Solar Rating and Certification Corporation OG-100 sheets are issued to solar manufacturers for certified collectors. These sheets contain lots of information, but most useful are the performance tables, which show predicted collector output in kilowatt-hours per day and Btu per day under 15 different conditions. Three insolation levels—a clear sunny day; a mildly cloudy day; and a cloudy day—are shown across the top of the chart. The insolation or amount of sun hitting the collector under each condition is shown in kilowatt-hours per square meter per day and Btu per square foot per day. Down the side are the temperature categories, shown in Celsius and Fahrenheit. Each category is classified by a letter and a temperature. The temperature shown is the difference between the incoming water temperature to the collectors versus the ambient air temperature around the collector. The key at the bottom of the chart suggests applications for each category. For example, for outdoor seasonal pool heating in a warm climate, you would likely use performance numbers for category A, since the incoming pool water temperature would be lower than the ambient temperature. If you were designing for winter water heating, you’d check out the numbers that correspond to category C or D since the outdoor ambient temperatures will be lower than the fluid entering the collectors.
When using the performance tables, remember that these ratings are for referencing and comparing different collectors. The given ratings are what the collector is making at that exact moment, under these exact conditions. In the field, a collector could experience all three insolation levels and three temperature categories in just one day.