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, 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 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
**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).
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)?)