A clear consensus exists in the German society that renewable energies have to play a dominant role in the future German energy supply system. However, many questions are still under discussion; for instance the relevance of the different technologies such as photovoltaic systems and wind energy converters installed offshore in the North Sea and the Baltic Sea. Also concerns exist about the cost of a future energy system mainly based on renewable energies. In order to be able to address the raised issues on a scientifically sound basis we have set up a new simulation model REMod-D (Renewable Energy Model-Deutschland) that models the energy balance of the electricity and heat sector including all renewable energy converters, storage components and loads for a future German energy system for a whole year based on an hourly energy balance. The target energy systems modeled use a high fraction up to 100% of renewable energies to cover the electricity and heat demand (heating and hot water). The model includes also energy retrofit of buildings as a measure to reduce future heat loads of the building sector. A mathematical–numerical optimizer is applied in order to identify system configurations with minimal overall annual cost. In this first part of a two-paper series we describe the methodology of the REMod-D model and discuss cost and performance values of all included components and in the second part we will discuss the results.