Considerations
Monitors running screensavers consume the same amount of power as when running normally, which can be anywhere from a few watts for small LCD monitors to several hundred for large plasma displays. Most modern computers can be set to switch the monitor into a lower power mode, blanking the screen altogether. A power-saving mode for monitors is usually part of the power management options supported in most modern operating systems, though it must also be supported by the computer hardware and monitor itself.
Additionally, using a screensaver with a flat panel or LCD screen instead of powering down the screen can actually reduce the lifetime of the display, since the fluorescent backlight remains lit and ages faster than it would if the screen were turned off completely. As fluorescent tubes age they grow progressively dimmer, and they can be expensive or difficult to replace. A typical LCD screen loses about 50% of its brightness during a normal product lifetime, if left on continuously. (In most cases, the tube is an integral part of the LCD and the entire assembly needs to be replaced.)
Thus the term "screen saver" is now something of a misnomer—the best way to save the screen (and also save electricity) would simply be to have the computer turn off the monitor.
Notice also that screensavers may utilize a significant amount of CPU time, increasing power consumption in the process.
Read more about this topic: Screensaver