With Quality of Experience (QoE) research having made significant advances over the years, increased attention is being put on exploiting this knowledge from a service/network provider perspective in the context of the user-centric evaluation of systems. Current research investigates the impact of system/service mechanisms, their implementation or configurations on the service performance and how it affects the corresponding QoE of its users. Prominent examples address adaptive video streaming services, as well as enabling technologies for QoE-aware service management and monitoring, such as SDN/NFV and machine learning. When considering the whole service, the question arises of how to properly evaluate QoE in a systems context, i.e., how to quantify system-centric QoE. The paper  provides fundamental relationships for deriving system-centric QoE,which are the basis for this article.