When designing a VM domestic solar water heating system, determine first how much hot water the family will be using per average day. Next find latitude, the amount of solar insolation, and number of hours of available sunshine at the job site location. Use the above chart to correct the effect of orientation of the roof (east/west orientation deviation).
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To heat 375 liters of water at a job site with a local Annual Insolation of 1400 hours, roof orientation of 30 degrees West of South, you need 43 VM tubes.
The Solar Insolation charts are based on a Solar Collector facing true South/North line. When Solar Collectors are mounted east or west of true south/north, there is some loss of insolation. But it is not measurable at 5 degrees east or west. By angling the Solar Collector to face westward in the Northern Hemisphere, a greater amount of solar insolation can be absorbed in the late afternoon. However, there is some loss of solar insolation during the course of the day, when the Solar Collectors are mounted east of true south. A good deal of morning solar insolation is lost at the same time.
The following above chart suggests a correction parameter for deviation from North/South line.
In the following four pages we are going to size a solar water heating system for a family of five. They have a hot water requirement of 50 liters per day, per person, with a local Annual Insolation of 1400 hours. Orientation of the roof is 30° West of South. The local Latitude is 45° and the roof angle 35° (inclination difference 10°). If your property is located in the US, go directly to US Insolation Charts.