  A large proportion of Africa's infrastructure is yet to be built. Where and
how these new buildings are constructed matters since today's decisions will
last for decades. The resulting morphology of cities has lasting implications
for a city's energy needs. Estimating and projecting these needs has always
been challenging in Africa due to the lack of data. Yet, given the sweeping
urbanisation expected in Africa over the next three decades, this obstacle must
be overcome to guide cities towards a trajectory of sustainability and
resilience. Based on the location and surface of nearly 200 million buildings
on the continent, we estimate the inter-building distance of almost six
thousand cities. Buildings' footprint data enables the construction of urban
form indicators to compare African cities' elongation, sprawl and emptiness. We
establish the BASE model, where the mean distance between buildings is a
functional relation to the number of Buildings and their average Area, as well
as the Sprawl and the Elongation of its spatial arrangement. The mean distance
between structures in cities -- our proxy for its energy demands related to
mobility -- grows faster than the square root of its population, resulting from
the combined impact of a sublinear growth in the number of buildings and a
sublinear increase in building size and sprawl. We show that when a city
doubles its population, it triples the energy demand related to commutes.
