We do not specify the confidence interval of the energy production estimates of PV Designer. Since our calculations are closely tied to PV Watts, it may be possible to leverage some of the work and expertise available for PV Watts. There is some literature available that shows example results for PV Watts, such as http://www.nrel.gov/docs/fy02osti/30941.pdf (an internet search on ?confidence interval pvwatts? provides other references as well.  However, we are not currently able to provide confidence interval calculations directly. I provide some background on the PV Designer?s methodology below and its relation to PVWatts. It may be possible to leverage from the methods for PV Watts and the TMY weather data sources to arrive at calculations of confidence interval for PV Designer. We would be interested in working with you if you decide to pursue this.   The Solmetric PV Designer software provides an estimate of the monthly and annual energy production. It uses calculation methods from PV Watts, the National Renewable Energy Lab program, with a few differences, including the following: –           Shading is considered across the area of the array on an hourly basis, vs. just a single derate factor. Modules that are shaded contribute less power. –           Solmetric Skyline data is used for the shading calculations across the array area. For any module location between skylines, an estimated probability of shading is calculated based on the distance weighting of the data from surrounding skylines. –           Calculations are based on power and actual temperature coefficients from the module specifications, instead of generic temperature coefficients. –           Inverter efficiency is taken from the inverter specification. –           Derating factor with all other derate values (not including shade or inverter efficiency) are combined by multiplying the coefficients together. This is summarized in the design. Different derate factors can be entered for each design to compare assumptions.   Another important contributor is weather data. We include information from various weather sources, including TMY3. Annual energy predictions are likely to reflect an average year. PV Watts uses TMY2 weather data, so differences may be observed with other data sources, eg. TMY3. The energy calculations within PV Designer are estimates, and do not provide any guarantee of actual production. 

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