Solar Collector Size Calculator

When you choose the size of solar collector, you must consider two key factors: insolation level and energy requirements. Energy requirement will usually take into account water volume and temperature rise needed. When you know these two factors you can determine the size collector you require. The bigger the collector you have, the more hot water you can possess, but you should make an economically sound decision. Generally speaking, it is wise to select a size which will provide you with 90% of your hot water requirements in summer.

Although it may seem strange to use a value of only 90% solar energy, it is for a good reason. It is normal to size energy requirement based on 100% of your summer hot water, with a percentage provided throughout other months, lowest obviously in winter. That is based on normal water usage, in summer, water usage patterns may not be that normal, with cooler than normal showers, and greater possibility of the house being vacant for one or two days each week (weekends). As such, using a value of 90% will probably actually result in a system that is able to supply more than 100% of your hot water requirement in summer, without excessive heat production, which may lead to water loss through pressure release and a waste of energy.

The calculator below can help to determine how many evacuated tubes you require according to your energy requirements. Solar collectors come in a set of standard sizing of 10, 20, 22 or 30, depending on your region. Of course you can also combine collectors to increase the size. If you get an answer that is not a standard size, as a general rule, select the next size down - this will prevent producing too much heat in summer.

Depending on your preference, either Metric or Imperial values may be used to calculate the number of tubes required.
Please note: 1 kWh/m2/day = 317.1 Btu/ft2/day

Metric Calculation

Insolation: kWh/m2/day
Water Volume:* Litres
Temp Rise:** oC
You Require:
Evacuated Tubes

Imperial Calculation

Insolation: Btu/ft2/day
Water Volume:* US Gallons
Temp Rise: ** oF
You Require:
Evacuated Tubes

*Water Volume = This should represent the actual volume of hot water used at the tap each day.
Although most hot water systems have target temps of 60oC / 140oF, when showering a temperature of between 42oC / 107oF and 45oC / 113oF is normally used. Therefore 300L of hot water at the tap may only draw 220L of hot water (at 60oC / 140oF) from the storage tank.

**Temperature Rise =tap hot water temp - average mains cold water temp.
Target hot water temp should usually be around 42oC / 107oF to 45oC / 113oF
Cold water usually fluctuates by about 10oC / 18oF between winter and summer. A check of your local weather records should provide you with an idea of average cold water temperatures (normall about 10oC / 50oF in winter and 20oC / 68oF in summer, in mild regions).

Apart from the three key factors used in the calculation above, you may also need to consider:
1. Annually/daily shade patterns
2. Angle/direction of installation (a lessideal angle will reduce efficiency)
3. Installation site (Do you have enough room for the collector(s)?)

The estimations above are just for your reference and are based on a average summer performance level of 70%. As explained above you'd better not to oversize the system. Your local distributor may need to complete a onsite inspection to accurately assess your requirements and design a solar water heating system .

For more information about performance and heat energy output please click here.


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