As per MSDN, O365 E1 license doesn't support Power BI Webpart. (https://technet.microsoft.com/en-us/library/sharepoint-online-service-description.aspx)
Can E1 licensed user view the power BI report(using Power BI webpart) added by E3 licensed user?
This will help us to take the licensing decision, e.g., we can have only 2-3 E3 licenses and rest of the user with E1 license.