The Energy Services Interface, or ESI, has been identified by NIST and the SGIP (Smart Grid Interoperability Panel) as an important interface between the customer's home, facility or factory and the SmartGrid. It consists of outward facing interfaces that are exposed to other domains such as operations and service providers along with a set of inwardly facing interfaces and functions that interact with a wide variety of internal systems and devices. The functions that an ESI performs vary based on the needs of different customers. An important question to answer is: how similar are customers and in what ways do they differ? The SGIP B2G (Building2Grid), H2G (Home2Grid) and I2G (Industry2Grid) Domain Expert Working Groups are exploring this issue and have been developing a white paper that discusses the definition, role and functions of an Energy Services Interface along with the similarities and differences that arise between residential customers, buildings and industrial facilities.