Extracting grid to excel for a silverlight web application

If your grid has a function built in to copy to Excel... then I'm not sure what you need to know... simply call the function with the necessary parameters within a JScript unit in TestComplete.

Generally speaking, there's pretty extensive documentation on working with grids at https://support.smartbear.com/testcomplete/docs/app-objects/specific-tasks/grids/basic-concepts.html