If you receive an error when interacting with a PowerPivot workbook in SharePoint, you may see the following error from the PowerPivot Service in the SharePoint ULS log:
EXCEPTION: System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: SetAuthContext need to be run as sysadmin.
This error refers to the Administrator permission on the PowerPivot Analysis Service instance. To fix the error, add the PowerPivot System Service account running the PowerPivot service application in SharePoint as an Administrator by connecting to the PowerPivot Analysis Services instance in SQL Management Studio. Right-click the instance, select Properties, select the Security page, click Add, add the user, click OK, and click OK again: