Climate change represents a fundamental challenge for human societies. The use of fossil fuels (e.g. coal, oil, gas and peat) in industry, transportation and energy production plays a substantial role in accelerating global warming. This qualitative field study draws on interview data to explore how Finnish municipal energy companies, which are often dependent on fossil fuels, take carbon into account when planning long-term investments. The study focuses on six middle-sized local energy companies, all of which have recently made substantial investments in their power plants. Our results indicate that carbon emissions have emerged as a significant environmental consideration for these organisations. Drawing on the theoretical approaches of institutional isomorphism and epistemic communities, we discuss how different institutional pressures, as well as social and technological carriers of ideas, have influenced the development of practices in the sector. Given the need for a major sustainability transition across the economy, the findings of this study point to the importance of understanding the role of epistemic communities and various carriers of ideas in different settings, as well as of exploring how these elements could be used to accelerate change in key organisational fields.