The Computer Revolution/Green Computing
Green computing refers to making computers and their production more environmentally friendly. The U.S. Department of Energy and the Environmental Protection Agency (EPA) developed the ENERGY STAR program; if hardware is ENERGY STAR compliant it displays the ENERGY STAR label. ENERGY STAR focuses on efficiency while devices are being used; as well as power management features that can put the device into sleep or hibernation mode after a set amount of time of not being used. 
Energy Consumption and Conservation
Today, because our computers are faster and more powerful than computers of old, the energy consumption and heat generation associated are higher than ever. Coupled with the high cost of energy, businesses are trying to find ways to reduce costs and energy consumption. A common energy saving tactic is the consolidation of servers. The Environmental Protection Agency (EPA) is also in the process of developing an energy performance rating system for data centers. Hardware manufacturers are developing energy efficient computers and components and energy-saving features. Because computers can consume a fair amount of energy despite in standby and sleep mode, businesses and schools are progressively using software to automatically shut down computers when not in use. Devices that draw power when turned off, (referred to as energy vampires) include computers, home electronics, and home appliances. It is estimated that the average U.S. household can save an average of $100 just by unplugging devices when you are not using them.
Reference: Morely, Deborah; Parker, Charles. Understanding Computers Today and Tomorrow. Course Technology, 2011, Page 666-667.