Hi,
When using your own ArcGIS Server services or ArcGIS Server services from the ArcGIS Data Appliance, you will not incur into any credit consumption. ArcGIS Online credits are only consumed when you use for fee ArcGIS Online services. Unless you configure Portal for ArcGIS to specifically hit ArcGIS Online services that consume credits, you will not see any credit consumption in your ArcGIS Online account driven from Portal.
For details regarding pricing and licensing, please contact your account representative. Alternatively, you can ask questions through the 'request pricing information' link at: http://www.esri.com/about-esri/contact
Thanks,
Ismael Chivite