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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 90% of your hot water
needs in the summer.

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 100% of your summer hot water
energy needs, with a percentage provided throughout other months, lowest obviously
in winter. That is based on 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.

The calculator below can help to determine how many evacuated tubes you require
given your energy requirements. Solar collectors come in a set of standard sizing of
10, 20, 25 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 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. 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
in total 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 = target 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).

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