Sometimes it can happen you configure the SharePoint Service Application for Excel Services and when you add the web part to show a Excel workbook you get a nice friendly error message about authorizations and all.
The reason for that is that the Application Pool account from the Excel Service SSA doesn’t have any authorisations on the web applications your trying to use it in.
The PowerShel script below will fix this. Just change the url in the one from your site collection.
$webApp = Get-SPWebApplication “http://UrlSiteCollection”
$webApp.GrantAccessToProcessIdentity(“DOMAIN\SPexcelSSA_Pool”)
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