Energy Star is an international accreditation launched in the United States in 1992 which aims to certify compliance with environmental standards on some products.
Since 2001, the EU is also involved in the limitation of greenhouse gas emissions, especially targeting copiers, among the highest energy consuming appliances.
The ENERGY STAR program mainly concerns the electricity consumption of office equipment during their whole life cycle. Thus, a photocopier which has energy management features can be awarded the Energy Star label.
A multifunction device will use 50% of the energy normally consumed by separate printer, scanner, fax and photocopier devices.
Furthermore, paper production actually consumes more energy than that required to print on it. So double-sided printing will save a lot of money.
Sources: Energystar.org, Wikipedia