The paper presents the initial version of an energy domain ontology intended to consistently describe active heating and electric distribution grids in the Industry 4.0 and energy transition context. The ontology applies as a semantic basis for digital twins of grid segments to reduce costs of integrating numerous disparate models that comprise the twin. Therefore, our ontology, unlike many others, is tailored for the convenience of modeling grid operation and management processes in all parts and aspects. Yet, top-level ontology concepts are made compatible with semantic models and ontologies existing in the energy domain, such as Common Information Model (CIM), Smart Appliances REFerence (SAREF), Domain Analysis-Based Global Energy Ontology (DABGEO). The taxonomy of processes to be twinned occupies a central place in our ontology. Taxonomies of other top-level concepts relate to them: events trigger processes, actors initiate processes and participate in them, equipment and other resources are used/affected, and models formally represent everything.