This paper focuses on the specific role of installers, a category of often overlooked diffusion intermediaries doing the actual implementation of energy transitions. We adopt an ecosystems perspective and aim to provide new knowledge on the installers' role in energy transitions, possible changes in this role, and the challenges installers face. Based on evidence from case studies in Gothenburg and Rotterdam, we first show how installers make or break energy transitions. They differ from other intermediaries in their long-term trust relations with customers, their deep contextual knowledge, and involvement in post-technology deployment. We unveil new nuances regarding downstream (installers deploy strategies to include budget-constrained customers in energy transitions) and upstream actors (installers face manufacturers' lock-ins and are trained by wholesalers and manufacturers). Secondly, we show challenges installers face caused by regulatory, market and technological dynamics in transitions, and identify new roles for them as IT-specialists, manufacturers and holistic advisors.