Demand response is seen as a key technology to help the introduction of large volumes of intermittent renewable energy and to help mitigate the effects of the increasing load on the electricity distribution grid. A large and diverse number of control mechanisms are proposed for demand response, while multiple industrial companies are designing their own proprietary solutions. This situation demands for standardized and generalized interfaces for smart devices, i.e., interfaces based on as few indicators as possible that reflect the flexibility state, while hiding all implementation details and specifics of the device. Electric domestic hot water buffers are devices that inherently contain a lot of flexibility, i.e., the charging can be shifted without impact on the comfort of the user. This article presents four key indicators for domestic hot water buffers that meet above requirements: P, the rated power, Emax, the energy required to fully charge the buffer, SoC, the state of charge, and Emin , the energy required before the SoC increases. The correct behavior of these key indicators has been validated by means of simulations and measurements on a lab prototype. The integration of the smart buffer into a time of use demand response system is demonstrated with a test setup and measurement results.