Economic and societal costs of the urban heat island are considered through the marginal effect of temperature increase on device efficiency and lifespan. Urbanization is virtually synonymous with the mechanization of human comfort systems, and the efficiency of these systems is subject to degradation from the urban heat island. The simplest way to model this degradation is an application of ideal device efficiencies, and the results of such an analysis are presented and considered in this paper. The magnitude of these costs and their avoidance or potential mitigation avenues are the principal topics of the work, and the technical underpinnings of the approach are presented in supplementary material available online. The self-reinforcing nature and economic scale of the urban heat island effect are thus approached from the first principles of thermodynamics and available data on relevant devices and systems. A global perspective on the phenomenon is presented, followed by a case study of the Phoenix, Arizona (US) metropolitan area to demonstrate the scale of these effects. This analysis synthesizes thermodynamic and economic approaches to the health and policy issues of the urban heat island, with particular consideration given to planning for minimization of these effects in low- and middle-income urban areas. This study first estimates the costs borne today by large urban centres, then highlights some of the risks that secondary cities will eventually face – and could potentially mitigate – as they undergo rapid growth and densification.