Use a computer with an Energy Star designation. It consumes 70% less electricity than other computers. If left inactive, Energy Star computers enter a low-power mode and use 15 watts or less. Spending a large portion of time in low-power mode not only saves energy, but helps equipment run cooler and last longer.
To maximize savings with a laptop, put the AC adapter on a power strip that can be turned off or will turn off automatically. The transformer in the AC adapter draws power continuously, even when the laptop is not plugged into the adapter.
Turn off equipment when it's not in use.
Set monitors to automatically switch to sleep mode when not in use, or manually turn them off. Screen savers don't reduce energy use by monitors.
For your next computer upgrade, consider buying a laptop, which uses much less energy than a desktop.
Source: U.S. Department of Energy