How much power does my computer consume?

This is a very interesting question that is becoming more current with all the green power awareness nowadays.

In average, an office desktop workstation will consume 130 to 175 watts per hour. That includes the PC (with a 65w based CPU, HDD drive and a basic video card) and a 40 watt LCD monitor.
The consumption is quite variable because the CPU consumes power based on its load, so at near 95% of CPU load this kind of station will consume around 175 watts, and when idle - it goes down to 130 watts. The monitor also varies in power consumption based on the backlight luminance, so at max luminance this monitor will consume 40 watts.

But does the station consume any power when in sleep mode or powered off completely? Yes it does. The PC itself may consume around 6-7 watts when turned off but plugged in (some motherboards power off completely, while others still provide +5V to USB ports). Same goes for the monitor in standby mode (3-6 watts).

Now, what is important when choosing or building a computer that is energy efficient?

First we start with the CPU, which are currently Haswell based CPUs by Intel, or Piledriver based Richland CPUs by AMD.
The lowest power consuming models of these CPUs (35-45watts) will have the letter T at the end of the model name (i.e. Pentium G3420T by Intel or A10-6700T by AMD).

Power Supply Unit - Power supply quality matters when we're talking about power efficiency, because a lot of energy is wasted when the PSU converts AC to DC necessary to power the computer.  So the better the PSU quality - the less power will be consumed. A good indication of power supply quality is the 80 Plus certification.

Motherboards - regular motherboards usually consume less power than high-end ones because they have less features (slots, ports, etc). Some motherboard manufacturers provide power saving technology built into their products such as Gigabyte's Dynamic Energy Saver or Asus' EPU technology.

Video cards - a video card is usually necessary for gaming or advanced image processing (CAD, 3D or video editing), and usually is power consuming, so unless there's a specific need for it - it may not be used.

Hard Drive - SSD drives consume about 70% less power than HDD drives. However, some energy efficient HDD drives will spin down when idle (such as the Green drives by WD), so that is a plus when building an energy efficient workstation.

Optical Drive - DVD drives generally consume a bit less power than Blu-Ray drives. An external USB optical drive may be used when necessary.

Cooling - A low power CPU will require less cooling, however cooling fans are not very power consuming, generally 1-6 watts.

Monitor - LED backlight based monitors generally consume less power (over 50%) than conventional LCD monitors, so replacing an older LCD monitor with a new LED based LCD monitor may be a good idea.

Speakers - Unless sound quality is of high importance, it might be quite efficient to use USB powered speakers, not only to use one less power socket, but also to save power by having small speakers that do not have their own power supply that wastes power.

So, to summarize - the most energy efficient component replacements are CPU, PSU, hard disk (SSD), and the monitor.

Click HERE for a sample configuration of an energy efficient station.