The best option was always and will always be to use hierarchical design for this. Just imagine your power supply lives in its own hierarchical sheet with a well defined interface then you simply copy the sheet file into your new project and include it in that new project like you would any other hierarchical subsheet.
What you need to take care of is that you have the same libraries active in both projects (both should only use global libs for best results) and that the source project is not in need of rescuing before doing this (open the source projects schematic in kicad to find out). Should the source sheet contain rescued symbols then you simply also need to copy the rescue lib and include it in your current projects lib table.
If the source project uses project local libs then the used libs need to be copied as well plus included in the local library table of the target project.
General tutorial about hierarchical design: Hierarchical or flat schematic design, what is best for me? (How to deal with multi page schematics?)
You can also use the append feature of standalone eeschema if you do not want to use hierarchical design in your target project (this can also only import a full sheets so you might need to delete unnecessary stuff if your source project had more stuff on the sheet you are interested in.)
And this also requires the same care with local libs and the rescue lib.