The growth of distributed generation requires analysis based on realistic forecasts of future scenarios. This paper presents an original methodology that has been employed to develop penetration scenarios to support further research. The methodology combines top-down and bottom-up approaches to produce robust scenarios. The top-down approach is based on forecasts of total distributed generation at a national level. The bottom-up approach exploits expert opinion to determine the most likely developments under different conditions. The two approaches are combined according to the objectives of the analysis to be supported. The methodology offers a useful tool for scenario development and supports ongoing research in distributed generation