You can run a PowerPivot 2012 (v2) Excel 2010 document up on Sharepoint 2013 server. The following read outlines a the pros and cons. Note Excel 2013 Powerpivot documents support Interactive refresh within the open workbook however PowerPivot v2 for Excel 2010 only supports Unattended Refresh. At this stage I havn’t any luck getting data refresh to work with Excel 2012 v2 Powerpivot documents and a bug is mentioned here http://technet.microsoft.com/library/jj218794.aspx. PowerPivot v1 does not support interactive nor unattended data refresh so you are best converting these.