Excel Services - The workbook cannot be opened

After embedding excel file via Excel Web Access webpart, an error "The workbook cannot be opened." may occur if the service account running the Excel Service Application has not been granted access to the SharePoint web application yet properly.

The powershell script below will handle permission to SharePoint Web Application for the service account.

# Service Application that runs Excel Service Application is "Contoso\SPAppPool"
#
$web = Get-SPWebApplication -Identity "https://sp.contoso.edu"
$web.GrantAccessToProcessIdentity("Contoso\SPAppPool")

In fact, what happens as a result of executing this script at the database level is that

1. The Excel Service Account (Contoso\SPAppPool) is granted  custom database role, "SPDataAccess", which basically provides READ access to all SharePoint objects.
2. If there are multiple content databases under https://sp.contoso.edu Web Application, then the database role, "SPDataAccess" will be granted to the Service Account on all content databases.