The native data format for the Spreadsheet widget is JSON.
There is also the built-in support for exporting to Excel that runs in the browser. To export huge datasets to Excel, a well-suited solution is the new RadSpreadStreamProcessing library which is part of Telerik Document Processing (TDP) by Progress.
For examples on how to export Excel files, refer to the RadSpreadProcessing library.
For more information on the Spreadsheet server-side processing, refer to this article.