Question-and-Answer Resource for the Building Energy Modeling Community
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As for the .ddy file, it is not something EnergyPlus really uses directly like it does the .epw. The .ddy file is just a handy collection of the climate design data, used for sizing, that are in E+ input format (see the object SizingPeriod:DesignDay). If you make your own SizingPeriod:DesignDay objects and put them in the .idf, then you don't also need to create a .ddy file. If you are not using autosizing, then you don't need any design day objects.
However, if the climate design data already exist for the location, then they don't necessarily need to be changed since they are a different sort of thing than hourly weather. To do better than handbook data you would need many many years of observations to capture extremes.
