Granting Excel Services access rights to your #PowerPivot for SharePoint Farm
In many of test PowerPivot for SharePoint deployments, the SharePoint Farm Administrator is used for all services including Excel Services during installation or configuration. This is a great and easy way to get yourself up and running fast. But a best practice is to use a different managed account for the application pool that Excel Services (and PowerPivot Services for that matter) is running under. This way Excel Services (and other services using that application pool) do not have Farm Administrator rights.