For the purpose of this discussion, we assume that your PC(s) were supplied with the Windows (tm) Operating System. The greatest opportunity for energy savings will be realized by retaining or resetting the ENERGY STAR default energy saving modes that place the PC and Display in a low power ("sleep" mode) after a period of inactivity. You can make sure that the default ENERGY STAR compliant power management settings are retained or reset through the Windows Power Management control panel. The net savings will vary depending on your use patterns when the PC is in operation. 

You can learn more about savings potential at the following ENERGY STAR official web page: https://www.energystar.gov/products/reduceitenergycosts

As to the potential energy savings that can be realized by shutting your PC off at night:

1. You can save approximately 0.5 W by shutting your PC off at night. This approximate 0.5 W energy savings is the difference in power consumed by the PC in the ENERGY STAR low power ("sleep" mode) ~3.0 W and ~ 2.5 W Off Mode (when still connected to AC mains). This assumes that the default ENERGY STAR compliant power management settings are in effect for this analysis.

2. Additional energy savings can be realized if your PCs are connected to a power switch with a "hard off" switch that when used, will further reduce power consumption to ~ 0.0 W.                                                                                      

For further advice and energy saving tips please visit the following website: http://www.hp.com/hpinfo/newsroom/feature_stories/2009/09energypersonal.html