As well as being the backbone of urban public transport systems, subways are also major consumers of energy. For example, the entire underground train network in Barcelona consumes around 63.1 million kWh a year. A third of the total energy is used to operate subsystems in the subway stations, such as air conditioning, escalators, elevators, and lighting. If it were possible to reduce energy consumption by just a few percent, this would save an impressive quantity of electricity.