Improving the energy efficiency of household appliances is a global strategy for mitigating the effects of climate change and ensuring energy security. Consumers, however, often underinvest in energy efficiency. This research draws on the theory of diffusion of innovation to develop an electronic word-of-mouth (eWOM) process model of energy-efficiency uptake in online marketplaces. Model estimation, based on room air conditioners’ data from a large online marketplace in India, uncovers underexploited eWOM pathways to accelerate the diffusion of energy efficiency innovations for the benefit of consumers and the health of the planet.