Hi @Terry Chan ,
For usage report is an internal function and we don't have an interface to directly download usage report from SharePoint, it is impossible to get usage reports in batches. You need to do it manually. As a workaround, you can loop all web applications and get all the site collections in the web applications, then export usage data to a .CSV file.
References:
Loop web applications and get site collections:
https://sharepointpixels.wordpress.com/2016/06/24/sharepoint-iterate-through-all-web-applications-site-collections-sites-lists-and-list-items-with-powershell/
Get usage data in SharePoint:
https://archive.codeplex.com/?p=sp13usagemonthlystats
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