Energy Flexibility is the ability of smart devices to deviate from their normal energy producing or consuming behaviour, while still fulfilling their intended purpose, with the goal of contributing to the operation of the energy system. Energy Flexibility has the potential to counteract the mismatch between production and consumption of energy as well as solving congestion problems in the grid therefore increasing the resilience of the energy system. There are many systems for utilizing and coordinating Energy Flexibility, varying in complexity, used incentives and technology. In this article, the Energy Flexibility Interface (EFI) is proposed. This interface, positioned between device and Energy Flexibility utilization system, provides a generic language for Energy Flexibility. The interface allows any device providing Energy Flexibility to work with any Energy Flexibility utilization system. Hence, Energy Flexibility can be utilized on a large scale while reducing implementation efforts and creating a level playing field for both devices and Energy Flexibility utilization systems. EFI is demonstrated in a dozen pilot projects and forms the basis of a European prestandard.