The energy for growing and maintaining cities.

Herein we develop a means to differentiate between the energy required to expand and the energy required to maintain the economies of cities. A nonlinear model is tested against historical data for two cities, Hong Kong and Singapore. A robust fit is obtained for Hong Kong, with energy for maintenance close to that for growth, while Singapore, with a weaker fit, is growth dominated. The findings suggest that decreases in either of the per unit maintenance or growth demands can simultaneously cause gross domestic product (GDP) and total energy use to increase. Furthermore, increasing maintenance demands can significantly limit growth in energy demand and GDP. Thus, the low maintenance demands for Hong Kong, and especially Singapore, imply that, all other things being equal, GDP and energy use of these cities will continue to grow, though Singapore's higher energy use for growth means it will require more energy than Hong Kong.