The importance of computer modelling in urban energy systems can be seen as twofold. First, it allows one to analyse and understand the current state of these systems. But also, and perhaps more importantly with regards to sustainability, it allows one to ‘predict, prescribe and invent’ the urban systems of the future (Batty 1976). Clearly however, ‘exact’ modelling of urban systems, on the scale of building-to-building or finer detail, is not feasible, as an enormous amount of information would be required to construct a complete urban model. It would also likely not be desirable, as it would limit the applicability of the derived model (e.g. to a specific city), with little scope for generalization. Therefore any urban energy systems model developed will be an idealization or simplification of reality; the extent depends on model complexity and the approaches used. Models can range from the highly aggregated to disaggregated, partial to general, static to dynamic, and so on, depending on their intended use.