The availability of sustainability indicators is a precondition for the conversion of the model of sustainable development into policy. Sustainability indicators define characteristics that are important for sustainable development. A single sustainability indicator defines a key issue, which characterizes a certain aspect of sustainability in the observed system. Sustainability indicator systems capture the complexity and order of the systems, and they offer new knowledge about the system that can be communicated to the general public. That is why in 2002 the German Government developed a sustainability indicator set for its sustainability strategy, including indicators for a sustainability energy system within this comprehensive strategy. The Government thereby defined a sustainable order for Germany based on the theme-based sustainability approach. The sustainability indicators thus have to measure the difference between the real and the normatively defined sustainability order: Do the individual preferences of society (households, enterprises) correspond to the sustainability order of the Government, is the implicit preference order of society congruent with the explicit order of the Government as expressed in the national sustainability strategy? The question of the measurability of sustainability is the key to the implementation of sustainable development of the energy sector. The index of sustainable development (ISUD) and the standardized sustainability index (SSEI), developed by the authors, calculate the degree to which sustainability is achieved. Both indices show whether Germany is on a sustainable energy path according to the goals set by the German Government in its strategy, but from a different perspective. The indices enable us to compare the normatively (politically) defined sustainability order of the German Government (goals) with the actual "behaviour" of German society. Both indices enable us to answer the question of whether the German energy sector is "better off" in sustainable categories. In a monitoring process, calculations of the sustainable indicators help us to understand where political action is needed. (C) 2012 Elsevier Ltd. All rights reserved.