As part of recent Power BI service update the ability to use excel files stored in SharePoint Online team sites was announced. This blog post explains the steps for creating Power BI report from excel file stored in SharePoint online tea site
1. Logon to PowerBi.com
2. Click on Get Data from left navigation
3. In the Get Data page click Get link in the files source
4. In the files page click SharePoint – Team Sites as the source
5. You will be prompted to enter the URL of the SharePoint site. Enter the URL of the SharePoint site like https://contoso.sharepoint.com/teams/sales and click Connect button.
6. You will be presented from the list of libraries to choose from. Click the library in which you have excel file to be used for Power BI. If you have the file within a folder then select the folder.
7. List of Excel files in the library or folder will be listed. Select the Excel file you want to use as source and click the Connect button at top right.
8. From the Choose how to connect page click Import button in Import Excel data into Power BI option
9. After processing your Excel file Power BI will create a new data set.
10. You will then be create charts and dashboards from the data set created from Excel file.