This makes intuitive sense since it would seem to maximize the amount of sunlight a panel will get as the sun tracks from one horizon to the other. But it isn’t true, at least according to a single study of homes in Austin, Texas. The Pecan Street Research Institute found that homeowners who aimed their panels toward the west, instead of the south, generated 2% more electricity over the course of a day.
The exact angle for best results depends on local conditions.
As the sun rises from the east and sets towards the west, then a bit west of south will give good results if the weather in the locality tends to be clearer in the afternoon.
Likewise in a locality with clear morning weather but regular afternoon cloud, then a bit east of south may be better.
Also some people confuse energy produced with achieving the best financial return. In many parts of the USA, electricity is sold at a higher price in the afternoons in summer, due to the widespread use of airconditioning. In such circumstances it is often worth maximising summer afternoon production to get the best return in money, even if total KWH production is slightly reduced.
For off grid applications it depends on for what purpose the electricity is to be used.
For lighting, due south and nearly vertical is often best in order to maximise winter production when lighting needs are greatest and sunlight limited.
For air conditioning, a fairly shallow angle towards the west of south will maximise production at the warmest time of day.
Finally, in the case of very large grid tied systems that cost millions, these are virtually certain to be correctly aligned for best results. There is a lot of money involved and those in charge have a clear interest in getting it just right.
Smaller installations are often atop existing buildings and may not be perfectly aligned, but still generaly produce acceptable results.
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