The calculator's a bit rough around the edges. There's no option to enter information about your WiFi card, for example. But it should give you a pretty good estimate. If you're not running Windows Vista, we'll just let you know that odds are your computer is using more than 100 watts per hour.

So what's the easiest way to reduce your computer's energy use? Turn it off. No, not all the time silly, but when you go to bed at the very least. Unless you need to leave your PC on all the time (to act as a web server or a personal video recorder), turning your computer off at night is probably the easiest thing you can do to save money and reduce your carbon footprint.

For example, check out your PC's power options and make sure that you've enabled sleep or hibernate modes. You can either schedule your computer to go to "sleep," at a scheduled time of day or night, or you can set it to sleep when your CPU is idle. Sleep and hibernate modes basically save data to RAM or to a hard disc and shut down most of your computer's processes so that it uses very little power. But when you go to use your PC again, it loads much faster than it would had you rebooted.

But you can also reduce your computer's power use without putting it to sleep. Some newer dual core processors allow you to shut off one of the cores if you don't need all that extra juice. If you're using a laptop, you'll also notice slightly longer battery life.

If you're in a well lit room, you might also want to dim your monitor or laptop display. And pay extra attention the power management settings. Windows Vista, for example, gives you three power options, "power saver," "balanced," and "high performance." But if you look at the advanced settings, you'll notice that you have much more control over your system settings. In general, the lower your CPU cycles and display brightness, the less power you'll be using.

If you'd rather not muck around with all those settings yourself, check out CO2 Saver. This little application works with Windows XP and Vista machines, and adjusts your power settings automatically. When you haven't used your computer in a while, it CO2 Saver will kick in and lower your PC's performance settings.

Another simple trick is to turn off your screen saver. Just select "blank" from your screen saver menu. Once upon a time, screen savers helped protect monitors from damage. Now they just needlessly draw energy to look pretty while you're proabbly not paying attention. Setting your screen saver to "blank" will result in a blank screen coming up after you haven't touched your keyboard or mouse for a specified period. Blank means very little power draw. But keep in mind, blank is not the same thing as "none." Setting your screen saver to "none" will prevent the screen saver from ever kicking in, and could actually increase your electric usage.