Washington  - At a time when few have money to spare, the cost of running a computer is not an insignificant concern.

A typical desktop computer, together with a monitor and printer, can consume as much electricity as five or six powerful incandescent household light bulbs.

If left running for long periods of time - or even 24 hours a day - a computer can easily cost you more in electrical bills than its original purchase price.

While you can use the power-saving features built in to today's operating systems to put a computer into a sleep mode when it's not being used or simply turn it off, many people don't.

Computers in Internet-connected households or in the workplace are frequently left running non-stop so that they're always available for research or play.

That's why it pays to look closely at how much electricity a computer is likely to use when you're on the hunt for a new machine or upgrading components within your existing one. Computers today run the gamut from power-sipping misers to unabashed energy hogs.

And it's not always clear from the packaging which one you're signing up for.

--- Notebooks

Notebook computers are built from the ground up to consume as little energy as possible - and therefore to save you money over desktop machines. Not all notebooks, though, are built with power- saving in mind. Models that are touted as "desktop replacements" typically are built around the same components that appear in full- sized PCs, and they therefore consume as much electricity - and cost almost as much to operate.

The key specification to pay attention to when evaluating the overall notebook package is battery usage. Generally, the longer a notebook will run on a fully charged battery, the more miserly are its components, and the more energy - and money - you'll save by running it full-time.

--- Under the hood

Whether you're evaluating a notebook or a desktop computer, the components in it determine how miserly it is, so it pays to know something about the power needs of the parts that make up any computer you're evaluating.

Central processing units (CPUs) - the main chips around which PCs are built - often use more energy than other components. While most processors today go into a low-power mode when they're not being taxed heavily by the demands of software, the maximum power usage is telling.

Intel's flagship Core i7 processor, for example, requires 130W of power, and AMD's Phenom X4 Quad-Core uses 140W. Intel's Core 2 Quad chips use 95W, while most of the Core 2 Duo chips consume significantly less, at 65W. What this illustrates is clear: today's powerful processors are also power-hungry, and the more powerful chips consume significantly more electricity and will therefore be costlier to operate.

Processors designed specifically for notebook computers fare much better. Intel's high-end Core 2 Duo chips designed for notebooks, which generally run at 2.1 GHz or better, consume either 25W or 35W, depending upon model. The Core 2 Duo U7700, which runs slower at 1.33 GHz, needs only 10W of power, while AMD's Sempron for notebooks runs at 1 GHz and requires just 8W of power.

Chips designed to run netbooks - the smallest of today's portable units - are the most miserly power users of all. Intel's Atom Z530, for instance, which runs at 1.6 GHz, sips just 2W of power.

Clearly, from a power perspective, cost of ownership of today's fastest desktop processors will be significantly higher than for lower-end desktop processors or notebook processors. Given that even today's low-end processors are sufficient for basic computing tasks - checking e-mail, writing documents, or surfing the Web - you may find that a less powerful processor makes much more sense.

--- Video cards

Today's fastest processors are typically paired with today's beefiest graphics cards, which use so much electricity and throw offso much heat that dedicated fans are required to keep them cool.

The graphics cards from Nvidia and ATI, built for running today's games and 3D applications, can consume anywhere from 117W when idle to well over 300W under load. "Under load" refers to the power draw of the cards when rendering complex objects or powering through a game, while "idle" refers to the power required just to have the video card turned on.

In either case, there's some serious power consumption involved, especially for graphics cards that are in computers left on all day. Video cards in notebook computers generally use 20 per cent less electricity than the CPUs in those same machines.

To save both electricity and money and still get good performance, go for video cards that are a step or two down from cutting edge, and prefer those that use passive cooling - without a fan.

--- Hard drives

Hard drives are on or at the ready almost always when a computer is powered up. So they're always drawing energy. As with other components in a computer, higher performing hard drives - those with rotational speeds of 10,000 revolutions per minute (rpm) or higher - consume the most electricity. To save money, look for hard drives - either desktop or notebook - that spin at 5400 or 7200 rpm. Western Digital's "green" drives, for instance, spin at a variable rate to conserve energy, and they consume about 40 per cent less power than comparable non-green drives.

Solid State Disks (SSDs) are increasingly popular as replacements for hard drives, both in desktop and laptop computers. While many assume that SSDs consume less power than traditional spinning- platter models, recent tests have shown that their power usage is about the same.

--- Monitors

If you're still using an old cathode ray tube (CRT) monitor, ditch it. Its power requirements are probably twice that of a similar-sized LCD panel. Most LCDs use fluorescent back lights, which are energy efficient. What's more, when in sleep mode, many LCDs consume just 2W or less. Better yet, monitors are easy to turn off completely. Do that if you won't be using it for a while.

--- Final decisions

The bottom line is that the energy efficiency of any computer - and by extension the cost of running it - boils down to how much power the individual components consume. In today's world of mix- and- match PC purchasing or building, you have a great deal of control over how energy-efficient your computer will be when armed with the knowledge of how power-hungry its parts are. (dpa)