Energyplus, Design day : date and solar radiation

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The solar radiation is not influenced by the weather file: of course it is right to take a look into the weather data to realize the highest values.

However, E+ usually uses a solar model to simulate the Summer Design Day (if you take a look in the solar profile during the design day you will get a perfectrly symmetrical profile): the standard is the AshraeClearSky method but you could choose other models already built it E+ like the AshraeTau (you should check the coefficients in the Ashrae reports here "http://ashrae-meteo.info/") or the Zhang Huang.

The solar model is selected in the object "SizingPeriod:DesignDay" in the field "Solar Model Indicator". Alternatively, you could use your own solar radiation profile selecting the option "Scheduled".

Therefore, the day you chose only influence the position of the sun (and the radiation power given by the model) during the design simulation and do not have any relation with the radiation in the corresponding weather file day.

Please note that this is really relevant since E+ can not foresee the shape and the solar overhangs of your building: the maximum solar radiation may not correspond to your building cooling peaks or maximum solar gain. In this case I advise adding more than one design day.

For the desing days you open the weater data in IDF Editor and in SizingPeriod:DesignDay you can see a list of the potentional SummerDesingDays and WinterDesingDays. Usually use the ones with the maximum/minimum (or the second ones) to size your heating and cooling capacity. Now for the solar radiation you can run your simulation and select the summary report from the output section. There you can see the values per month about solar radiation in the sections about outdoor enviroment.