How accurate are the output data?
The basic solar heat gain factor (SHGF) calculation is fairly accurate…the calculated values are within 1% of (and generally within 0.1% or identical to) the tabular SHGF data published in the ASHRAE 1997 Fundamentals volume. Further adjustments to the SHGF introduce various inaccuracies, primarily averaged climate data (which may vary substantially from actual, realized weather, and which may be based on weather stations some distance from the actual location) and, as mentioned above, the use of the percent of possible sunshine factor to deprecate the solar gain. The author estimates (rather subjectively) that the overall calculation is accurate to within about 20% as an average value. The actual monthly heat gain may vary much more substantially, of course, depending on the volatility of local weather compared to the average monthly values used in this tool. The output values are probably more useful in comparing relative performance in different designs, than in obtaining an exact
