The user-defined project database is created from projects saved by the user (or others) over time. To open user-defined projects, the same steps are taken as for the built-in case studies, but the user must first open a RETScreen ".ret" file. A sample version is provided to the user ("RETScreen.ret"), but the user may create (i.e. save) as many ".ret" files as preferred, and in fact, may save multiple versions of a project, or multiple projects, within one single ".ret" file. The ".ret" files are created by the user by using the "Save" or "Save as" Excel functions, and then selecting the special RETScreen file type (*.ret). In addition to the advantage of allowing the user to customise the project database with these user-defined ".ret" files, the other advantage is that the ".ret" file size is about 100 KB as opposed to 8 MB for a conventional ".xls" file that is normally created when saving the RETScreen Excel workbook file. And each additional project saved within a ".ret" file is only about 10 KB. This helps free up considerable hard-drive space and allows for easier transmission of project files over the Internet, to be shared with other RETScreen users working on a particular project.

The Delete button in the dialogue box allows the user to delete projects from the user-defined project data within an individual ".ret" file. This is provided because each time the user clicks on "Save" or the save icon, the file is saved within the ".ret" file. The delete function allows the user to delete versions that are not required. This is purely an optional housekeeping function. The user will typically create additional ".ret" files to simply file projects under different categories (e.g. wind.ret, hydro.ret, etc.).