There is an increasing focus in research and policy on cities as sites where low carbon transition can and should be driven (Barber 2017; Coenen et al. 2012). This paper contributes to the emerging field of urban sustainability transitions (Frantzeskaki et al. 2017; Hodson et al. 2017) by examining how the scaling up of decarbonisation is achieved across space, beyond local niche experimentation. This paper identifies a critical limitation of existing multilevel frameworks which recognise non-linearities and multiplicity of pathways, but maintain an a-spatial diffusion or aggregation model of scaling (Geels and Johnson 2018; Geels et al. 2018). To address this gap, the paper analyses financial relations underpinning urban renewable energy interventions in order to illuminate spatial difference in scaling processes. Drawing on empirical cases across national contexts, the paper defines four ‘financial rules’ through which to understand the spatial and scalar boundaries of urban sustainability interventions. These financial rules have distinct implications for the possibilities and limits of ‘just’ transition in cities.