Chapter 6: Where can you obtain solar radiation data?
"Utility engineers need solar radiation data accurate to within plus or minus 5% to assess the resource, estimate the output of a solar system, and determine whether the system can reliably and economically meet daytime demand and energy requirements. Because there are few sites with data of this accuracy, we need monitoring stations to collect the data at proposed PV sites."For large-scale applications of solar energy conversion technologies, most experts agree that solar radiation data should be accurate to within 5% so they can make reasonable assumptions concerning energy output to evaluate the performance and economics. Unfortunately, not much available solar radiation data are accurate to within 5%. This is due to the measurement uncertainties of the instruments used and the limited number of measurement sites. Consequently, designers today have to apply these data more conservatively than is ultimately desirable. The SOLMET/ERSATZ data are the most widely used solar radiation data. On an average for all sites, they are accurate to within about 10% for average daily values on an annual basis. But for average daily values on a monthly basis for an individual site, they can be in error by 20% or more. For interpolating data for sites between SOLMET/ERSATZ stations, microclimate differences due to terrain and local weather conditions can also increase the uncertainty of the data.
--J.E. Bigger, Electric Power Research Institute
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