Two windmills on the roof of a grey building in Marsh Road, Causeway Bay, might not mean much to the casual observer but they are clues to the unique character of what Hongkong Electric calls the 'first green substation in Hong Kong'. The Marsh Road substation, which came into use in August this year, serves Wan Chai, Admiralty and Central, including the Tamar site where the new government headquarters will be built. Hongkong Electric said the project cost HK$100 million, of which 2 per cent was spent on environmentally friendly facilities. The six-storey substation uses 15 per cent less electricity each year than traditional substations. That amounts to 250,000 kWh, or the equivalent of a month of electricity used by 714 families, according to Tso Che-wah, the power company's general manager for projects. The British-made windmills and 16 solar panels provide enough energy to light the entire substation. Meanwhile, the building uses light-emitting diode tubes and energy-saving 16mm fluorescent lights, which consume less energy, have a longer lifespan and lead to lower carbon dioxide emissions. Dr Tso said the substation was characterised by its rooftop garden, whose plants helped to lower the interior temperature by 2 degrees Celsius - in turn saving 10 per cent of the energy used each year on air conditioning. The plants are watered mainly by rain, which is collected and stored in a tank on the roof. That saved about 180,000 litres of water a year, Dr Tso said. The substation had been built in an environmentally friendly way, he said. It used concrete that contained 5 per cent fuel ash, from the company's power station on Lamma Island. Further, the rooftop was paved with recycled plastic waste. Inside the building, elevators are switched off if left unused for more than 30 minutes, while extra windows for ventilation in the corridors reduce the need for air conditioning. The Marsh Road substation was built to 'accommodate community development in the next decade', Dr Tso said. Hongkong Electric has about 40 substations on Hong Kong Island. Some older substations had been renovated to include more environmentally friendly features. For example, a rooftop garden was added to the Connaught Road zone station, which is next to the Harbour Building. Future substations would all be green, according to Dr Tso.