What is solar insolation?The amount of electromagnetic energy (solar radiation) incident on the surface of the earth. Basically that means how much sunlight is shining down on us.Why is knowing the insolation level useful?By knowing the insolation levels of a particular region we can determine the size of solar collector that is required. An area with poor insolation levels will need a larger collector than an area with high insolation levels. Once you know your regions insolation level you can more accurately calculate collector size and energy output. What units are used to express Insolation levels?The values are generally expressed in kWh/m2/day. This is the amount of solar energy that strikes a square metre of the earths surface in a single day. Of course this value is averaged to account for differences in the days length. There are several units that are used throughout the world. The conversions based on surface area as follows:1 kWh/m2/day = 317.1 btu/ft2/day = 3.6MJ/m2/dayThe raw energy conversions are:1kWh = 3412 Btu = 3.6MJ = 859.8kcalIs my regions insolation level low, moderate or high?The following scale is a basic guide for insolation levels. Although a value of 5 is not considered very high during the summer months, as an average annual value this is very high. You will see that in central Australia, which is a hot, sunny place, the annual average insolation is 5.89. You may compare you location to the following two extreme locations.Average annual insolation levels:Central Australia = 5.89 kWh/m2/day - Very HighHelsinki, Finland = 2.41 kWh/m2/day - Very LowNow I know my insolation level, how do I calculate my collectors size? 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 http:/www.apricus.com/html/solar_collector_size.htm 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 / 113oFCold 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)?) The estimations above are just a guide and are based on a average summer performance level of 70%. As explained above it is best not to oversize the system. Your local distributor may need to complete a