Urban energy consumption plays an ever increasing role, as more than half of the world's population already lives in cities. Many municipalities have decided on ambitious goals to become climate neutral within the next decades, however, very limited experience is available of how demand can be significantly reduced on a city scale and what contributions local renewable energy sources can bring to cities. Many cities are starting to analyse their current CO2 emission levels, although continuous monitoring is very rarely available.

In the current paper, two case studies were carried out in Southern Germany, which cover an entire medium sized city (Ludwigsburg) as well as a city quarter in Ostfildern. Although in both cases solar technologies have the highest potential for decentral renewable supply, the most significant contribution to cover the demand is by central power plants based on biomass. In the low energy building quarter of Ostfildern, a biomass cogeneration plant covers 80% of the heating and 50% of the electricity consumption. The remaining electricity consumption could be provided by photovoltaics on easily available roof tops, however, less than 5% of consumption is currently covered by installed PV systems. The studies show that the main challenge is to reduce emissions caused by individuals in their buildings and transport activities and to implement solar technologies in buildings. The municipalities themselves can be front-runners with highly efficient buildings and central renewable plants constructed by their municipal energy supply companies. However, they are directly responsible for less than 5% of all urban emissions and their energy supply companies only provide a limited fraction of households with heat and electricity.