Mental accounting refers to the fact that people create mental budgets to organize their resource use and to create linkages between specific acts of consumption and specific payments. Research on financial decision-making and consumer behaviour shows that these mechanisms can have a large impact on decisions and behaviours, deviating from normative economic principles. Here we introduce a theoretical framework illustrating how mental accounting mechanisms may influence individual decisions and behaviours driving energy consumption and carbon emissions. We demonstrate the practical relevance of mental accounting in the context of designing carbon pricing mechanisms and discuss the ethical dimensions of applying the concept to intervention design. By bridging the mental accounting literature and research in the energy domain, we aim to stimulate the study of the cognitive mechanisms underlying energy-relevant decisions and the development of novel theory-based interventions targeting reductions of energy use and carbon emissions.