Home computers are multiplying as they become the family entertainment and business center. And with more people operating businesses from their homes, the impact these computers and all of their peripherals like printers, scanners, and large color monitors are having on utility bills is increasing. If the computer set-up uses 300 watts for 4 hours per day, at $.10 per kWh, that's about $45 annually or less than $4 dollars a month. So you are not likely to limit your time on the computer to save on your electric bills, but you might thing twice about leaving it on 24 hours a day. That would be 6 times more usage, and more like $270 per year.
Besides just having them on when needed, what you purchase next is the place to focus your attention for energy efficiency. Look for the EnergyStar logo to find ones that use less energy.