Feedback interventions have proved to be effective at promoting energy conservation behaviour, and digital technologies have the potential to make interventions more powerful and scalable. In particular, real-time feedback on a specific, energy-intensive activity may induce considerable behaviour change and savings. Yet the majority of feedback studies that report large effects are conducted with opt-in samples of individuals who volunteer to participate. Here we show that real-time feedback on resource consumption during showering induces substantial energy conservation in an uninformed sample of guests at 6 hotels (265 rooms, N = 19,596 observations). The treatment effects are large (11.4% reduction in energy use), indicating that the real-time feedback induced substantial energy conservation among participants who did not opt in, and in a context where participants were not financially responsible for energy costs. We thus provide empirical evidence for real-time feedback as a scalable and cost-efficient policy instrument for fostering resource conservation among the broader public.