with standard water flow. One person takes a bath per week. They run their dishwasher once per day and they do 2 loads of laundry a week with a top load washing machine.

Example of Hot Water Usage for a 2 Person Home
Times per Week
Consumption per Activity
Consumption Per Week
10 minute shower
95 Liters
665 Liters
5 Minute Shower
47 Liters
329 Liters
159 Liters
159 Liters
57 Liters
399 Liters
67 Liters
134 Liters
1686 Liters
Per Day
241 Liters

As per the above example, this household could use a 200 Liter solar system as an undersize, which will require some backup heating, or it can use a 300 liter system, which will allow for a little extra usage and hardly any backup heating. Most people tend to oversize their systems, so for the sake of continuing the example further, we will assume a 300 liter system is chosen.


General Collector Sizing

Once the needed tank size is known, then the solar collector can be sized accordingly to the amount of water needed to be heated. The number of Duda Solar tubes needed depends entirely on solar insolation and weather.

Using the solar insolation map, an approximate amount of yearly insolation for your area can be determined. Find your location on the map, and then use the chart to determine how many tubes are needed. The chart includes approximate weather data based on location on the map.


Liters of Water Heated to 50°C / 122°F based on Insolation
Assumed Inlet Water Temperature
Liters of Water
6.5 Liters
7.5 Liters
8.5 Liters
9.5 Liters
10.75 Liters
11.5 Liters
13 Liters
14 Liters

Example of Sizing the Solar Collector

Let's assume that our example household is located in Chicago, IL. This area is about a 6.0 kw-hr/m2 area. They would require 1 tube per 8.5 liters of water to be heated which comes to 300 liters / 8.5 liters = 35.3 tubes. For the sake of even numbers, the household would most likely choose 2 of the 20 tube collectors, for a total of 40 tubes or if perfection is not necessary on the roof, a 15 tube and 20 tube collector could be chosen.

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System Sizing

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