The use of transient computer simulations for quantifying energy use of individual buildings is now standard in both research and industry. However, their use has been computationally prohibitive at the larger scales of districts and cities. We present a new simulation platform that offers a spatially differentiated, hourly analysis of energy consumed by the built environment. The City of Westminster, within central London, was chosen for the first pilot application due to diversity of building types and high-energy demand. This seminar highlights the challenges associated with its development, as well how it supports the assessment of energy systems in cities.