PowerPivot for Sharepoint 2013 supports a variety of different installation architectures – one of which allows you to run the PowerPivot Analysis Services instance on a server other than your Sharepoint 2013 server. You still need to install PowerPivot software on your Sharepoint server, however only the spPowerpivot.msi package (or on all Sharepoint servers if you have a multi node Sharepoint farm) – this can be found on your SQL Server install media or located on the SQL 2012 feature pack http://go.microsoft.com/fwlink/p/?LinkID=263965. Make sure the same version of msPowerPivot.msi is used on all servers in farm.
Make sure you specify the Excel Services Data Model settings in the configuration of your Excel Services Service Application – this is the instance name of your PowerPivot instance e.g. HOSTNAME\POWERPIVOT.
For Kerberos configuration you will need to get the Port number of your POWERPIVOT instance (as well as the port number of the browser service), create your MSOLAPsvc.3 and MSOLAPDISCO.v3 SPNs then add these to your constrained delegation configuration.
Finally, you need to run the PowerPivot configuration tool to complete your installation – because you installed msPowerPivot.msi on the Sharepoint server, you can run the PowerPivot configuration tool and finish the PowerPivot integration.