Power consumption of PC components had never been a real issue for most users - until it started to reach levels at which it impacted cooling, noise and reliability. Processors such as the the Intel Pentium 4 family at 3.6 and 3.8 GHz or the first Pentium D dual cores drove the maximum power requirements of desktop processors well beyond the 100 Watt level, and hence attracted negative attention. Cooling down a component that runs hot requires extensive cooling, which in turn means more noise that fans generate. Lastly, energy costs have been an issue for enterprises, and are becoming important for an increasing number of end users, as energy costs, including the price of air conditioning, will only go one way: up.
The issue of high-energy requirements isn't only related to processors, but to most system components found in modern computers. Once processors were identified as potential troublemakers, the term "Thermal Design Power", or TDP, quickly moved from datasheets into our everyday hardware lingo, and the media as well as end users requested TDP specifications for other components such as chipsets, graphics processors and other silicon products. Graphics cards, motherboards and even hard drives are being watched suspiciously from the power consumption standpoint, as well.
However, looking at the maximum power a component may convert into heat may not provide more than a theoretical piece of information: You have to consider that no processor or graphics chip will run at maximum load all the time, but most likely will spend a lot of time running idle or at low loads. Hence, the idle or minimum power requirement is at least equally interesting, if not more important. Finally, the actual power requirement of the entire system highly depends on how many hours per day the system is powered on or on standby.
You can now visit your favorite computer store and shop for the lowest-power components available. This will give you peace of mind, but it isn't necessarily the best option. A system based on a VIA C7 processor or special solutions using AMD or Intel mobile and high-efficiency processors will provide minimum system idle power requirements of 40 W or less, and a maximum requirement of no more than 50-60 W (without 3D graphics). But components such as an ultra-low voltage processor or sophisticated motherboards are expensive, and they do not even remotely provide the performance level we're used to. What sense does a low-power system make if it has to use a maximum amount of resources for a long time to complete a task, while a mainstream PC might provide much higher performance to complete the same task in a fraction of the time, allowing it to go back to idle much sooner. The best low-power computing options, which are energy efficient and offer high performance, can often be found in mainstream component options.