excel services install

https://www.youtube.com/watch?v=ifSRe6rCd28

for installing the powerpivot service
https://www.youtube.com/watch?v=Vhr34aSrLIA

it seems that powerpivot works without the powerpivot service but when it has to refresh the data it has to have 
a SSAS server that installs the powerpivot service

in the configuration of the excel service the last part is about the SSAS server...

for later investigation
https://blogs.msdn.microsoft.com/analysisservices/2012/08/02/verifying-the-excel-services-configuration-for-powerpivot-in-sharepoint-2013/