You would have to file an amended return. In order to receive a refund you must file an amended return within three years from the date you filed your original return or within two years from the date you paid any tax due, whichever is later. If you filed your original return before the due date (usually April 15), it's considered filed on the due date.
The credit is only available if you installed the panels, or purchased a new home with solar panels and the builder did not claim a credit.
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