Demonstration projects and trials (DTs) are an extension of the prototyping process into next phases of development and are widely used in reducing uncertainty for new technologies. During the last few decades, DTs have been extensively used to help overcome innovation uncertainties in renewable energy for electricity supply systems in the US. EU and Japan. However, there is still relatively little attention to this "uncertain middle" phase in accelerating complex, large-system innovation, particularly as to what companies actually value, as distinct from what advocates suggest they should gain and what policy makers believe publicly funded DTs should achieve. Following development of a comprehensive database of DTs, fifteen company cases have been developed on solar PV with nine on wind turbines to establish what benefits they deliver. These provide comprehensive lessons for the design, management and coordination of future DT programmes in low carbon energy technology. (C) 2010 Elsevier Ltd. All rights reserved.