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 AP Solar Hot Water Collector Sizing

When determining what size collector you need, you must consider two key factors: insolation level and energy requirements. Energy requirement will usually take into consideration the volume of water and rise in temperature required. Once you know these factors you can determine the size collector you require. The bigger the collector you have, the more hot water, but you should make an economically sound decision. Generally it is wise to select a size which will provide you with 9

Although it may seem strange to use a value of only 90% for summer solar contribution, it is for good reason. It is normal to size based on &quot0% of your summer hot water energy needs, with a percentage provided throughout other months, lowest obviously in winter.; normal" water usage, but often, and particularly in the summer, water usage patterns may not be that normal, with cooler than normal showers taken in hot weather, and greater possibility of the house being vacant for one or two days each week (weekends). As such using a target value of 90% will probably actually result in a system that is able to supply more than 100% of your hot water needs in the summer, without excessive heat production, which can lead to water loss via pressure release and a waste of energy.

If you get an answer that is not a standard size, as a general rule, select the next size down - this will prevent having too much heat in the summer.

Depending on your preference, either Metric or Imperial values may be used to calculate the number of tubes required. One 1.8m tube = 1.2 x 1.5m tubes.

Metric Calculation

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

Imperial Calculation

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

*Water Volume = This should represent the actual volume of hot water used at the tap.
Although most hot water systems have target temps of 60
oC / 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 = target tap hot water temp - average mains cold water temp.
Target hot water temp should usually be around 42
oC / 107oF - 45oC / 113oF
Cold water usually fluctuates by about 10
oC / 18oF between winter and summer. A check of your local weather records should provide you with an idea of average cold water temperatures.

Metric to Imperial Conversion

kWh/m2/day = Btu/ft2/day


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 less than ideal angle will reduce efficiency)
6. Installation site (Do you have enough room for the collector(s)?)

The estimations above are just a guide and are based on a average summer performance level of 65%. As explained above it is best 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 which suits your needs. For more information about performance and heat energy output please click here.

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