Regardless of the source, energy is a major factor for development. It is needed for transport, industrial and commercial activities, buildings and infrastructure, water distribution, and food production. Most of these activities take place in or around cities, which are on average responsible for more than 75 per cent of a country’s Gross Domestic Product (GDP) and therefore the main engines of global economic growth. To run their activities, cities require an uninterrupted supply of energy. They consume about 75 per cent of global primary energy and emit between 50 and 60 per cent of the world’s total greenhouse gases.

This figure rises to approximately 80 per cent when the indirect emissions generated by urban inhabitants are included. Buildings also consume vast amounts of energy at all stages of their existence. Energy is needed for the raw materials, construction process, and maintenance and daily operational needs such as lighting, air conditioning, and cleaning. In addition, urban sprawl, increasing distances between destinations, and inefficient public transport systems prompt overall reliance on private motorized transport, such as cars, which have a high energy consumption, mostly of petroleum products.

In 2012, global energy supply consisted of 81.3% fossil fuels (oil, coal and gas), 9.7% nuclear power, and only 9% renewable energy sources (such as hydro, wind, biomass and solar). Unfortunately, this widespread use of fossil fuels causes a number of challenges. Carbon-based energy generation has a large ecological footprint, not only due to rising greenhouse gas emissions and pollution caused by burning fuels, but also because of extraction techniques that contaminate the environment, and frequent production or delivery accidents.

Furthermore, because of the current mono-dependency on fossil fuels, supply drops or price hikes can easily disrupt economies. Fossil fuels are also all too often a source of regional conflicts and are misused as a means of political pressure. Besides, fossil fuel resources are not infinite, and their depletion is a near reality.

A sustainable urban energy system will need low carbon technologies on the supply side, and efficient distribution infrastructure as well as lowered consumption on the end-user side. Cities therefore need to shift from the current unsustainable fossil fuel energy generation towards using renewable energy sources, not only because of looming resource depletion but also to curb the negative externalities such as pollution and greenhouse gas emissions. At the same time, energy consumption must be reduced by changing consumption patterns and adopting energy saving techniques. 

Lastly, because energy is paramount to revenue generation, its distribution needs to become more inclusive and fair to foster universal development, especially for the urban poor. Although renewable energy technologies (RET) such as wind, water, solar, and geothermal are becoming more accessible -- and already cover the energy demands of some neighbourhoods if not whole cities in certain areas -- intermittence of supply and high upfront costs are the main deterrents of a wider adoption. Nevertheless, benefits in the long run will outweigh the initial challenges, both from an environmental and economic perspective.

For instance, once a renewable energy generation system is in place, future running costs are usually very low due to an inexpensive and abundant supply of the energy source. Cities also need to assess their meteorological and geographical specificities in order to best transform the surrounding natural resources into power. To tackle intermittency, several renewable energy sources should be combined to overcome source-specific shortages, such as solar at night, or wind during doldrums. Solutions can also come from waste and heat recovery technologies that can be used to bridge supply gaps.

Smart grids – electric grids that harmonize supply and demand – provide another solution for the intermittent power supply by helping to balance variable power generation and end-user needs. These grids are also more efficient in transmission and distribution, thus reducing energy loss. Machine shifts can be automated to run during hours of the day when there is enough power to meet demand (for example, washing machines do not need to run at a specific time, so they can turn on automatically while the customer is asleep, or at work).

Cities need to establish strong policies and standards to develop sustainable urban energy systems and to reduce the use of unsustainable technologies and practices. Governments must not only institute legislation to regulate energy use and consumption, but must also set up incentive measures that promote research, innovation, and, most importantly, the adoption of greener and more efficient technologies. Sound collaboration and mutual understanding between the private sector -- which runs most of the world’s energy systems -- and overseeing authorities is therefore paramount for short-term commercial interests not to overshadow long-term environmental concerns and sustainable development opportunities.

Governments should also pursue collaboration between local and international partners in order to enable local companies to strengthen their knowledge, expertise, and market reach. Governments of developing countries should consider private-public partnerships to develop their energy systems, as current costs cannot be carried by a country alone. For each city to be able to adapt to its own local particularities, authorities need to design decentralized energy systems and infrastructure, and also be permitted to have specific legislation and tax systems to either promote the use of sustainable energy, or to curb and dissuade the use of polluting, inefficient technologies and consumption habits.