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.
But in the process of doing this, one might have forgotten to grant Excel Services rights to the SharePoint Content databases. If you did, here’s a quick Powershell script that grants db_owner access to the SharePoint Foundation content databases.
PS> $w = Get-SPWebApplication -identity <your web application>
PS> $w.GrantAccessToProcessIdentity(“<insert service account>”)
where the service account is the managed account that is running the application pool that Excel Services is using. To identify the service account, go to Central Administration > Security > Configure service accounts and under Credential Management go through the different application pools that is running the Excel Services Application. For PowerPivot Single Server install, this is the Service Application Pool – SharePoint Web Services System as noted in the Figure below.