  The global energy system is undergoing a major transition, and in energy
planning and decision-making across governments, industry and academia, models
play a crucial role. Because of their policy relevance and contested nature,
the transparency and open availability of energy models and data are of
particular importance. Here we provide a practical how-to guide based on the
collective experience of members of the Open Energy Modelling Initiative
(Openmod). We discuss key steps to consider when opening code and data,
including determining intellectual property ownership, choosing a licence and
appropriate modelling languages, distributing code and data, and providing
support and building communities. After illustrating these decisions with
examples and lessons learned from the community, we conclude that even though
individual researchers' choices are important, institutional changes are still
also necessary for more openness and transparency in energy research.
